Negative emissions—Part 2: Costs, potentials and side effects

Abstract

The most recent IPCC assessment has shown an important role for negative emissions technologies (NETs) in limiting global warming to 2 °C cost-effectively. However, a bottom-up, systematic, reproducible, and transparent literature assessment of the different options to remove CO2 from the atmosphere is currently missing. In part 1 of this three-part review on NETs, we assemble a comprehensive set of the relevant literature so far published, focusing on seven technologies: bioenergy with carbon capture and storage (BECCS), afforestation and reforestation, direct air carbon capture and storage (DACCS), enhanced weathering, ocean fertilisation, biochar, and soil carbon sequestration. In this part, part 2 of the review, we present estimates of costs, potentials, and side-effects for these technologies, and qualify them with the authors' assessment. Part 3 reviews the innovation and scaling challenges that must be addressed to realise NETs deployment as a viable climate mitigation strategy. Based on a systematic review of the literature, our best estimates for sustainable global NET potentials in 2050 are 0.5–3.6 GtCO2 yr⁻¹ for afforestation and reforestation, 0.5–5 GtCO2 yr⁻¹ for BECCS, 0.5–2 GtCO2 yr⁻¹ for biochar, 2–4 GtCO2 yr⁻¹ for enhanced weathering, 0.5–5 GtCO2 yr⁻¹ for DACCS, and up to 5 GtCO2 yr⁻¹ for soil carbon sequestration. Costs vary widely across the technologies, as do their permanency and cumulative potentials beyond 2050. It is unlikely that a single NET will be able to sustainably meet the rates of carbon uptake described in integrated assessment pathways consistent with 1.5 °C of global warming.

Full text

Environ. Res. Lett. 13 (2018) 063002 https://doi.org/10.1088/1748-9326/aabf9f
TOPI CAL REVIEW
Negative emissions—Part 2: Costs, potentials and side
effects
Sabine Fuss1,11 , William F Lamb1, Max W Callaghan1,2,J
´
erˆ
ome Hilaire1,5, Felix Creutzig1,3,Thorben
Amann4,TimBeringer
1, Wagner de Oliveira Garcia4, Jens Hartmann4, Tarun Khanna1, Gunnar
Luderer5, Gregory F Nemet6,JoeriRogelj
7,8,PeteSmith
9,Jos
´
eLuisVicenteVicente
1, Jennifer Wilcox10,
Maria del Mar Zamora Dominguez1and Jan C Minx1,2
1Mercator Research Institute on Global Commons and Climate Change, Torgauer Straße 12–15, EUREF Campus #19, 10829 Berlin,
Germany
2School of Earth and Environment, University of Leeds, Leeds LS2 9JT, United Kingdom
3Technische Universit¨
at Berlin, Straße des 17. Juni 135, 10623 Berlin, Germany
4Universit¨
at Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
5Potsdam Institute for Climate Impact Research, D-14473 Potsdam, Germany
6La Follette School of Public Affairs, University of Wisconsin–Madison, 1225 Observatory Drive, Madison, WI 53706, United States of
America
7ENE Program, International Institute for Applied Systems Analysis (IIASA), Laxenburg, Austria
8Institute for Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
9Institute of Biological and Environmental Sciences, University of Aberdeen, 23 St Machar Drive, Aberdeen, AB24 3UU, Scotland,
United Kingdom
10 Department of Chemical and Biological Engineering, Colorado School of Mines, Golden, CO, United States of America
11 Author to whom any correspondence should be addressed.
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RECEIVED
20 October 2017
REVISED
30 March 2018
ACCEPTED FOR PUBLICATION
20 April 2018
PUBLISHED
22 May 2018
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Attribution 3. 0 licence.
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E-mail: fuss@mcc-berlin.net
Keywords: climate change mitigation, negative emission technologies, carbon dioxide removal, scenarios
Supplementary material for this article is available online
Abstract
The most recent IPCC assessment has shown an important role for negative emissions technologies
(NETs) in limiting global warming to 2 ◦C cost-effectively. However, a bottom-up, systematic,
reproducible, and transparent literature assessment of the different options to remove CO2from the
atmosphere is currently missing. In part 1 of this three-part review on NETs, we assemble a
comprehensive set of the relevant literature so far published, focusing on seven technologies:
bioenergy with carbon capture and storage (BECCS), afforestation and reforestation, direct air carbon
capture and storage (DACCS), enhanced weathering, ocean fertilisation, biochar, and soil carbon
sequestration. In this part, part 2 of the review, we present estimates of costs, potentials, and
side-effects for these technologies, and qualify them with the authors’assessment. Part 3 reviews the
innovation and scaling challenges that must be addressed to realise NETs deployment as a viable
climate mitigation strategy. Based on a systematic review of the literature, our best estimates for
sustainable global NET potentials in 2050 are 0.5–3.6 GtCO2yr−1 for afforestation and reforestation,
0.5–5 GtCO2yr−1 for BECCS, 0.5–2 GtCO2yr−1 for biochar, 2–4 GtCO2yr−1 for enhanced
weathering, 0.5–5 GtCO2yr−1 for DACCS, and up to 5 GtCO2yr−1 for soil carbon sequestration.
Costs vary widely across the technologies, as do their permanency and cumulative potentials beyond
2050. It is unlikely that a single NET will be able to sustainably meet the rates of carbon uptake
described in integrated assessment pathways consistent with 1.5 ◦Cofglobalwarming.
1. Introduction
The Paris goal of holding global warming ‘well below
2◦C’and to ‘pursue efforts’tolimititto1.5◦Cimply
a starkly limited remaining CO2budget (IPCC 2013,
2014b,Rogeljet al 2016). Considering the lack of deep,
near-term decarbonisation, negative emission tech-
nologies (hereafter referred to as NETs) will evidently
© 2018 The Author(s). Published by IOP Publishing Ltd

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
need to play a progressively more important role in cli-
mate stabilization strategies (Rogelj et al 2015a, Luderer
et al 2013).
Studies applying global Integrated Assessment
Models (IAMs) have highlighted the strategic and
long-term importance of CDR for cost-effectively lim-
iting global warming to 2 ◦C(Kriegleret al 2014), the
key technology being Bioenergy with Carbon Capture
and Storage12 (BECCS) (Fuss et al 2014,Clarkeet al
2014)13.Rogeljet al (2018) analyse the most recent and
comprehensive set of 1.5 ◦C scenarios, which remove
about 15 GtCO2yr−1 (median, 3–31 full range) by 2100
through BECCS. This corresponds to 175 (median,
54–404 full range) EJ yr−1 of bioenergy. Such large
amounts of bioenergy can however imply trade-offs
with other land-based policy goals such as biodiver-
sity conservation and food production (e.g. Kraxner
et al (2013), see Creutzig (2016) for a discussion of
the associated views). Recently, afforestation and refor-
estation have been added to many of the IAMs, which
explicitly model the land use sector or are coupled to
large-scale, geographically explicit land use models (e. g.
Humpen¨
oder et al 2014,Calvinet al 2014).
However, in order to perform a high-quality inte-
grated assessment of NETs, a characterization of the
different options is needed. A variety of reviews on
NET technologies have taken on this task over the
years (Smith et al 2016a, National Academy of Sci-
ences 2015,Caldecottet al 2015,Lenton2014,2010,
McGlashan et al 2012,McLaren2012, Vaughan and
Lenton 2011, The Royal Society 2009). From the
approximately 3000 articles on NET technologies and
measures identified by Minx et al (2017), more than 200
are classified as review articles. However, the available
assessments have three shortcomings: first, they insuf-
ficiently bridge the divide between strategic evidence
from long-term climate change mitigation models and
the technological and institutional bottom-up evidenc e
from the engineering and social science disciplines
(Minx et al 2018). Second, the assessment of the
entire NETs portfolio has been very fragmented so
far, with only one publication assessing a full set of
options (Friends of the Earth 2011), and other impor-
tant efforts missing out technologies such as biochar
and soil carbon sequestration (National Academy of
Sciences 2015). Third, none of the available NETs and
geoengineering assessments provide a systematic, com-
prehensive and transparent analysis rooted in a formal
review methodology.
As in the two companion papers to this piece
(Nemet et al 2018,Minxet al 2018), our review is
12 Although the term storage might imply accumulation for future
use, we use it here interchangeably with the term ‘sequestration’in
accordance with the literature reviewed.
13 Notable exceptions are Marcucci et al (2017), Chen and Tavoni
(2013)andStrefleret al (2018b) for DACCS and St refler et al (2018a)
for terrestrial enhanced weathering. Strefler et al (2018b)combined
three NETs (AR, BECCS and DACCS) for the first time.
formalized according to standard systematic review
procedures (such as those more frequently employed
in the medical and social sciences): (1) a search query
is defined for each NET to transparently identify
the relevant literature; (2) studies are then individ-
ually excluded according to pre-defined eligibility
criteria; (3) qualitative and quantitative evidence is
extracted and synthesized from the final document
set (see the supporting information (SI) available
at stacks.iop.org/ERL/13/063002/mmedia for the full
protocol). Such a procedure is necessary to ensure
reproducibility, avoid systematic omissions or biases in
literature selection and to deal with a rapidly expanding
base of knowledge (Minx et al 2017).
This paper is divided into two main parts. The
first section proceeds with a review of low-stabilization
scenarios from the IAM literature, examining the
role of negative emissions in the mitigation portfo-
lio and the magnitudes of CO2that would be removed
from the atmosphere. The second part comprises our
bottom-up review of individual NETs technologies
and options, with a particular focus on magnitudes,
costs and side-effects—both negative (e.g. competi-
tion for land, biodiversity loss or increased ocean
acidification) and positive (e.g. health benefits from
reduced air pollution, reduced ocean acidification,
energy access—particularly off-grid). While the sce-
nario literature in the past has mostly incorporated
negative emissions in the form of BECCS and afforesta-
tion and reforestation, we here consider a larger
range of negative emissions options, including biochar,
enhanced weathering, ocean fertilization, direct air car-
bon capture and storage, soil carbon sequestration and
some further options with smaller literature bases.
2. Scenario evidence on the role of negative
emissions
The IPCC’s Fifth Assessment report highlighted a
potentially important role for NETs in keeping global
temperature rise below 2 ◦C with a probability greater
than 66% (IPCC 2014a,Clarkeet al 2014). More
recently the ambition of the Paris Agreement not
only to keep warming well below 2 ◦C, but to pur-
sue further efforts to limit warming to below 1.5 ◦C
(UNFCCC 2015) has pushed NETs into the spotlight
of discussions on viable mitigation pathways (Hulme
2016, Peters 2016,Rogeljet al 2015a, Hallegatte et al
2016, Luderer et al 2013). A series of high level com-
mentaries and recent articles further elevated the issue
and emphasized the controversial nature of NETs
deployments featured in long-term mitigation sce-
narios (Geden 2015,Anderson2015, Anderson and
Peters 2016, Gasser et al 2015, Peters and Geden 2017,
Lomax et al 2015, Williamson 2016,Parson2017,
Field and Mach 2017). We engage with this discus-
sion directly, including its ethical foundations, in part 1
of the review series (Minx et al 2018).
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Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
In this section we review publicly available data
from multi-model inter comparison studies14 in order
to understand the role of NETs in climate change miti-
gation (Riahi et al 2015,Kriegleret al 2015,GEA2012,
Riahi et al 2017, van Vuuren et al 2017a,Kriegler
et al 2013b,2016b). We supplement this data with
further scenario evidence on the 1.5 ◦C limit (Lud-
erer et al 2013,Rogeljet al 2015a,2013a,2013b,
2018). Hence the comprehensiveness and transparency
of our review in this section is related to pooling
the available data from recent studies. While many of
the recent IAM scenarios include negative emissions,
we only systematically review the literature that give
sufficient importance to NETs, i.e. where NETs are
mentioned in abstract, keywords or title. Importantly,
the vast majority of mitigation scenarios considered
here only features negative emissions via bioenergy
with carbon capture and storage (BECCS). We inter-
pret this evidence as a lower-bound estimate of negativ e
emission potentials in these models, since the introduc-
tion of additional NETs seems to consistently increase
cumulative NETs deployment (Chen and Tavoni 2013,
Humpen¨
oder et al 2014, Marcucci et al 2017).
2.1. Understanding the role of negative emissions for
achieving alternative long-term climate goals
The carbon budget has been established in IPCC AR5 as
a fundamental concept to understand human-induced
(long-term) warming. It is defined as the cumulative
amount of net CO2emissions that can be released while
still limiting warming with a specific minimum proba-
bility to below a given temperature threshold (IPCC
2013,2014b,Rogeljet al 2016). In principle, gross
CO2emissions can be larger than the carbon bud-
getaslongastheyaresimultaneouslycompensatedby
negative emissions (figure 1)(Kriegleret al 2014,Riahi
et al 2015,Eomet al 2015, van Vuuren et al 2013,
van Vuuren and Riahi 2011, van Vuuren et al 2007,
Azar et al 2006,2010). Yet the geophysical limits of
negative emissions are currently not well understood
(Rogelj and Knutti 2016), although they are starting
to be explored more rigorously (Keller et al 2018). A
recent study asserts that the carbon budgets associ-
ated with the Paris Agreement temperature goals can
be revised upwards from AR5 estimates (Millar et al
2017). This discussion is still new and unresolved. In
the absence of a broader body of evidence we maintain
the estimates from the IPCC AR5 (IPCC 2014b).
Looking across the available scenario evidence,
two major purposes of negative emissions in climate
change mitigation can be identified: first, NETs are
deployed in scenarios for biophysical reasons, because
the carbon budget consistent with a given tempera-
ture target is exceeded (van Vuuren et al 2007,van
Vuuren and Riahi 2011,Clarkeet al 2014). The ‘pay-
back’for this temporary exceedance is the required
amount of cumulative net negative emissions, i.e. the
total global net removal of carbon dioxide from the
atmosphere towards the end of the 21st century when
NETs draw global emission levels below zero (Blan-
ford et al 2014,Kriegleret al 2013a). Compensation
of excess positive emissions by negative emissions can
come with a penalty (or ‘interest’), because the cooling
from net negative anthropogenic emissions may only
offset part of the warming from earlier positive emis-
sions (Zickfeld et al 2016). Second, negative emissions
are deployed in scenarios for intersectoral compensa-
tion, i.e. to offset residual emissions that are difficult
to mitigate, such as transportation—especially emis-
sions from aircraft—and industrial emissions (Rogelj
et al 2015a), or non-CO2GHGs from agriculture (Ger-
naat et al 2015). This occurs particularly in the second
half of the 21st century when high carbon prices are
realized in integrated assessment models. In figure 2
this can be observed when the total removal of CO2
by NETs—henceforth referred to as gross negative
emissions—is much larger than cumulative net neg-
ative emissions (Kriegler et al 2013a, van Vuuren and
Riahi 2011,Kreyet al 2014a, van Vuuren et al 2013).
It is important to note that in some scenarios NETs
are predominantly deployed because they are econom-
ically attractive (Azar et al 2006,Luckowet al 2010,
Lemoine et al 2012), while in others they are biophysi-
cally required. For instance, some scenarios show that
with immediate strengthening of climate policies it is
still possible to limit warming to below 2 ◦Cwithout
NETs (Kriegler et al 2014). However, further delay of
action or the tighter 1.5 ◦CclimategoalrendersNETs
indispensable in the currently available scenarios.
Figure 3provides an overview of emission pathways
for achieving alternative climate targets and the role of
negative emissions therein (Panel A). Scenario ev idence
available on the 1.5 ◦C warming limit remains compar-
atively limited. For IPCC AR5, evidence from only two
models was available (Luderer et al 2013,Rogeljet al
2015a,2013a,2013b). We complement these studies
with more recent 1.5 ◦C scenarios that span a variety of
socio-economic conditions (Rogelj et al 2018).
Temperature overshoot is a typical feature in avail-
able 1.5 ◦C scenarios although the current scenario
literature has not specifically focused on avoiding it15.
All available scenarios hence show net negative cumu-
lative emissions budgets for the second half of the 21st
century (2050–2100) (Rogelj et al 2015a,2018)oreven
in the longer run until 2300 (Akimoto et al 2017).
14 LIMITS (https://tntcat.iiasa.ac.at/LIMITSDB/), AMPERE
(https://tntcat.iiasa.ac.at/AMPEREDB/), RoSE (www.rose-
project.org/database) provide results for hundreds of scenar-
ios from roughly a dozen of models in the absence of explicit
information on NETs in the even larger IPCC scenario database
(https://tntcat.iiasa.ac.at/AR5DB/). We also include the SSP scenar-
ios (https://tntcat.iiasa.ac.at/SspDb/). A description of the models
that produced the scenarios analysed in this section is available in
the SI.
15 Azar et al (2013) reported that their IAM is unable to produce
a1.5
◦C scenario without overshoot. A working paper by Holz
et al (2017) suggests that it could be possible although they operate
a system dynamics model that allows for very rapid decarbonisation
and use a climate model that allows for a large carbon budget.
3

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Figure 1. Positive and negative CO2emission components for reaching alternative temperature goals. Basic descriptive statistics of
the underlying data is provided under the bar plot; more detailed data is available in the SI. Values in the row labelled ‘pathways’
indicate the number of available individual pathways. Those in the rows ‘models’and ‘scenarios’indicate the number of available
individual models (see SI for model description) and scenarios. Important note: median values for gross negative CO2emissions can
be considerably lower when a larger ensemble of scenarios is considered, as can be seen in figure 3. Yet not all emission categories were
available from all modelling teams (e.g. gross positive emissions from fossil fuels and industry and net land-use change), so we had
totakeasmallersample(seetableS16inSI).Inthefollowingparagraphsofthissectiononscenarioshowever,wefocusonnegative
emissions and so we use the more complete dataset from figure 3.
Box 1. Defining climate policy scenarios in terms of warming limits.
We analyze the role of negative emissions for keeping temperature rise below alternative warming thresholds—namely 1.5 ◦C, 2 ◦Cand
3◦C. For this purpose we define scenarios in terms of a minimum probability (usually 66%; sometimes 50%) for temperature rise not
to exceed a certain warming threshold (Rogelj et al 2016), as conventionally done in the literature (Rogelj et al 2015a, Luderer et al
2013,Rogeljet al 2011,Clarkeet al 2014):
∙1.5 ◦Cscenarios: Scenarios with a probability greater than 50% of reverting warming below 1.5 ◦C by 2100.
∙Likely 2 ◦Cscenarios: Scenarios that keep warming below 2 ◦C with a greater than 66% probability throughout the 21st century.
∙Medium 2 ◦Cscenarios: Scenarios that keep warming below 2 ◦C with a greater than 50% probability throughout the 21st century.
Most of the scenarios that meet this criterion introduce adequate climate policies after an initial delay (delayed action scenarios).
∙Likely 3 ◦Cscenarios: Scenarios that keep warming below 3 ◦C with a greater than 66% probability throughout the 21st century.
For most scenario studies the reduced-form carbon-cycle and climate model MAGICC was used in a probabilistic setup to determine
implied warming levels (Meinshausen et al 2011,Rogeljet al 2012, Schaeffer et al 2015). While some of these scenario categories are
more in line with the Paris Agreement long-term temperature goal, they do not represent a formal interpretation of the UNFCCC
temperature goal.
All these 1.5 ◦C scenarios are fundamentally depen-
dent on the global-scale availability of NETs (figure 3).
Scenarios that restrict16 the availability of NETs sub-
stantially often result in model infeasibility (Luderer
et al 2013). However, recent structured explorations of
various socioeconomic contexts, the so-called Shared
Socioeconomic Pathways (SSPs) (Riahi et al 2017,
O’Neill et al 2017), have shown that NETs use
can be restricted to some degree in 1.5 ◦Cscenar-
ios if specific socioeconomic conditions are met,
such as low energy demand, sustainable consump-
tion patterns and high crop yield improvements (SSP1)
(Rogelj et al 2018).
Transition pathways limiting climate change to
1.5 ◦C are consistently characterized by sharp imme-
diate reductions of net CO2emissions (3%–7% yr−1
on average between 2030 and 2050, taking 2030 as
a reference) that lead to a fully decarbonized world
(net zero emissions globally) between 2046 and 2056
(15th and 85th percentiles), with a sustained period of
annual net negative emissions17 ranging between 1.3–
29 GtCO2yr−1 during the second half of the century.
In general, NETs deployment throughout this period
is large-scale in currently available scenarios. By 2050
NETs deployment is already between 5 GtCO2yr−1 and
15 GtCO2yr−1 in most scenarios. The associated scale-
up of NETs between 2030 and 2050 therefore takes
placemuchmoreswiftlythaninmost2◦Cscenarios,
removing an additional 0.1–0.8 GtCO2every year on
average. Total NETs deployment across the 21st cen-
tury is associated with a cumulative removal of carbon
of 150–1180 GtCO2.
16 Typical restrictions imposed are the unavailability of CCS and a
restriction of the annual bioenergy potential to 100 EJ (Krey et al
2014b, Luderer et al 2013,Clarkeet al 2014).
17 Please note that this analysis only considers gross negative emis-
sions from BECCS as those from AR are n ot available in the datasets.
4

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
(a) (b)
Figure 2. The role of negative emissions in climate change mitigation. The graph juxtaposes emission reductions from conventional
mitigation technologies (panel A) with the removal of carbon dioxide via negative emissions technologies (panel B) in an exemplary
scenario con sistent with a 66% chance of keeping warming below 2 ◦C relative to a baseline scenario. Global emission levels turn net
negative towards (hatched blue area) the end of the century to compensate for earlier carbon budget overshoot. Cumulative gross
negative emissions represented by the entire blue area. The exemplary scenarios ‘business as usual’and ‘below 2 ◦C’were constructed
using data from the LIMITS database (https://tntcat.iiasa.ac.at/LIMITSDB/). They correspond to the LIMITS-RefPol and LIMITS-
RefPol-450 scenarios produced with the MESSAGE model. Gross positive an d negative CO2emissions from land-use changes labelled
as ‘land use’(bottom grey shaded area) and ‘afforestation/reforestation’(bottom blue shaded area) were inferred from net land-use
changes emissions by using data in figure SI13 in Popp et al (2017). This manual edit was done to account for current afforestation
and reforestation efforts and differentiate between negative emissions from land-use changes and other NETs.
Our ranges describe the statistics of an ensem-
ble of opportunity, which has not been designed
to span all possible outcomes in terms of NETs
deployment. It mostly represents dynamics that are
considered cost-effective by models in absence of
particular societal preferences. The ranges thus repre-
sent characteristics of the currently available literature.
Additional research needs to confirm that these can
also be interpreted as requirements in a more formal
sense.
Compared to 1.5 ◦C scenarios, the literature on the
role of NETs in 2 ◦C scenarios is much more mature
and rooted in a series of inter-model comparison exer-
cises (Riahi et al 2015,Kriegleret al 2013b,Kreyet al
2014b,Kriegleret al 2016b,2016a,2014,Fusset al
2014,TavoniandSocolow2013,Kriegleret al 2013a,
van Vuuren et al 2013). These have been summarized
in IPCC AR5 (Clarke et al 2014). In 2 ◦C scenarios
NETs are primarily deployed for limiting overshoot
in atmospheric concentrations rather than tempera-
tures (Riahi et al 2015,Blanfordet al 2014,Tavoni
and Socolow 2013, van Vuuren et al 2013). While this
still leads to a significant reduction in the probabil-
ity of reaching the long-term climate goal (Schaeffer
et al 2015,Riahiet al 2015,Eomet al 2015), tem-
perature overshoot carries additional risks associated
with higher levels of warming and the resulting impacts
and climate feedbacks that could occur (van Vuuren
et al 2013, Solomon et al 2009, Friedlingstein et al
2006,Clarkeet al 2014).
In general, 2 ◦C scenarios feature much more flex-
ibility in NETs deployment, covering a wide range
from zero to levels comparable with higher bound
deployments in 1.5 ◦C scenarios. Hence, it is impor-
tant to highlight that while many 2 ◦C scenarios deploy
NETs at large scale, there are also scenarios that do
not deploy NETs at all, or at very low levels (e.g.
Eom et al 2015,Kriegleret al 2014, Luderer et al
2013,Riahiet al 2015,Rogeljet al 2013a)—an aspect
that is often sidelined in discussions but is crucial
for understanding the policy option space (Eden-
hofer and Kowarsch 2015,Minxet al 2017). For 2 ◦C
scenarios featuring NETs deployment, it also points
towards a strong economic rationale within models,
as towards the end of the 21st century NETs become
economically attractive if a temporary overshoot of the
CO2budget is allowed, or if residual GHG emissions
from other sectors are highly expensive to mitigate
(Kriegler et al 2013b,Kreyet al 2014a,Kriegleret al
2014).
In the 2 ◦Cscenarios
18 with immediate implemen-
tation of climate policy and no additional technological
constraints, net CO2emission reductions between 2030
and 2050 take place at an average rate of 1%–4%
peryear(Riahiet al 2015). After 2080 more than
two thirds of these 2 ◦C scenarios have completed
the decarbonisation of the world economy, i.e. they
18 Here we account for both likely 2 ◦Candmedium2◦Cscenarios.
5

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
(a)
(b)
(c)
(d)
Figure 3. The role of negative emission s for achieving alternative long-term clim ate goals. Climate goals are indicated with b lue shades.
The more stringent the climate goal, the darker the blue colour. Net CO2emissions are displayed in panel (a) (top-left). Ribbons
indicate the 15th and 85th percentiles for each climate goal. The original RCPs and the SSP2–2.6 marker scenarios are provided for
orientation purposes. The boxplots in panels (b)–(d) provide the same statistics. The range between the minimum and maximum
values is indicated with a vertical solid line. The range between the 15th and 85th percentiles is indicated by a blue-filled rectangle.
The median is shown with a solid horizontal line whereas the mean is indicated by a white point. NETs deployments in 2030, 2050
and 2100 are shown in panel (b). Cumulative gross negative CO2emissions between 2011 and 2100 are shown in panel (c). Annually
averaged gross negative CO2emissions between 2030 and 2050 are displayed in panel (d). Descriptive statistics of the underlying data
are provided under panel (d) and more detailed data is available in the SI. Statistical differences between figures 1and 3arise because
a few modelling teams did not report all variables necessary to plot figure 1(i.e. gross positive emissions from fossil fuel and industry,
net land-use emissions). A description of the models is provided in the SI.
transition from net positive to net negative CO2emis-
sions (Rogelj et al 2015b).Whiletherearesome
scenarios available without net negative emissions at
the end of the century, most scenarios feature consid-
erable NETs deployment ranging from 5 GtCO2yr−1
to 21 GtCO2yr−1 at the end of the 21st century. These
annual deployment ranges are therefore not much
lower than for 1.5 ◦C scenarios. In 2 ◦C scenarios
with limited or no negative emissions (labelled with
‘limited bioenergy’or ‘no CCS/BECCS’), decarboni-
sation (including fossil fuel phase-out) occurs more
rapidly than in the most cost-efficient 2 ◦C scenarios
(full portfolio), but at a higher overall cost (Kriegler
et al 2014,Kreyet al 2014a,Riahiet al 2015, Luderer
et al 2014). For instance, Luderer et al (2013)—based
on the REMIND model—show that mitigation costs
defined as the ratio of discounted19 and aggregated
19 Luderer et al (2013) applied a discount rate of 5% to both con-
sumption losses and GDP.
consumption losses over discounted and aggregated
GDP increase from 1.4% to 1.9% if BECCS (as the only
explicit NETs option in the model) is limited, and to
2.3% in the absence of BECCS in 2 ◦C scenarios. Lim-
iting BECCS in 1.5 ◦C scenarios increases costs from
2.3%–4.1%, while the absence of BECCS makes the sce-
narios infeasible. Similarly, multi-model results from
EMF27 highlight the most significant cost mark-up for
imposed technology constraints when CCS remains
absent and bioenergy is limited to 100EJ (Kriegler
et al 2014). One key reason for the larger cost mark-ups
could be the constraints imposed on the NETs deploy-
ment potentials in the scenarios. Klein et al (2014)
show that the negative emissions value of biomass tends
to dominate over its energy value in low stabilization
scenarios.
Low energy demand trajectories (low energy
intensity) with more aggressive energy savings are
an important option for providing further flexibil-
ity in 2 ◦C scenarios for achieving the climate goal
with lower negative emissions deployments (figure 4)
6

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
(a) (c)
(d)
(b)
Figure 4. Negative emissions have a distinct role in 2 ◦C scenarios depending on the technological options and policy timing.
Technological options and policy timing are indicated with various colours (dark blue for full technological portfolio, light blue for
low energy intensity, green for limited biomass and no CCS/BECCS, and red for delay action until 2030). The cases full portfolio, low
energy intensity and limited biomass or no CCS/BECCS assume climate action from 2010 onward. Net CO2emissions are displayed
in panel (a) (top-left). Ribbons indicate the 15th and 85th percentiles for each pathway category. The original RCP-2.6 (also called
RCP-3PD) and the SSP2–2.6 marker scenarios are provided for orientation purposes. The boxplots in panels (b)–(d)providethesame
statistics. The range between the minimum and maximum values is indicated with a vertical solid lin e. The range between the 15th and
85th percentiles is indicated by a blue-filled rectangle. The median is shown with a solid horizontal line whereas the mean is indicated
by a white point. NETs deployments in 2030, 2050 an d 2100 are shown in panel (b). Cumulative gross negative CO2emissions between
2011 and 2100 are shown in panel (c). Annually averaged gross negative CO2emissions between 2030 and 2050 are displayed in panel
(d). Basic descriptive statistics of the underlying data are provided under panel (d), and more detailed data is available in the SI. 2 ◦C
scenarios include both likely 2.0 ◦Candmedium2.0◦C scenarios. A description of the models is provided in the SI.
(Rogelj et al 2013b,Kreyet al 2014a,Eomet
al 2015). In particular, gross cumulative negative
emissions deployment (290–760 GtCO2)tendstobe
lower in these scenarios driven by lower deployment
rates (0–7 GtCO2yr−1) and upscaling (0–0.3 of addi-
tional GtCO2yr−1) between 2030 and 2050. Further
delay in adequate global climate action swiftly locks
2◦C pathways with NETs in, including delayed action
until 2030 (as implied by current NDC ambitions).
Like most 1.5 ◦C scenarios, these 2 ◦C pathways can no
longer be achieved without any or even limited amounts
of NETs (figure 5)(Ludereret al 2013,Riahiet al 2015).
Deployment and upscaling rates also increasingly mir-
ror those seen in the available 1.5 ◦Cscenarios.
The CO2removal ranges presented in this review
are much wider than those reported in (Clarke et al
2014)and(Rogeljet al 2015a). The underlying scenar-
ios are almost exclusively assuming middle-of-the-road
and do not consider systematically socio-economic
variations in future conditions. Here, such variations
are considered via the new shared socio-economic
pathways (SSPs) (Riahi et al 2017,O’Neill et al 2017,
Rogelj et al 2018). One important insight from this new
evidence is that the role and importance of NETs in cli-
mate change mitigation scenarios depends crucially on
socio-economic developments (Riahi et al 2017,Bauer
et al 2017,Poppet al 2017, van Vuuren et al 2017b,
Rogelj et al 2018). For optimistic storylines following
a sustainability narrative (SSP1). NETs requirements
can be substantially lower than in middle of the road
scenarios (SSP2) (Riahi et al 2017,Rogeljet al 2018).
Conversely, the dependence on NETs increases in sce-
narios characterized by high energy demand and a
strong preference for using fossil fuels (SSP5). For
instance, to keep global warming below 1.5 ◦Cthe
required cumulative removal of carbon over the 21st
century can decrease by up to 67% in an SSP1 scenario,
but increase by up to 32% in an SSP5 scenario (both
compared to SSP2 scenarios). Likewise, scenarios char-
acterized by strong regional rivalries across the world
7

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Figure 5. Model feasibilityfor different climate policy scenarios. Scenarios differ with respect to the climate policy ambition, the timing
of climate policy and the assumptions of the available technology portfolio. All scenarios share the same exogenous socio-economic
assumptions for GDP, population and energy demand. ‘Many’means that less than 50% of the model runs were feasible whereas
‘some’means that less than 33% of the model runs were feasible. Data come from REMIND model runs (Luderer et al 2013). 2 ◦C
scenarios include both likely 2 ◦Candmedium2◦Cscenarios.
(SSP3), or strong inequality within and between world
regions (SSP4), would feature different NETs require-
ments. Beyond discussions of how to organize climate
and energy policies with regard to negative emissions
(Peters and Geden 2017), it is therefore also crucial to
start a discussion on how the general development tra-
jectory affecting consumption patterns, energy demand
and international cooperation can be changed in light
of its impact on NETs reliance to achieve stringent
climate objectives.
While the vast majority of studies feature BECCS
as the only explicit NET in the portfolio, a num-
ber of studies examine the role other NETs, often in
small portfolios of two NETs that include BECCS.
These studies looked at afforestation and reforesta-
tion (AR) (Humpen¨
oder et al 2014,Kreidenweiset al
2016,Tavoniet al 2007, Edmonds et al 2013, Reilly
et al 2012,Poppet al 2017,Roseet al 2012,Calvin
et al 2014), enhanced weathering (EW) and direct
air carbon capture and storage (DACCS) (Marcucci
et al 2017, Chen and Tavoni 2013, Strefler et al
2018b). The IAM community is currently investigat-
ing the role of larger NETs portfolios including AR,
BECCS, DACCS and EW.
The IAM literature on AR has now become sub-
stantial. It shows an average cumulative potential
for AR over the 21st century and across models
ranging from 200–860 GtCO2(Humpen¨
oder et al
2014, Kreidenweis et al 2016, Rao and Riahi 2006,
Calvin et al 2014,Tavoniet al 2007, Edmonds et al
2013, Reilly et al 2012).Theupperendoftherange
is computed by models that include endogenous
technological change (Humpen¨
oder et al 2014,Krei-
denweis et al 2016, Edmonds et al 2013). Yet it is
interesting to note that a model that does not consider
this effect, but includes the impacts of climate change
on crop yields, still reports estimates up to 650 GtCO2
(Reilly et al 2012). The lower end of the range of
results is associated with modelling constraints (e.g.
limits on bioenergy production or the availability of
BECCS). Maximum annual deployments over the 21st
century range between 0.5–10 GtCO2(Humpen¨
oder
et al 2014,Poppet al 2017, Reilly et al 2012). A com-
mon finding to all the selected studies is the low cost of
implementing AR compared to that of other NETs. For
instance, Strengers et al (2008) estimated that about
50% of the potential would be available at costs below
55 US$/tCO2,while Humpen¨
oder et al (2014)note
that AR starts at carbon price as low as 6 US$/tCO2.In
terms of policy costs, AR can decrease the costs of mit-
igating climate change by about US$3 trillion (Tavoni
et al 2007).
Chen and Tavoni (2013), Marcucci et al (2017)and
Strefler et al (2018b) provide the only assessments of
DACCS in a full-fledged integrated assessment model.
They find that DACCS may only be profitable for very
stringent climate policies. DACCS is only phased in
after 2065, but then scales up rapidly to annual removal
rates of 35–40 GtCO2yr−1 by the end of the century.
The availability of DACCS in model runs eliminates
sharp emission reductions in the short-term and com-
pensates these delayed reductions via large amounts of
net negative emissions towards the end of the cen-
tury. Despite the relatively short application period
8

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
the cumulative removal is large-scale, up to about
500 GtCO2, and hence is associated with a sharp decline
of atmospheric CO2concentrations. Uncertainty sur-
rounding the development and implementation costs
of this technology currently remains a major barrier
(see section 3.3). Finally, Strefler et al (2018a) assess the
techno-economic potential for land-based enhanced
weathering at about 5 GtCO2for both basalt and fos-
terite. Adding it to the technology portfolio reduces
the carbon price (Strefler et al 2018a). Because EW
does not compete with BECCS, this technology might
be particularly valuable if other NETs options are
limited.
In sum, a common picture of NET portf olios seems
to emerge from the integrated assessment evidence.
Adding a second NET to the mitigation portfo-
lio increases the negative emission potentials while
reducing mitigation costs. In scenarios produced by
inter-temporal optimization frameworks that hav e per-
fect knowledge of future technological availability and
costs, these benefits accrue at the expense of weak-
ened incentives for short-term emission reductions.
We address the issue of potential moral hazard in
paper 1 of this series (Minx et al 2017a). Additionally,
these results suggest that expanding the NETs portfolio
can hedge against risks associated with the large-scale
deployment of BECCS (e.g. biodiversity loss and food
price increase). Finally, the levels and timing of NETs
deployments differs across technologies, as would be
expected.
3. Potentials, costs and implications of
large-scale NET deployment
Whether NETs perform at the levels of deployment
envisioned in integrated assessment scenarios depends
on three crucial features: their biophysical potentials for
carbon sequestration (including storage and its perma-
nence), their economic costs, and the social, economic,
and environmental side-effects of their deployment—
which in turn may pose limitations on potentials
and costs20. Existing assessments suggest that NETs
range widely along these dimensions (Royal Society
2009, National Academy of Sciences 2015)andthat
20 Evidently, for any niche technology to achieve wide-scale adop-
tion, basic research and development needs to take place, and a
specific set of political and institutional conditions must exist to gen-
erate demand. We review NETs along these lines in paper 3 of this
series (Nemet et al 2018).
21 Cost estimates come in a variety of different forms, including (1)
establishment or capital costs (e.g. the cost of converting land use
to forestry, or installing a direct air capture unit); (2) opportunity
costs (e.g. the lost revenue from competing land uses, principally
agriculture); (3) the carbon price required to deploy a NET at a given
scale; (4) the normalized average carbon price required to sequester a
unit of carbon for a given NET. Clearly (4) is the most desirable cost
unit for technology comparison, but it is rarely reported and often
derived from widely varying assumptions. In the following, we will
make sure to highlight which cost type we are putting forth in order
to avoid inducing inappropriate comparisons.
large-scale deployment will indeed have non-trivial
impacts on water use, land footprints and nutrient
use (Smith et al 2016a). In this section we proceed
with an exhaustive review of the bottom-up litera-
ture on seven NETs. We aim to be transparent and
comprehensive in our selection of literature (see SI).
Where global deployment potentials or costs exist for
a given NET, they are reported in the text and tran-
scribed into the SI. This data is used to present ranges
visually, and forms the basis of our synthetic compar-
ison between different technology options. A variety
of drivers may explain differences between reported
ranges, including study methodology (e.g. empirical
research, modelling), scope (technology type, system
boundaries), and constraints (e.g. social, environmen-
tal). Where possible we group costs and potentials by
these drivers, thereby explaining the wide differences
that can be observed between studies. Finally, each NE T
is assessed by a subset of authors with the correspond-
ing expertise. They make an overall judgement of a cost
and potential range for each technology, taking into
account our current understanding of the literature
and social, economic and environmental constraints to
deployment21.
3.1. Bioenergy with carbon capture and storage
(BECCS)
The concept of BECCS rests on the premise that
bioenergy can be provided with zero or at least low
carbon emissions, i.e. about as much additional CO2
is sequestered above baseline when growing additional
biomass as feedstock, as is released during its combus-
tion or other energy conversion processes. If the latter
emissions are then also captured and stored (e.g. in
geological formations), it is effectively taken out of the
carbon cycle and the system generates negative emis-
sions (Creutzig 2016,Creutziget al 2015,Smithet al
2013,2014). As section 2has shown, BECCS features
prominently in the IAM scenario literature and has
been subject to considerable scrutiny since the IPCC’s
Fifth Assessment Report (AR5) in 2014 (Fuss et al
2014, Anderson and Peters 2016). In this section, we
provide a comprehensive overview of the literature on
global potentials and costs, and some key side-effects
of BECCS are considered.
In this assessment, we focus on bioenergy as well
as geological storage potentials as limiting factors and
consider costs and additional aspects from the liter-
ature on the entire BECCS chain in order to keep
this multi-technology review manageable. There is a
lot of additional literature, for example, relevant to
the CCS component of BECCS. This covers many
specific technological aspects related to the capture,
transportation and storage components. A recent com-
prehensive review of this literature is provided in
Bui et al (2018).
Global bioenergy potentials: The availability of
biomass and land is seen as the fundamental lim-
iting factor, structuring discussions about BECCS
9

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
0
0
2500
% of Studies
5000
7500
10000
2005
Aquifers
[4 studies]
Coal beds
[5 studies]
100 75 50 25 0
DNG
[4 studies]
DOF
[3 studies]
DOG
[4 studies]
Tot a l
[4 studies]
2010
Publication Year
Sink
2015
0
Bioenergy
Crops
[15 studies]
500
1000
1500
100
Cost [US$(2011)/tCO2]Total Storage Potential [Gt CO2]
Bioenergy Potential [EJ/year]
200
300
400
BECCS - Costs
CO2 Storage Potential
Bioenergy Potential
Forestry
[7 studies]
Residues
[7 studies]
Waste
[2 studies]
Tot a l
[10 studies]
Resource
Figure 6. Costs and potentials for BECCS. The heat bar panel for costs is shaded according to the proportion of the ranges overlapping
at each cost value (studies are depicted as lines plotted by the year of publication, or as dots where only a single estimate was available).
The heat bar panels depicting negative emissions potentials are shaded according to the proportion of studies whose maximum
estimate (depicted as dots) is greater or equal to each potential value. Where no maximum estimate was available, the estimate was
taken. For BECCS, we show bioenergy potentials categorised by feedstock, and storage potentials by sink (DNG: Depleted natural gas
fields, DOG: Depleted oil fields, DOG: Depleted oil and gas). Estimates and ranges at the top and bottom end of the distribution are
labelled; the data can be further explored in our online supporting material available at https://mcc-apsis.github.io/NETs-review/.
Technology development and transfer
Employment
Market opportunities
Economic diversification
Direct GHG emission substitution
Displacement of activities or land uses
Biodiversity
Soil and water
Use of fertilizers with – impacts on soil and water
Deforestation or forest degradation
Health impacts
Food security
Total No.
Studies
Impact
40
56
24
46
48
82
44
65
74
33
63
43
39
040
Technological
Economic
Environmental
Social and Health
Impact Category
No. of Studies
Figure 7. Distribution studies discussing negative and positive impacts for key side-effects. Adapted from Robledo-Abad et al (2017).
potentials (Krey et al 2014a,Smithet al 2016a,
Creutzig et al 2015). 1 EJ of biomass typically yields
around 0.02–0.05 GtCO2worth of negative emissions.
Total bioenergy potential estimates for 2050 range
from 60–1548 EJ yr−1 . Estimates at the lower-end of
this range (Kraxner and Nordstr ¨
om 2015,Searleand
Malins 2015, Smith et al 2012) all provide minimum
estimates of around 60 EJ yr−1. Bioenergy crop deploy-
ment is limited by land allocation for natural parks
(Kraxner and Nordstr¨
om 2015,Fieldet al 2008 (a 2030
estimate)), or when deployed only on degraded (Wicke
et al 2011, Nijsen et al 2012) or marginal land (Searle
and Malins 2015), which will lead to lower yields. Other
conservative estimates for 2050 consider only residues,
either immediately available (Smith et al 2012)orin
line with 2050 estimates (Tokimatsu et al 2017).
Potentials increase as deployment constraints are
relaxed to include more productive land, with mini-
mum potentials of 130 and 160 EJ yr−1 and maximum
estimates of 216 and 267 EJ yr−1 (Beringer et al 2011,
Rogner et al 2012) and similar estimates for 2055
(Popp et al 2014,Kleinet al 2014). Higher estimates
10

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
are characterized by more limited cropland expansion
and lower nature conservation criteria.
The group of optimistic estimates start at around
350 EJ yr−1 (Cornelissen et al 2012,Fischerand
Schrattenholzer 2001, Smeets et al 2007). Although
Cornelissen’s calculations are limited to rain-fed agri-
culture, and apply a food-first principle, they still
estimate 340 EJ available/yr by 2050. This is partly
due to their inclusion of algae as a feedstock, which
contributes 90 EJ yr−1 to their estimate and the use
of fertilizers over a relatively large deployment area
(673 Mha). Fischer and Schrattenholzer (2001) assume
limited agricultural land expansion due to increasing
yields. Bioenergy crops here are deployed on grassland
rather than constrained to marginal or degraded lands
as in the more conservative estimates above. Smeets
et al (2007) provide the most optimistic estimates
between 370–1500 EJ yr−1.Muchofthispotential
(215–1272 EJ yr−1) comes from dedicated bioenergy
crops and the wide range reported reflects different
factor yield increases (2.9 vs 4.6), area available for
deployment (729 Mha and 3585 Mha respectively) and
rainfed vs irrigated agriculture. Smith (2012)provides
an estimate of biospheric capacity of 727.5 EJ yr−1 over
all vegetated land (11 000 Mha). Rogner et al (2012)
assess a theoretical bioenergy potential of 793 EJ yr−1
if all aboveground net primary production (NPP)
that is not used for food, feed or fiber is devoted to
bioenergy production.
Bioenergy estimates from dedicated crops offer
wide discrepancies, from conservative estimates of
approximately 20 EJ yr−1 (Erb et al 2012a,Thr
¨
an et al
2010, Hakala et al 2008, Nijsen et al 2012,Beringer
et al 2011), to middle ranges of 70–180 (Erb et al
2012b, Yamamoto et al 2000, Hoogwijk et al 2009,
Cornelissen et al 2012,Beringeret al 2011,Thr
¨
an et
al 2010,Rogneret al 2012) to high estimates above
200 EJ yr−1 (Hoogwijk et al 2005, Smeets et al 2007,
Cornelissen et al 2012). Variance depends largely on
available land and yields (Dornburg et al 2010,Boysen
et al 2017), which in turn can be driven by assump-
tions regarding future population and diet (Haberl
et al 2011, Hakala et al 2008), biodiversity and con-
servation restrictions (Erb et al 2012a,Beringeret al
2011,Poppet al 2011), or land quality and technology
improvements (Smeets et al 2007). Hakala’slowesti-
mates use current global statistics rather than projected
yields to account for social and institutional condi-
tions, and they further reduce their estimates when
considering global affluent diets (Hakala et al 2008).
High estimates are derived from scenarios of large-
scale deployment on abandoned agricultural land,
where yield factor increases are far higher than on
marginal lands (Smeets et al 2007, Hoogwijk et al
2009). Higher estimates are commonly grounded in
economic analysis, involving factors such as tech-
nological change to improve yields, whereas lower
estimates focus on ecological and biophysical concerns
and natural limits to sustainable bioenergy deployment
(Creutzig 2016).
Estimates for forestry-sourced bioenergy range
from 38–165 EJ yr−1 (Smeets and Faaij 2007,Lauri
et al 2014, Smeets et al 2007, Cornelissen et al 2012,
Rogner et al 2012).Theonlyestimateabove200EJyr
−1
comes from an aggressive deployment of afforestation
and reforestation activities. Smeets’central estimate
sees forests being deployed on 292 Mha of land, while
Obersteiner et al (2006)’s lower deployment scenario
starts at 290 Mha and goes up to 660 Mha for the
extreme estimate of 1250 EJ yr−1 . By contrast, the
lowest estimate comes from the application of strict
sustainability criteria that excludes consideration of
protected, inaccessible and undisturbed forests, as well
as non-commercial species and traditional-use biomass
resources (Cornelissen et al 2012).
Although not fully assessed here, algae has been
proposed as an alternative source of biomass for
BECCS. Due to its high photosynthetic efficiency and
high yields (Moreira and Pires 2016), its capacity
to co-produce protein and its potential to decrease
land competition (Beal et al 2018), algae may
address some of the sustainability concerns raised by
BECCS.
Global storage potentials. The second major factor
that could restrict BECCS deployment is the availabil-
ity of storage. There is little doubt in the literature
that there is, in principle, sufficient potential avail-
able across the globe to geologically store vast amounts
of CO2permanently, as required by many 1.5 ◦Cand
2◦C scenarios (Dooley 2013). Yet in individual regions
there could be storage bottlenecks that would limit
the BECCS potential in that region (Calvin et al 2009,
Edmonds et al 2007, Dooley 2013).
Global estimates of total storage potential span
a massive range—from 320 (Koide et al 1993)to
50 000 GtCO2(Hendriks and Blok 1995). Global esti-
mates using top down approaches grow as more
storage options are considered. The low estimate of
320 GtCO2conservatively assumes that 1% of all sed-
imentary basins might be suitable for storage (Koide
et al 1993). This more than doubles (to 777 GtCO2)
when proven depleted oil and gas reserves are included
(Ormerod et al 1993), then roughly doubles again to
2065 GtCO2(Hendriks and Blok 1995)whenconsider-
ing undiscovered oil and gas reserves. The assessment
increases dramatically to over 50 000 GtCO2when
other trapping mechanisms allow storage to occur
in aquifers without a structural trap (Hendriks and
Blok 1995). Later estimates make use of more detailed
information from regional and national studies to
generate global estimates (Selosse and Ricci 2017, Doo-
ley 2013). Dooley’s estimate of theoretical capacity
is in the same order of magnitude (35 000 GtCO2),
but is significantly reduced by physical and practi-
cal constraints to 13 500, 3900 and 290 GtCO2of
effective, practical, and matched potential worldwide,
11

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
respectively22 . The effective capacity estimate is in line
with estimates reached by integrating global IEA GHG
data with data from national and site specific esti-
mates, and other sources such as Total Petroleum
System and the United States Geological Survey for
a total potential of 10 000 GtCO2(Selosse and Ricci
2017).
Global estimates for depleted oil and gas fields range
from 458 (Ormerod et al 1993) to 923 GtCO2(IEA
Greenhouse Gas R&D Programme 2000) (IEA Green-
house Gas R&D Programme 2000). This relatively
narrow range likely results from thorough documen-
tation of structures during exploration and extraction.
Despite wide differences in total potentials, the broad
studies with breakdowns provide a narrower range of
458–801 for oil and gas fields (Hendriks and Blok
1995, Selosse and Ricci 2017,Ormerodet al 1993).
IEA GHG estimates are based on a detailed database
of 155 geological provinces (IEA Greenhouse Gas
R&D Programme 2000). Regional assessments provide
insight into the geographical distribution of resources.
North American estimates range from 40 (Dooley
et al 2005) to 136 GtCO2(Wright et al 2013). The low
estimate only considers the CO2sequestration poten-
tial of depleted gas fields and oil fields with enhanced
oil recovery (EOR). European estimates range from
the effective capacity evaluated by the GeoCapacity
project of 20 GtCO2(Vangkilde-Pedersen et al 2008)
to 280 GtCO2(Hendriks and Blok 1995). The latter
estimate can likely be attributed to the former Soviet
Union nations, which Selosse and Ricci (2017)estimate
have 277 GtCO2of capacity. Estimates that exclude
this region cluster are between 20 and 60 GtCO2
(Vangkilde-Pedersen et al 2008,2009,IEAGHG2005,
Selosse and Ricci 2017). Lower estimates exclude some
countries and present effective capacities with site-
specific information. Middle Eastern estimates range
from 208 (Selosse and Ricci 2017) to 250 GtCO2(Hen-
driks and Blok 1995), but only EOR estimates were
found at national or site specific levels (Jaju et al 2016,
Movagharnejad et al 2012,Mortensenet al 2016,Has-
sani et al 2016).
Estimates of the storage potential of coal beds
range from 60 (Gale and Freund 2001,Gale2004)to
700 GtCO2(Kuuskraa et al 1992). Lower estimates con-
sider the economic constraints (Dooley et al 2005)of
10 high potential countries, while a more comprehen-
sive assessment of 24 countries expands the potential
to 487 GtCO2(Godec et al 2014). Kuuskraa et al
(1992)’s estimate appears to be based on theoretical
analysis by the authors leading to a higher range. The
early estimate of 150 GtCO2considers few countries
and subsequent regional estimates have revised this
22 The types of potential correspond to estimates of capacity increas-
ingly constrained by physical (theoretical), technical (effective),
regulatory, econom ic (practical) barriers as well as detailed matching
with large CO2sources (matched) (Bradshaw et al 2007,Bachuet al
2007).
upward. In North America, estimates have increased
from 47 GtCO2(IEA GHG 1998,Gale2004)to
65–120 GtCO2(Godec et al 2014, Dooley et al 2005,
Wright et al 2013). Lower-end estimates tend to con-
sider specific basins with high potential and favorable
market conditions while higher estimates reflect theo-
retical global potentials.
Most of the potential and variability in esti-
mates comes from estimates of potentials in aquifers.
Hendrik’s broad estimate of 200–50 000 GtCO2cov-
ers all other estimates in the literature. The lower-end
considers aquifers only with a structural trap, while
the high end integrates other trapping mechanisms
allowing much wider deployment23 . Early estimates
are explicitly conservative, but it is unclear whether
they are considering structural traps in their constraints
(Koide et al 1993). Although the 50 000 estimate is
explicitly theoretical, regional estimates have provided
support to it. High potentials have been estimated
for North America (Dooley et al 2005,Wrightet al
2013), China (Li et al 2009)andOECDEurope
(IEA GHG 2005).
Costs. Cost estimates through the entire literature
range from US$15–400/tCO2. Estimates that cover
BECCS generally estimate prices of between US$30
and 400/tCO2(Luckow et al 2010, Koornneef et al
2012,Arastoet al 2014). However, most sources focus
on a specific source for CO2capture. Many papers
explore the potential of capture from ethanol fermen-
tation and find ranges of US$20 to 175/tCO2(de Visser
et al 2011, Fabbri et al 2011,Fornellet al 2013,Laude
et al 2011,M
¨
ollersten et al 2004, Johnson et al 2014,
Rochedo et al 2016). Low values within the studies
represent deployment in the most suitable plants with
easy access to abundant biomass and short distances
to storage sites. Capturing CO2emissions from both
ethanol fermentation and cogeneration units increases
costs (US$40–120 vs US$180–200/tCO2avoided)
but also increases avoidance potential (Laude et al
2011). Combustion BECCS has higher costs ranging
from US$88 to US$288/tCO2(Akgul et al 2014,Al-
Qayim et al 2015,K
¨
arki et al 2013). Low estimates
in combustion come from utilizing oxy-fuel technolo-
gies (Al-Qayim et al 2015,K
¨
arki et al 2013). The
lowest estimate for this technology group (US$14–
77/tCO2avoided) comes from a variation of oxy-fuel
combustion that is still unproven (Abanades et al
2011). Biomass gasification technologies are estimated
between US$30 to US$6/tCO2(Gough and Upham
2011,RhodesandKeith2005, Sanchez and Call-
away 2016). However, Ranjan provides much more
pessimistic estimates of US$150–400/tCO2avoided,
but these might be due to extremely large land
requirements for the production of biomass. The cost
23 Structural traps refer to geological structures capable of retain-
ing hydrocarbons, sealed structurally by a fault or fold (IPCC
2005). For an overview of trapping mechanisms see (Bradshaw et al
2007).
12

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
of CO2avoidance via BECCS utilizing black liqu or pro-
duced by pulp and paper mills has been estimated to
range between US$20 and US$70/tCO2when using
recovery boilers (Onarheim et al 2015,M
¨
ollersten
et al 2004)andUS$20–55 when using gasification
technologies (M¨
ollersten et al 2006,2004). Other
technologies have been estimated at US$86–167/tCO2
avoided (Carbo et al 2011)andUS$20–40/tCO2(John-
son et al 2014) for BioSNG and biomass FT diesel,
respectively.
Low cost estimates typically start with a coal-CCS
configuration and assume biomass fuel costs lower than
those of coal, as at least partially available, e.g. in the
US-Midwest. Transport costs of biomass are included
in some (e.g. Sanchez and Callaway 2016)butnotall
studies. Importantly, biomass is nearly always assumed
to be produced at zero life-cycle emissions. But life-
cycle emissions related to direct or indirect land use
pose a 10%–30% efficiency penalty on carbon abate-
ment, and hence on costs of negative emissions, even in
the optimistic cases where biomass is derived from cel-
lulosic sources or dedicated bioenergy crops. It may also
be relevant to price in indirect externalities, mediated
via land markets, e.g. on food markets, ecosystem ser-
vices, and livelihoods (see below). This is a contentious
exercise with little agreement and large parameter
uncertainties.
Side effects. Side effects can be broadly cat-
egorized into climate effects induced by biomass
provision, resource needs, and broader environmental
and sustainability effects transmitted via the coupled
land-energy system (Creutzig et al 2015,Robledo-
Abad et al 2017). An exhaustive and comprehensive
literature review of 1175 publications on side effects
and sustainable development contributions of bioen-
ergy published in a recent study revealed that side
effects can be in general both positive and negative;
however, negative effects are more often observed
in the literature in social and environmental dimen-
sions, whereas positive effects are more often observed
in economic and technological dimensions (Robledo-
Abad et al 2017).
Climate effects belong to the categories of direct
land use change, indirect land use change, and albedo
effects. Land use change emissions include those from
change in previous use, such as deforestation, and
changes in global land use induced by economic
markets. These are generally high for first-generation
biofuels, such as corn ethanol, which are derived from
food markets; while overall emissions are still rele-
vant but in lower ranges for bioenergy from cellulosic
or woody sources, and from food waste and forest
residues (Plevin et al 2010,Smithet al 2016a)24.(Some
24 Although achievable scales are not clear yet, there is also research
on third-generation biofuels, derived from algal biomass (Brennan
and Owende 2010) with the potential to enhance yields by improv-
ing microalgal biology through genetic or metabolic engineering
(Tandon and Jin 2017).
specific albeit relatively low-yield choices can gener-
ate carbon-negative bioenergy, see Tilman et al 2006).
Low emissions also translate into a significant effi-
ciency loss in bioenergy for climate mitigation or for
BECCS as negative emissions technologies. Calcula-
tion of indirect land use effects is subject to parameter
and structural model choice rather than accounting
only and leads to considerable uncertainty in estimates
and abatement effects (Plevin et al 2010,2014).
The global albedo effects of cultivating biomass for
bioenergy are also relevant and vary with geograph-
ical location. Higher latitudes, where biomass might
replace reflective snow cover, are more prone to an
albedo effect that offsets climate mitigation (Bright et al
2015). Land use and land cover change forcing ranges
from −0.06 to −0.29 W m−2 by 2070 depending on
assumptions regarding future crop yield growth and
whether climate policy favors afforestation or bioenergy
crops (Jones et al 2015).
Required resources may include fertilizer use
(which in turn lead toGHG emissions and must be fac-
toredin)andwateruse.If170EJyr
−1 were produced
by a 2 ◦C-compliant BECCS infrastructure by 2100,
the water footprint would amount to 59.5 km3/GtCO2
by 2100 (Smith 2016), which corresponds to 1.5% of
global yearly freshwater withdrawals.
Bioenergy is confronted with substantial con-
cerns regarding competition for land, including impact
on food prices, biodiversity, water and nutrients
(Williamson 2016,Smithet al 2013,Haberl2015,
Robledo-Abad et al 2017,Edenhoferet al 2013). A
major concern is the effect that large-scale deployment
poses on food security. Although many studies apply
a food-first principle to limit deployment, increased
land competition could lead to increased global food
prices (Popp et al 2011, Reilly et al 2012) and regional
resource shortages (M¨
uller et al 2008). Some biofu-
els (such as corn ethanol) impact food prices, but
others that do not directly compete with food (such
as sugarcane) have a lower impact—yet often these
price impacts are dwarfed by exogenous factors like
economic growth (Zilberman et al 2013,Robertsand
Schlenker 2010, Timilsina et al 2012). These con-
cerns can be alleviated by limiting deployment to
marginal land, but this is often associated with detri-
mental impacts on biodiversity (Dale et al 2010,Wiens
et al 2011). Conversely, deployment on degraded
lands could contribute to protection from erosion and
soil restoration (Lemus and Lal 2005).
More than 1 billion small-holder farmers could
also be directly or indirectly subjected to changing
agricultural practices and bioenergy systems, both pos-
itively and negatively (Mutopo et al 2011,Creutzig
et al 2013). BECCS has the potential to increase and
diversify rural income, but also at the risk of displac-
ing small-holders or exposing them to the volatility of
world markets (Buck 2016). Current practice is com-
monly not concurrent with livelihood concerns; instead
research points to the global commodification of
13

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
a local energy supplement and the consolidation of cor-
porate power in agribusiness and energy sectors (Borras
and Franco 2010,Ristet al 2010). Case study analy-
ses demonstrate that while some local actors are likely
to profit from bioenergy deployment schemes, oth-
ers, often starting from an institutionally disadvantaged
position, can lose out (Creutzig et al 2013, Schoneveld
et al 2010). Distributional issues are hence a crucial
dimension in designing the governance of bioenergy
(Hunsberger et al 2014).
CCS poses its own set of risks. Overpressure could
lead to the pollution of potable water, to seismic activity
or to leaks, which could not only rapidly reverse pos-
itive mitigation effects, but cause environmental and
health damage at the leakage sites (Holloway 2009,
National Academy of Sciences 2015,Smithet al 2016a,
Bruckner et al 2014).
Permanence and saturation. In principle, once the
CO2removed from the atmosphere via BECCS is geo-
logically stored, it is one of the NET options that is less
vulnerable to reversal. Most importantly, stored CO2is
not subject to further management decisions like other
land-based NETs. While leakage can be an issue, it is not
widely perceived as a major hurdle to safe and perma-
nent storage. Moreover, there is significant research on
monitoring and verification as well as on leak detection
and remediation (Bui et al 2018). However, consider-
able concerns with BECCS are associated with its level
of effectiveness, which can be compromised by sig-
nificant amounts of emissions from indirect land-use
change (Plevin et al 2010).
Authors’assessment. Overall, by 2050 we see
BECCS at costs of US$100–200/tCO2that accrue inter
alia from thenecessity to guarantee limited sustainabil-
ity and land-use carbon cycle effects, and which will
require high management intensity on a case-by-case
basis. Our estimate of 2050 potentials ranges is 0.5–
5GtCO
2(considering here a technological potential
that remains cognizant of other sustainability aims). As
for all land-intensive options, we remain conservative
in our suggested values as they refer to mid-century
where population pressures are highest according to
recent projections (Samir and Lutz 2017). A range of
5GtCO2and possibly higher requires global land gov-
ernance, integrating multiple land use concerns for the
global common good.
3.2. Afforestation and reforestation (AR)
Afforestation refers to planting trees on land that has
not been afforested in recent history (a reference value
of at least 50 years is commonly used). Reforestation,
on the other hand, refers to the replanting of trees
on more recently deforested land (IPCC 2000). Neg-
ative emissions can arise from both practices, as the
growth of additional biomass sequesters CO2from the
atmosphere. The distinction between afforestation and
reforestation is often not clean in the literature and we
therefore categorize them jointly.
Global sequestration potentials and costs. Out of
12 previous assessments of different NETs, seven offer
yearly potentials at either mid-century or 2100. The
2050 range is 0.5–7 GtCO2yr−1 (Lenton 2014), which
encompasses the ranges given in earlier assessments
(Friends of the Earth 2011,McLaren2012,Lenton
2010). In 2100, this range widens to 1–12 GtCO2yr−1,
covering the ranges given by Smith et al (2016a),
the National Academy of Sciences (2015) and Lenton
(2010,2014). In addition, some assessments give poten-
tials in cumulative terms with the lowest 2100 estimate
of 80 GtCO2coming from the Oxford University’s
Stranded Assets Programme (Caldecott et al 2015)and
the highest estimate being the upper end of the IPCC
AR5 range with 260 GtCO2(IPCC 2014a). There is
high agreement on the maximal costs of AR being
around US$100/ton of sequestered CO2and less agree-
ment on the lower-end of the range, with the National
Academy of Sciences (2015)quotingUS$1andtherest
being in a range of US$18–20/ton CO2.TheRoyal
Society Report (2009) does acknowledge AR as an
option to remove carbon, but does not give potentials.
Their assessment points to ‘low costs’as well.
Taking the systematically scoped literature (see
section 3.2.1) into account, the upper end of the
2100 sequestration potential remains at just above
12 GtCO2yr−1 in 2100. The lower-end is slightly more
conservative at 0.54 GtCO2yr−1 (Liu et al 2016)25.New
estimates from Integrated Assessment Modeling com-
bined with more detailed bottom-up land use models
give a range of 5.83–9.56 GtCO2yr−1 in 2100 when
2580 Mha are afforested (Kreidenweis et al 2016), with
a lower potential of 3.53 GtCO2yr−1 for afforesta-
tion of 1489 Mha at a carbon price of US$24/tCO2
(Humpen¨
oder et al 2014). Earth System Modeling
mimicking the afforestation rates in an RCP4.5 path-
way finds 6.64 GtCO2yr−1 in 2100 (Sonntag et al 2016).
Houghton et al (2015) estimate that about 500 Mha
could be available for the re-establishment of the
world’s tropical forests on lands previously forested but
not currently used productively. This would sequester
at least 3.7 GtCO2annually for decades, even though
they raise the important caveat that forests need both
time to grow and will eventually suffer from saturation
and thus assume a linear decline in productivity from
3.7 GtCO2in 2065 to 0 by 2095. Earlier estimates lie
in between 0.47 and 4.88 GtCO2yr−1 by 2100 (Sohn-
gen and Mendelsohn 2003, Cannell 2002, Canadell
and Raupach 2008,Strengerset al 2008,vanMin-
nen et al 2008,Thomsonet al 2008) with estimates
depending on various assumptions, most notably the
amount of land available. For example, many stud-
ies assume that only abandoned or low-productivity
land can be used for AR. For example, Thomson et al
(2008) use an area of 120 Mha of unproductive land
25 Assuming that Liu et al (2016) provide a 2100 potential, which
seems likely, but the main body of their article is in Chinese.
14

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Afforestation and Reforestation - Costs Afforestation and Reforestation - Potentials
Publication Year Publication Year
1990 1995 2000 2005 2010 2015 1995 2000 2005 2010 2015
Cost [US$(2011)/tCO2]
Sequestration Potential [Gt CO2/year]
200
150
100
50
0
8
6
4
2
0
% of Studies
100 75 50 25 0
Figure 8. Costs and potentials for afforestation. The heatbar distribution of literature estimates in each panel are calculated as in
figure 6, with individual publication cost ranges represented by lines (costs panel); and maximum estimates of negative emissions
potential plotted by publication year (potentials panel). Afforestation potentials are constrained to global studies for the year 2050 (or
proximate to 2050, e.g. Houghton et al 2015). Cost estimates include both regional and global studies. Estimates and ranges at the
top and bottom end of the distribution are labelled; the data can be further explored in our online supporting material available at
https://mcc-apsis.github.io/NETs-review/.
arriving at a maximum potential of 1.14 GtCO2yr−1
by 2100, while van Minnen et al (2008) start from
abandoned land of 831 Mha thus also having higher
maximum potentials (4.88 GtCO2yr−1 by 2100). 2050
potentials range from 0.44 GtCO2yr−1 (van Minnen
et al 2008)to6.16GtCO
2yr−1 (Dixon et al 1994).
A number of publications using different methodolo-
gies fall in between those (Brown et al 1995,Kaiser
2000,Karnoskyet al 2003, Nilsson and Schopfhauser
1995,Thomsonet al 2008). Ben´
ıtez et al (2007)
use a 20 year time frame to arrive at a sequestra-
tion potential of 1.3 GtCO2yr−1. (Richards and Stokes
2004) review older literature and find that more than
7GtCO
2could be sequestered yearly for decades (IPCC
2000,Nordhaus1991, Sedjo and Solomon 1991,Sohn-
gen and Mendelsohn 2003). Griscom et al (2017)
assess a wider set of conservation, restoration, and
improved land management actions that increase car-
bon storage and/or avoid GHG emissions across global
forests, wetlands, grasslands, and agricultural lands.
The maximum potential of the AR component of
these actions is 17.9 GtCO2, where all grazing land in
forested ecoregions is reforested—however this would
require substantial global dietary shifts away from
grass-fed beef. Note that it is not possible from the
studies identified to conclude whether different mod-
eling and estimation techniques lead to systematically
higher or lower potentials.
Global costs range between US$2and
US$150/tCO2for the scoped articles (Humpen¨
oder
et al 2014, Richards and Stokes 2004,Sohngenand
Mendelsohn 2003,Brownet al 1995). This range
includes almost no estimates from the Integrated
Assessment Modeling literature for 2100 and is mostly
based on bottom-up estimates and establishment
costs. As more IAM literature becomes available for
sequestration through AR (as is happening now),
the upper range can be expected to shift upwards. In
addition, conserving forests as long-term sinks will
still require management after the actual afforestation
process, an additional cost that is often not taken into
account in the reviewed estimates.
Richards and Stokes (2004) provide an overview of
AR cost studies from the 1990s and early 2000s, identi-
fying substantially lower costs in developing compared
to industrialized countries. This is in line with our
observations from the scoped literature: most of the
cost studies originate in the USA, Australia, Canada,
or are global studies. There are only few cost studies
in Latin America, two for India, none for Southeast
Asia and none for Africa. Out of the full sample of the
scoped literature, 17% present cost estimates. Multi-
ple factors may drive differences in cost-effectiveness
between regions, such as yield rates, land prices, trans-
action costs, and reporting differences. Many more
studies based in developing countries would be needed
to clarify these differences. In addition, estimates differ
in methodology and scope. For instance they may be
the (exogenous) prices at which a potential was cal-
culated, or the bottom-up establishment costs for a
pre-specified sequestration target, or the cost at which
AR becomes profitable, or a combination of these. This
15

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
has to be borne in mind as a major caveat when exam-
ining the cost ranges across regions. For those studies
that follow an optimization approach, the cost range
is US$10–237/tCO2. Bottom-up studies relying on the
valuation of the different cost components range from
US$0.1–15/tCO2. Those that use opportunity costs
correspond to a range of US$3–160, but are obviously
very location-specific. Studies relying on reviews of pre-
vious literature, where it is impossible to track down
the type of costs surveyed, lie between US$7.50 and 50.
Side effects. A wide variety of biophysical, social
and economic side-effects are considered in the AR
literature. One of the most prominent issues from a
climate perspective is albedo change, which finds signif-
icant attention particularly in global studies (Anderson
et al 2011, Arora and Montenegro 2011, Betts et al
2007,Jacksonet al 2008,Wanget al 2014). There is
high agreement that the low albedo of boreal forests
renders AR in high latitudes counterproductive, accel-
erating local warming and speeding ice and snow
cover loss; similarly, temperate AR has uncertain or
net neutral benefits for global temperature reduction,
particularly if substituting for relatively high albedo
agricultural land uses. Tropical AR, due to higher pro-
ductivity, moderate albedo effects, and its potential
to generate evaporative cooling, i.e. the local cool-
ing effect resulting from evapotranspiration, holds the
greatest potential for net temperature reduction—up
to three times that of boreal forests per unit of land
area, according to Arora and Montenegro (2011). A
second major consideration is the association between
AR and biodiversity. The literature is currently lacking
a comprehensive review on this topic, which is pre-
dominantly investigated on a case study basis, with
ensuing variety in terms of local system conditions.
Nonetheless, afforestation using native species is gen-
erally regarded as superior compared to plantations for
habitat quality and species diversity (Hall et al 2012,
McKinley et al 2011); and although they may per-
form less well in terms of carbon sequestration, diverse
afforestation plots are less vulnerable to climatic per-
turbations (Locatelli et al 2015)andprovideagreater
variety of subsistence products and services, enhancing
local management and acceptability (D´
ıaz et al 2009,
Locatelli et al 2015,Venteret al 2012).
Other issues addressed were local livelihoods, par-
ticularly for developing and middle-income regions
(which are inevitably matters of design, ownership and
appropriate payments in afforestation schemes) (Greve
et al 2013, Locatelli et al 2015, Renner et al 2008);
AR effects on soil organic carbon, for which Laganiere
et al (2010) provide a meta-analysis of varying species
and site conditions; and questions of broader resource
limits to large-scale AR schemes, including nutri-
ent cycling and water consumption (Deng et al
2017,Jacksonet al 2005,SmithandTorn2013).
Permanence and saturation. Biogenic CO2stor-
age has a much shorter permanence than CO2stored
in geological formations. Forest sinks saturate within a
period of decades to centuries, compared to thousands
of years for geological storage (Smith et al 2016b);
forests are also subject to natural and human distur-
bances,e.g.drought,forestfiresandpests(potentially
exacerbated by climate change), or sudden reversals
in land use. These issues require careful forest manage-
ment long after theactual afforestation process, making
AR a less attractive NETs option over time. Ultimately,
total long-term afforestation (storage) potential is con-
strained by land area, so new land will need to be freed
up for additional negative emissions in the 22nd cen-
tury, for instance by shifting global diets away from
meat products (R¨
o¨
os et al 2017,Griscomet al 2017).
Upscaling. Although AR does not involve ramping
up large infrastructures like BECCS (see section 3.1)
and DACCS (see section3.3),thepaceatwhichremoval
will be taking place will still be slow, as forests need to
grow to their full potential. Upscaling and diffusion
will be analyzed and discussed in more detail in Nemet
et al (2018).
Authors’assessment. Albedo effectively constrains
afforestation as a mitigation strategy to the tropics—
and within these regions it will have to compete with
agriculture and other sectors for land (particularly
under a portfolio of NETs). The estimate by Houghton
et al (2015) for a total area of 500 Mha of marginal
land in the tropics is therefore a feasible, yet ambi-
tious boundary limit for global afforestation. Note
that an earlier study by Zomer et al (2008)found
only 760 Mha of globally available land that satisfied
UNFCCC accounting conditions. The 500 Mha con-
straint constitutes approximately 3.6 GtCO2yr−1 of
carbon removal by 2050, albeit declining to 0 by the
end of the century (Houghton et al 2015). Under these
conditions (marginal land in the global South), costs
will tend towards the lower-end of the global range,
likely not exceeding US$5–50/tCO2,withthecaveat
that very few cost studies exist for tropical countries in
the past decade (Ben´
ıtez and Obersteiner 2006, Torres
et al 2010).
3.3. Direct air carbon capture and storage (DACCS)
Direct air CO2capture and storage, also known as CO2
capture from ambient air, comprises several distinct
technologies to remove dilute CO2from the surround-
ing atmosphere.
There is a plethora of different materials and pro-
cesses under investigation. Most attempts have focused
on hydroxide sorbents, such as calcium hydroxide.
More recently a stream of research on solid materi-
als has emerged, mostly involving amines. Engineering
problems involve enlarging the contact surface to
increase CO2withdrawal and dealing with moisture.
A key issue is the energy needed. This includes the
energy for releasing CO2from the sorbent, regener-
ating the sorbent, for fans and pumping, as well as
for pressurizing the CO2for transportation. For exam-
ple, temperatures greater than 700 ◦C are required to
separate CO2from the calcium compound and to
16

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
2005 2010 2015
0
25
50
75
100
0
250
500
750
1000
Cost [US$(2011)/tCO2]
Publication Year
% of Studies
DACCS - Costs
Figure 9. Costs for direct air capture. The heatbar distribution of literature estimates are calculated as in figure 6for costs only, with
individual publication cost ranges represented by lines. No literature estimates for potentials exist, but are often implicitly assumed to
be unlimited. Estimates and ranges at the top and bottom end of the distribution are labelled; the data can be further explored in our
online supporting material available at https://mcc-apsis.github.io/NETs-review/.
regenerate calcium hydroxide. Readers interested in t he
specific processes, materials and options are referred to
(Sanz-P´
erez et al 2016, Barkakaty et al 2017).
Potential and costs. Generally, potentials remain
largely ignored, in part because they are implicitly
assumed to be unlimited. Yet, many of the available
NETs assessment have provided estimates—most rang-
ing somewhere between 10–15 GtCO2annually in 2100
(Fuss 2017,Smithet al 2016a,McLaren2012, National
Academy of Sciences 2015) with some seeing much
higher potentials beyond 40 Gt CO2(Lenton 2014). The
limited evidence from long-term mitigation scenarios
are at the higher end of this range (i.e. 40 GtCO2)by
end of the century (Chen and Tavoni 2013). However,
potentials have not been systematically investigated,
and some critical perspectives voice doubts on the
scalability of DACCS (Pritchard et al 2015).
Most of the discussion around DACCS potential
has been dominated by cost considerations as the key
parameter determining the viability of the technology.
A recent study by Sanz-P ´
erez et al (2016) has reviewed
the available literature comprehensively and provides
much of the basis associated with the DACCS costs
presented here.
Costs of DACCS incur mainly from (1) capital
investment, (2) energy costs of capture and operation,
(3) energy costs of regeneration, (4) sorbent loss and
maintenance. Additional costs occur for CO2com-
pression, transportation and storage and are similar
to those studied in the CCS literature. However, a
main difference is that DACCS can be deployed prox-
imate to storage facilities, and can be co-located with
attractive sites for renewable energy, thus minimizing
transport and grid costs (Goldberg et al 2013). Depend-
ing on location and grid demand, there may exist
opportunities where renewable energy is abundant and
cost-competitive and accessed directly thereby circum-
venting the grid. Given the energy requirements of
DACCS, coupling these plants with cheap renewable
energy could be a method of bringing down the oper-
ating costs of the plant. It is important to note that
if DACCS is powered with coal, the CO2emissions
from fueling the plant would be greater than the CO2
captured (National Academy of Sciences 2015).
Asshownintable1, in general, cost estimates range
from US$30-$1000/tCO2(Sanz-P´
erez et al 2016), see
also figure 9. It is difficult to compare the costs of DAC
reported in the literature due to their differing bound-
ary conditions in addition to the fact that many of the
reported estimates are the costs of CO2capture and not
the costs of capturing the avoided CO2.Morespecif-
ically, a significant amount of thermal energy is often
required for DAC due to the requirement of strong
binding of the capture material because of the extreme
dilution of atmospheric CO2. The use of natural gas to
provide the thermal requirements for regenerating the
capture material, results in CO2emitted into the atmo-
sphere. Hence, if a DAC plant is designed to capture
on the order of 1 Mt CO2yr−1, it may ultimately avoid
only a fraction of this due to the emissions generated
from the use of natural gas to provide energy to the
plant. The use of renewable energy or in the case of
Climeworks, low-grade waste heat—provided the sep-
aration process allows, for DAC will lead to the greatest
impacts since the maximum amount of CO2will be
captured and avoided. For example, House et al 2007
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Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Table 1. Cost estimates of complete26 DACCS systems reported in the literature, concentrated from 400 ppm to 98+%purity
27.
Cost
[US$(2011)/tCO2]
Assumptions References
<140 ∙1/3 cost capital and maintenance
∙2/3 cost carbon-neutral electricity +natural gas
∙Sodium hydroxide solvent approach followed by causticization and calcination
∙Efficient heat exchange
∙Contactor design based on cooling tower technology
Keith et al 2006
∼600 ∙Modeling Carbon Engineering’s approach
∙Potassium hydroxide solvent approach followed by causticization and calcination
∙Conventional contactor design based on postcombustion CO2capture
APS Report 2011
∼1000 ∙Theoretical estimate based upon minimum work calculations combing with
second-law efficiencies ranging between 2%–5% and energy cost estimates ranging
between 80–103 $/MWh for natural gas, excluding capital costs
House et al 2011
<500 ∙2nd-law efficiency of 10%
∙Contactor design based on cooling tower technology
∙Inexpensive contactors, i.e. $0.5 M to capture 1tCO2/day
Simon et al 2011
∼300 ∙Contactor design based on cooling tower technology
∙Plastics in place of stainless steel for contactor packing
Zeman 2014
60–190 ∙Capture based on solid sorbents rather than solvents for CO2capture
∙Estimate does not include compression for transport
∙Temperature vacuum swing adsorption process
Sinha et al 2017
600 ∙Capture based on solid sorbents rather than solvents for CO2capture
∙Amine-functionalized solid sorbents
∙Temperature and vacuum swing adsorption process
Climeworks
www.climeworks.com/
n/a ∙Capture based on solid sorbents rather than solvents for CO2capture
∙Humidity swing adsorption process
∙Concentrating to 3%–5% purity only
Lackner 2009
provide a range of energy required for DAC between
500–800 kJ molCO2. For many processes, this consists
of a combination of electricity for fans and pumps and
thermal energy for regeneration of the capture material.
Based on a carbon intensity of 490 g CO2per kWh for
natural gas, leads to emissions of 0.7–1.2 MtCO2yr−1,
resulting in CO2avoided of 0.3 MtCO2peryearin
the best case and net emissions of 0.2 MtCO2yr−1 in
theworst-casescenario.IfthecostofCO
2capture is
$200/tCO2, this scenario would lead to a lower-bound
avoided costs of CO2capture of $600/tCO2.There-
fore, depending on how one chooses to provide energy
to the DAC plant will ultimately determine the cost of
avoiding CO2in the atmosphere.
Low-cost estimates tend to come from sources
closer to industry (Ishimoto et al 2017), but they
also often do not include all cost components and
are therefore difficult to compare. The upper range
estimate of US$1000/tCO2is derived from thermody-
namic considerations without an explicit consideration
of a particular technology. For instance, Ranjan and
Herzog (2011) argue that such thermodynamic con-
26 Complete indicates, contactor, regeneration, and compression,
ready for pipeline transport; approximate cost of CO2transportation
via pipeline is US$2.2-$8.9/tonne CO2per 250 km of dedicated
pipeline, range capturi ng a capacity of 3–10 MtCO2yr−1 for onshore
and offshore pipelines. (IPCC 2005); storage costs range due to the
heterogeneity of the reservoi r, 7–13 2011 USD/tCO2(USDOE 2014).
27 With the exception of Lackner et al (1999)wheretheendproduct
is 3%–5% for algae cultivation applications.
siderations rule out estimates below US$500. These
calculations include costs for capture and regenera-
tion. Some judge such high cost estimates as more
reliable (Socolow et al 2011); they are also pro-
posed more frequently as outcomes in the available
scientific assessments(National Academy of Sciences
2015,Smithet al 2016a,Caldecottet al 2015,
McLaren 2012).
Socolow and colleagues (Socolow et al 2011)used
a simplified factored estimation approach consist-
ing of the dominant pieces of equipment used in a
solvent-based separation process. They focused on a
two-loop hydroxide-carbonate system, similar to that
which has been proposed by the first DACCS study by
(Lackner et al 1999), and relying on processes also
used in the pulp industry. Under optimistic techno-
logical assumptions for this process they obtain costs
of US$600/tCO2. They also point out that in the early
stages of deployment costs are likely to be substan-
tially higher. About 30% of the costs originate from
aCO
2-penalty as the process is heated by natural gas
combustion. This a key source for efficiency improve-
ment and cost reduction.
Using the APS estimate as benchmark, a num-
ber of options might reduce costs. Mazzotti et al
(2013) investigate optimization at the front-end that
amongst other effects increase the fraction of CO2cap-
tured. That could reduce costs by 10%–20% down to
around US$520/tCO2.AnotherstudybyZeman(2014)
further optimized the design by APS and Mazzotti
18

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
(e.g. substituting certain stainless steel components for
plastics), to obtain about US$310/tCO2.
Holmes and Keith (2012), associated with the air
capture company Carbon Engineering, suggested a
cooling tower design, where air flows orthogonal to
a downward flowing hydroxide solution. Holmes et al
(2013) presented then a prototype with >1000 hours of
operation, validating the cross-flow contactor design.
However, the authors and their company did not dis-
close the costs and energy requirements of regenerat ion,
which are estimated to be substantial. As a compari-
son, APS calculations would result in ca. US$230/tCO2
(neglecting CO2-penalties if heated by natural gas, and
neglecting storage costs).
An alternative design is based on solid sorbents
(specifically: anionic-exchange resin) (Lackner 2009).
Solid sorbent systems might be cheaper as less energy
is required for regeneration. A preliminary calcu-
lation yields estimates of US$200/tCO2, costs that
could decrease down to US$30/tCO2with technologi-
cal development. It is important to note that the desired
CO2application requires very low purity CO2,i.e.3%–
5%, which means the capture technology may be wea kly
binding with an elegant regeneration approach such as
humidity swing. In the situations in which high-purity
(i.e. 95%) CO2is required as a chemical feedstock,
a weak-binding low-cost approach would likely not
be feasible. An alternate solid sorbent system based on
amines with porous oxide supports found US$95/tCO2
but excluding capital costs (Kulkarni and Sholl 2012).
A similar approach, based on monolithic honeycombs
finds similarly plausible costs of around US$100/ tCO2
(Sakwa-Novak et al 2016). Again, all of these costs are
not taking into account the avoided emissions, but are
reflective of only costs of capturing CO2.
Wastewater treatment is being explored as a means
to capture ambient CO2.Huanget al (2016)demon-
strated a moisture-driven capture process via an
ion-exchange resin and subsequent microbial electro-
chemical carbon capture, capable of a capture efficiency
of 0.40 g CO2g−1 of chemical oxygen demand (COD)
or biochemical oxygen demand (BOD). Assuming an
average BOD of 0.35 and 0.5 g L−1 for domestic and
industrial wastewater, respectively, global potential for
CO2storage via wastewater treatment is estimated at
220 MtCO2per year (Sato et al 2013). This figure is
based on numbers reported for wastewater treated in 55
of 181 countries, including the North America, South
America, most European nations, China, Japan, India,
South Korea, and the Russian Federation. This figure
does not reflect the amount of wastewater generated,
and it is estimated that while high-income countries
treat 70% of the wastewater generated, this drops to
28%–38% for middle-income countries and as low as
8% for low-income countries. Further, only 37% of the
data reported could be considered recent (2008–2012).
Thus, though the global estimate provided above is
non-conservative and defines a theoretical upper-limit
based on best-available current wastewater treatment
data, the amount of treatable wastewater is expected to
be much larger, resulting in a greater theoretical global
capacity. Yet, this greater upper-bound remains limited
by the financial constraints.
Within the field of Industrial ecology there is sup-
port to capture and store CO2in materials, such as
polymers, rather than underground (Meylan et al 2015,
Barbarossa et al 2014,Bringezu2014). A significant
breakthrough was the proof that CO2from ambient air
can be converted to methanol (Kothandaraman et al
2016).Howeverifmethanolisusedasafuel,thisprocess
is at best carbon neutral, not carbon negative. Concerns
are that above ground storage of CO2in polymers may
be substantially less than that of CO2underground in
addition to the potential nature of shorter timescales of
CO2storage in materials.
In a modeling study with mass production and
technological learning, cost floor estimates of US$60/
tCO2were found for 2029, and possibly even lower
with time (Nemet and Brandt 2012).
Side effects. The literature has so far not dis-
cussed side-effects systematically. While the physical
scale would be impressive if DACCS were deployed at
relevant GtCO2scales, limited land resources are not
much of a concern (Keith 2009,Lackneret al 2012)
nor is storage capacity (de Coninck and Benson 2014)
(see section on BECCS). However, geological storage
is associated with a string of side-effects, as described
in the section on BECCS. In the case of solvent-based
separation for DACCS, the use of potassium hydrox-
ide (Holmes and Keith 2012) is well-studied and have
been used for industrial applications (e.g. pulp and
paper industry) for decades with minimal wastewa-
ter produced. Solid waste build-up in the recovery
cycles of these separation processes will have similar
environmental implications and disposal guidelines as
the reclaimer waste in conventional amine scrubbing
operations.
Permanence and saturation. The implicit under-
standing of the literature is that DACCS can be
scaled-up solely subject to technological learning but
not subject to biophysical constraints. DACCS has,
with the exception of one small plant (Magill 2017),
not been deployed, and hence has yet to receive the
same level of scrutiny as other technologies. Currently
its biggest stated drawback is the cost; there is a wide
range of estimates with several important publica-
tions emphasizing the high end. A plethora of more
cost-effective and more conventional options exist;
however, the examples of photovoltaics and batteries
(Kittner et al 2017,Creutziget al 2017)havedemon-
strated that an order of magnitude in costs can be
bridged within one or two decades via manufacturing
scale; and DACCS potentially could also be produced
at high volumes.
Permanence and saturation is mostly subject to
geological storage underground, similar to those noted
in the section on BECCS. Building a DACCS plant that
captures 1 MtCO2yr−1 , requires a significant surface
19

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Figure 10. Costs and potentials for enhanced weathering and ocean alkalinisation. The heat-bar distribution of literature estimates
in each panel are calculated as in figure 6, with individual publication cost ranges represented by lines (costs panels); and maximum
estimates of negative emissions potential plotted by publication year (potentials panels). All estimates are global. Note that published
numbers only consider inorganicsequestration effects, neglecting any additional potentialof biomass production increase by improve-
ment of soil conditions and provision of geogenic nutrients. This is an important and remaining information gap. Estimates and
ranges at the top and bottom end of the distribution are labelled; the data can be further explored in our online supporting material
available at https://mcc-apsis.github.io/NETs-review/.
area for the contactor alone—on the order of 38 000 m2
for 75% capture. The materials and labor to build such
an operation would be significant, making the siting
of DACCS plants of significant scale in remote loca-
tions challenging. As discussed previously, the energy
demands due to pressure drop considerations and
material regeneration requirements, CO2-free energy
sources will be essential for DACCS to be considered a
NET. Hence, a careful approach to the siting of DACCS
plants is needed.
Authors’assessment. Based upon our literature
review, it appears that a first-of-a-kind plant may be on
the order of US$600–1000/tCO2initially, but that as
advances are recognized through the building of more
plants, this cost may decrease to US$100-300/tCO2.For
instance, Climeworks has built the first commercial-
scale DACCS plant and suggests current costs on the
order of US$600/tCO2with anticipated costs of nth
plants being on the order of US$200/tCO228.Costsare
initially high because of the up-front expenses of sourc-
ing supply chains and resolving infrastructures issues,
and because of lack of experience with the technology.
It is also important to note that since the regenera-
tion approach of Climeworks is based upon low-grade
waste heat, the cost of CO2capture is similar to that
of CO2avoided.
Our judgement on potential is 0.5–5 GtCO2yr−1 by
2050. Main constraints may be storage and unexpected
28 www.climeworks.com/our-technology/.
environmental side-effects, as well as moderate land
demand. However, if these constraints can be proven
unjustified or can be overcome, potentials of up to
40 GtCO2maybepossiblebytheendofthecentury.
3.4. Enhanced weathering (terrestrial and ocean)
Weathering is the natural process of rock decomposi-
tion via chemical and physical processes. It is controlled
by temperature, reactive surface area, interactions
with biota and water solution composition. Enhanced
weathering (EW) aims to artificially stimulate one or
more of these variables to speed up rock decomposi tion
while increasing the cation release to produce alkalinity
and geogenic nutrients. This purposeful acceleration
of biogeochemical cycling transforms the process of
weathering from geological to humanly relevant time
scales by favoring chemical reactions that have the
potential to sequester relevant amounts of atmospheric
CO2. This is done by grinding selected rock material
into rock powder with a suitable grain size distribution
to facilitate a maximum reactive surface area. In addi-
tion to the use of natural rocks, some authors report
the use of other materials like mine waste material
(Power et al 2013), concrete (Yamasaki et al 2002)
or alkaline waste (Morales-Florez et al 2011).
Besides being a CDR strategy, EW can ameliorate
soil and act as a long-term nutrients source (Leonar-
dos et al 1987,Nkouathioet al 2008). Many tropical
regions have nutrient poor soils, e.g. oxisols and ulti-
sols and due to their high precipitation rates and
20

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
temperature represent areas of high potential for EW
implementation. Considering the world’soxisolarea
and an application rate of 900 t km−2, similar to lim-
ing rates at Brazilian Cerrado (Lopes 1996), a total
amount of 8 Gt of rock material would be needed to
cover the world’s oxisol area. For comparison, world
lime production from 2005–2014 averaged 0.34 Gt yr−1
(Corathers 2015). According to Strefler et al (2018a)the
annual application of 3 Gt yr−1 basalt might sequester
1GtCO
2yr−1.
Ocean alkalinisation (or ocean liming) considers
the addition of alkalinity, e.g. via Ca(OH)2to marine
areas to locally increase the CO2buffering capacity
of the ocean (Gonz´
alez and Ilyina 2016,Renforth
and Henderson 2017). While not strictly a weathering
method, it is a further technology being incorporated
in this section as similar geochemical principles apply.
Atmospheric carbon can be sequestered via EW
in an inorganic or organic form. Inorganic C is
sequestered through the production of alkalinity
(bicarbonate and carbonate ions) while anions are
counterbalanced by the release of cations from the
rock products. If the solution is supersaturated with
respect to a chemical element, precipitation of sec-
ondary minerals can occur, for example, in the
form of carbonate minerals (Manning and Ren-
forth 2013,Poweret al 2013,Washbourneet al
2012). Organic C is sequestered when CO2is
reduced and incorporated in biomass and addi-
tional carbon sequestration potential can be expected
from the release of rock derived geogenic nutrients
(i.e. potassium, phosphorus, several micronutrients)
enhancing biomass production above previously
limiting conditions (Hartmann et al 2013).
ThemethodofEWcanbeappliedtodifferentEarth
compartments like soils (and also mining waste rock)
(Hartmann and Kempe 2008,K
¨
ohler et al 2010,Man-
ning and Renforth 2013,Renforth2012,Tayloret al
2016,tenBergeet al 2012,Wilsonet al 2009), the open
ocean (Hauck et al 2016,Houseet al 2007,K
¨
ohler
et al 2013), and coastal zones (Hangx and Spiers 2009,
Montserrat et al 2017). The chemical weathering of the
rock powder material in different Earth compartments
is conceptually the same and involves the release of
cations, nutrients like phosphorus or silica, and pro-
duction of alkalinity, for example as bicarbonate29 .
The largest research gap is missing field experi-
ments that consider real scales, which evaluate the full
impact of EW on biogeochemical cycles, biomass and
carbon stocks in the soils, and the plants. Mineral dis-
solution kinetics in the soil-ecosystem of applied rock
products containing fresh surface areas and a wide
range of grain sizes should be included in studies. The
field reaction kinetics will be different from labora-
tory studies, which may not consider all effects like
29 A model reaction for the mineral Forsterite is: Mg2SiO4+4CO2
+4H
2O⇒2Mg2+ +4HCO
3−+H4SiO4.
the freshness of rock surfaces, topography, groundwa-
ter table variation, soil profile heterogeneity, grain size
and unsaturated hydraulic conductivity variations. In
addition, the grain surface evolution is essential, also
with respect to clay mineral production or mineral-
root interaction, to understand the element release
patterns and potential for plants to utilize those released
elements.
Areas in which biomass is under nutrient limi-
tation conditions are the most attractive targets for
implementing EW (Garcia et al 2018). Rock prod-
uct weathering processes might supply nutrients to the
environment, which potentially can increase biomass
production (Anda et al 2015,2012,d’Hotman and Vil-
liers 1961, Hartmann et al 2013, Strefler et al 2018a).
Nevertheless, studies quantifying the effect s on biomass
increase due extra geogenic nutrient input or soil ame-
lioration by EW are scarce. Only a sufficient amount
of field studies and data collection on nutrient appli-
cation rates for certain climate-soil-plant conditions
could enable the development of management plans
to optimize CO2sequestration via additional biomass
growth.
Sequestration potentials and costs. The reported
sequestration potential considers theoretical (Hangx
and Spiers 2009, Hartmann and Kempe 2008,Man-
ning and Renforth 2013,Renforth2012,Renforthet al
2011), and observational assessments (Morales-Florez
et al 2011,Wilsonet al 2009,Houseet al 2007),
as well as regional to global scale model assessments
(Hangx and Spiers 2009,Haucket al 2016,Houseet al
2007,K
¨
ohler et al 2010,Tayloret al 2016,K
¨
ohler et al
2013, Strefler et al 2018a) and plot-scale experiments
(Montserrat et al 2017,tenBergeet al 2012). Reported
potentials range widely (see figure 10), depending on
the compartment type assessed, such as local soils,
coastal zones, or the open ocean. The highest reported
regional sequestration potential is 88.1 GtCO2yr−1 for
spreading pulverized rock over a very large surface area
in the tropics (Taylor et al 2016). Considering crop-
land areas only, the potential carbon removal might
be 95 GtCO2yr−1 for dunite and 4.9 GtCO2yr−1 for
basalt (Strefler et al 2018a). Other assessments for land
application range between these estimation approaches
and are highly uncertain due to a variety of assump-
tions and unknown parameter ranges in the applied
upscaling procedures, which still need to be verified by
field experiments. A global CO2sequestration potential
of organic biomass increase due to geogenic nutrient
fertilization and improved soil conditions is not avail-
able, due to missing upscaling studies, and therefore
are not represented in figure 8.
Costs are closely related to the chosen technol-
ogy for rock grinding, material transport and the rock
source (Hartmann et al 2013,Renforth2012,Stre-
fler et al 2018a). As costs are related to application
site characteristics and purpose (for example, whether
inorganic or organic sequestration is favored), most
reported back of the envelope calculations found in
21

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
literature are highly uncertain. They range for inor-
ganic CO2sequestration from US$15–40/tCO2to US$
3460/tCO2(K¨
ohler et al 2010,SchuilingandKri-
jgsman 2006,Tayloret al 2016). Renforth (2012)
conducted a regional cost assessment for implementing
inorganic EW in the UK, reporting operational costs
applying mafic30 rocks being US$70–578/tCO2and
for ultramafic31 rocks being US$24–123/tCO2.These
numbers can be taken as a reference for global appli-
cation, considering the relative cost levels of regional
economies. Variables like depth of rock extraction,
technical, economic, socio-environmental drivers, and
transport impact the costs of EW. Transportation
costs depend on the means of transportation, with
the cheapest costs for inland waterway and large
ship distribution (US$0.0016/t rock/km) and the most
expensive for road transport done by heavy vehicles
(US$0.07936/t rock/km) (Renforth 2012). This high-
lights that infrastructural conditions are relevant for
implementation and global cost estimates, which are
normally not considered in global assessments.
A detailed global cost assessment (Strefler et al
2018a) points out that EW is a competitive option
for carbon dioxide removal at US$60/tCO2−1 for
dunite and US$200/tCO2−1 for basalt. The upper
global limit of inorganic CO2sequestration including
forested areas (Taylor et al 2016) might be only reached
if very cost-intensive spreading by planes is considered.
The potential of other more unconventional tech-
nologies like dirigibles or slurry pipelines were not
studied so far. No study explicitly identifies the cost-
effectiveness of sites globally and spatially-explicitly at
a high spatial resolution. The costs change for differ-
ent rock sources and for the considered region (Strefler
et al 2018a), and estimates for organic CO2sequestra-
tion via the fertilization effect are missing for different
types of biomass, as for example afforestation or bioen-
ergy purposes, which demand optimization of the
soil for the best sequestration potential.
The amount of rock products to be moved, given
the above scenario of tropical soil in the introduction
of this section (8 Gt rock/year) is comparable to the
amount of coal mining and transport (IEA 2017)and
appears to be low if compared to the 2010’sglobal
material consumption (biomass, fossils, and minerals)
of70Gtperyear(Krausmannet al 2017). A detailed
spatially explicit global analysis suggests that in humid
and tropical areas about 80% and 95% of the applicable
crop areas for EW are within a distance of 300 km
from potential source rocks, respectively (Strefler et al
2018a).
Application costs and potentials based on tech-
nological considerations of ocean liming, ocean EW
30 A rock that has high magnesium and iron silicate minerals con-
centration.
31 A rock which is rich in magnesium and iron silicate minerals
but with very low silica content. The low silica content influences
weathering rates in a positive way.
or electrochemical weathering are less researched.
A recent review suggests the potential for Ocean
Alkalinization to be between 100 MtCO2yr−1 and
10 GtCO2yr−1 with costs ranging between US$14 to
more than US$500/tCO2(Renforth and Henderson
2017).
Side effects. The assessment on side effects of EW is
complex and challenging as they depend on exchanged
matter between different compartments of the Earth
System (Hartmann et al 2013,Tayloret al 2016,ten
Berge et al 2012). The main factors controlling the side
effects are: rock powder source, soil, ecosystem and
climate characteristics. Application to soils alters the
soil physical and chemistry properties, with impacts
on groundwater, river water, and coastal zone water.
In addition, the released material and changes in soil
properties influence ecosystems and biomass carbon
contents. Application in the coastal zone and the open
ocean impacts the marine water chemistry and ecosys-
tems.
The main side effects of land application are an
increase in water pH (K ¨
ohler et al 2013,Tayloret al
2016), the release of heavy metals (e.g. Ni and Cr) in
case of inappropriate material use, the release of plant
nutrients like K, Ca, Mg, P, and Si (Hartmann et al
2013), as well as hydrological soil property changes,
which could be designed to be favorable depending
ontheecosystem.Respirableparticlesizeswhichmay
contain asbestos-related minerals need to be avoided
(Schuiling and Krijgsman 2006,Tayloret al 2016)by
appropriate application procedures (e.g. water based
slur application).
K¨
ohler et al (2010) point out the complexity of
predicting the impact of mineral dissolution rates on
the carbon cycle due to changes in dissolved inor-
ganic carbon and total alkalinity. Hartmann et al
(2013)andTayloret al (2016) emphasize the additional
potential for improved CO2drawn-down by marine
diatoms due to the increased land-to-ocean silica fluxes
and enhanced alkalinity fluxes, increasing the oceans
aragonite saturation state32 .Tayloret al (Taylor et al
2016) point out the potential increase in atmospheric
CO2drawdown by coupling EW with AR.
EW application in the open ocean is less researched,
and Hauck et al (2016)andK
¨
ohler et al (2013)found
that the efficiency of the method is closely related to the
rock powder particle size. K¨
ohler et al (2013)recom-
mend a maximum particle size of 1 𝜇m to prevent early
sedimentation, but this would demand high energy
costs for grinding rock products (Strefler et al 2018a).
The change in regional export production of organic
matter is supposed to be less than 10%. Release o f heavy
metals from ultramafic rock products like Ni and Cr
32 The aragonite saturation state (ASS) is obtained by the product
of dissolved calcium and carbonate ions in seawater divided by the
aragonite solubility in seawater. If ASS is higher or lower than one sea
water is respectively over- or unde rsaturated with respect to aragonite.
If ASS is equal to one, the sea water solution is saturated.
22

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
is a potential negative side effect and a detailed study
on the marine biology related impacts and risks to its
ecosystems is missing (K ¨
ohler et al 2013). Montserrat
et al (2017) simulated, based on laboratory experi-
ments, coastal zone conditions for EW using olivine
dominated rock powder. They confirmed an increase
in Mg2+, Si, total alkalinity, and dissolved inorganic
carbon, Fe2+,andNi
2+ in the aqueous solution con-
centrations. A better comprehension of heavy metal
ecotoxicological effects on coastal environments for
large-scale application of olivine is missing.
Permanence and saturation. The sequestered CO2
by EW on land can be stored in several pools. In the soil
pore solution and groundwater, it remains first as dis-
solved inorganic carbon (or alkalinity). If the solution
gets supersaturated, carbonate minerals can precipitate
in the soil (Manning and Renforth 2013)andmean
residencetimescanbeintheorderof10
6years or more
(Wilson et al 2009). If carbonate precipitation does not
occur in the land system, and the solution is trans-
ported to the ocean by rivers, the dissolved weathering
products would be stored as ocean alkalinity (Taylor
et al 2016,K
¨
ohler et al 2010, Hartmann et al 2013,
Manning and Renforth 2013).
The fertilization effect of released nutrients can
cause additional biomass production, and the fate of
this additional carbon pool would be comparable to
that reported in the section for afforestation, soil car-
bon and bioenergy. Hence these methods are connected
and other land-based NETs could rely on EW to create
the optimal soil and nutrient supply conditions. How-
ever, this connection is not to be found in the literature
(at a global scale) at the time of publication.
Authors’assessment. So far, publications on ter-
restrial EW are mainly comprised of model studies or
theoretical discussions. The aforementioned research
gaps leave large uncertainties in the potential to
sequester CO2, which can only be overcome by field
studies because of the manifold influencing param-
eters and their interdependencies, which can, so far,
only be roughly considered in models. Current pub-
lished estimates of CO2removal potentials and costs
should be seen as boundary values, while more progress
is evident considering the cost estimation (Strefler et al
2018a).
The largest CO2penaltyaswellascostsarecre-
ated from the energy demand of rock grinding (Strefler
et al 2018a). The penalty will decrease significantly in
the future due to the expected transition to renewable
energies and technological advances (Napier-Munn
2015). If prices for fertilizers rise, and resources are
expected to decrease (Manning 2015), the EW side
effect of geogenic nutrient release and soil ameliora-
tion potential may become one of the strong suits of
this technology, potentially making it a valuable asset
in global agriculture (van Straaten 2006), irrespec-
tive of its potential to sequester CO2into alkalinity.
Including these aspects in detailed techno-enonomic
assessments may render EW more attractive (Strefler
et al 2018a). Accordingly, the cost range of our
assessment is US$50–200/tCO2for a potential of 2–
4GtCO
2yr−1 from 2050, excluding biological storage.
The cost range is high due to economies of scale
and highly variable cost-incurring parameters like
source rock properties, transport distances and field
application technology.
Ocean alkalinization has been discussed only in very
few global modelling studies. Given results are abstract
and provide upper limits. The costs can only be esti-
mated after clarification of how alkalinity is produced
and distributed at the global scale.
3.5. Ocean fertilization
Ocean fertilization (OF) is based on the effect of bio-
logical production increase, which is macro- (Harrison
2017,Matear2004) or micronutrient (Gnanadesikan
et al 2003, Raven and Falkowski 1999) limited, by
deliberately adding nutrients to the upper ocean waters.
Efficiency of the method is determined by the chemi-
cal form of the added nutrient (Harrison 2017,Matear
2004). Often, iron is the limiting nutrient in the ocean,
so that deliberate iron fertilisation is well discussed
(Strong et al 2009b, Markels and Barber 2001). The
algal bloom resulting from artificial OF leads to car-
bon fixation and subsequent sediment sequestration,
or sequestration on shorter time scales in the water
column. Due to the low iron requirement of phyto-
plankton, the ratio of CO2uptake per iron application
is high (2600–26600 C per added amount of Fe, de Baar
et al 2008). The increase in the biological production
(phytoplankton) would reach a maximum until further
nutrients become limited (Markels and Barber 2001).
K¨
ohler et al (2013)andHaucket al (2016)pro-
posed a coupling OF with EW, because the studied
mineral olivine releases iron and silicic acid during
dissolution. However, slow dissolution reactions and
the sinking of the particles remain a limiting factor
(Hauck et al 2016). OF can also be achieved by artificial
upwelling of nutrient-rich deep ocean water (Oschlies
et al 2010). Some early work authors doubt that OF
is feasible due to the large area needed to sequester
substantial amounts of CO2(Zeebe 2005).
Sequestration potentials and costs. Different fac-
tors control the atmospheric CO2uptake and storage
by OF, like duration of the experiment (Jin et al
2008), carbon export (Bakker et al 2005), changes in
mixing layer depth (Bozec et al 2005), mixing, lat-
eral and vertical transport (Jin et al 2008, Joos et al
1991), and wind speed (Bakker et al 2001). The
global atmospheric CO2drawndownpotentialswere
obtained from model simulations based on experi-
mental data. The obtained values can be divided in
three main approaches: modelled CO2sequestration,
process-based/experimental results and literature esti-
mates. The overall reported minimum sequestration
value for OF is 1.52 ×105tCO2yr−1 (Bakker et al
2001) for a spatially constraint field experiment while
the maximum reported value is 9.8 ×1010 tCO2yr−1
23

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Figure 11. Costs and potentials for ocean fertilization. The heat bar distribution of literature estimates in each panel is calculated as
in figure 6, with individual publication cost ranges represented by lines (costs panel); and maximum estimates of negative emissions
potential plotted by publication year (potentials panel). Estimates and ranges at the topand bottom end of the distribution are labelled;
the data can be further explored in our online supporting material available at https://mcc-apsis.github.io/NETs-review/.
(Oschlies 2009) using a modelling approach (see also
figure 11). The latter consider upscaling local experi-
mental process to global boundaries in order to predict
the potential CO2sequestration. Different authors
point out the low efficiency of OF in general (Rem-
bauville et al 2018,AumontandBopp2006,Jin
et al 2008, Zahariev et al 2008, Zeebe 2005)orthatOF
efficiency has a high degree of uncertainty (Aumont
and Bopp 2006).
Ocean fertilization costs depend on nutrient pro-
duction and its delivery to the application area (Jones
2014). The costs range from US$2/ tCO2(Boyd and
Denman 2008)toUS$457/tCO2(Harrison 2013). A
detailed economic analyses for macronutrient appli-
cation reports US$20/tCO2(Jones 2014), whereas
Harrison (2013) details that costs are much higher
due to the overestimation of sequestration capacity and
underestimation of logistic costs.
Side effects. OF is expect ed to alter local to regional
food cycles by stimulating phytoplankton production,
which is the food cycle’s basis. Long-term reductions
in ocean productivity could also occur (Matear 2004,
Denman 2008) and a more rapid increase in ocean
acidity (Cao and Caldeira 2010) due to fast CO2
dissolution and dissociation into bicarbonate and car-
bonate ions (Denman 2008). Impacts on the food
cycle would be unpredictable (Strong et al 2009a).
Extensive blooms may cause anoxia (Matear 2004,
Russell et al 2012, Sarmiento and Orr 1991)inthe
surface ocean due to the remineralization of sink-
ing organic matter and probable toxic algal blooms
(Bertram 2010,Tricket al 2010). Deep water oxygen
decline, as a potential side effect, has been observed
in the Baltic sea due to anthropogenic nitrate inputs
(Matear 2004). Nutrient (iron, silica or phosphate)
inputs potentially cause a shift in ecosystem production
from an iron-limited system to a phosphate-limited,
nitrogen-limited or a silicate-limited system, depend-
ing on the location (Bertram 2010,Matear2004). An
increase in the production of further greenhouse gases
may occur, including N2O (Bertram 2010,Matear
2004, Cullen and Boyd 2008,Denman2008)orCH
4
(Bertram 2010,Matear2004, Sarmiento and Orr 1991,
Cullen and Boyd 2008).
Permanence and saturation. Ocean CO2perma-
nence is rather controversial and depends on whether
the carbon remains dissolved in the different ocean
layers (short-term pool) or if it sediments as organic
carbon to the ocean abyssal plains, or to other
ocean compartments as long-term pool. Authors like
Williams and Druffel (1987) in Markels and Barber
(2001) suggest residence times of sinking carbon to the
deep waters being around 1600 years, or millennia in
the deep ocean (Jones 2014), while Aumont and Bopp
(2006) state that the sequestered carbon is rapidly re-
exposed to the atmosphere after cessation of OF, due
to the low final sedimentation rate (only 10%–25%)
(Zeebe 2005).
Authors’assessment. The high recycling rate of
organic carbon that stores the CO2leads to very low
overall potentials to sequester CO2on a longer time
scale. This meager efficiency as a NET, combined
with wide impacts on ecosystems, e.g. food web dis-
turbances, suggests that OF is not a viable negative
emissions strategy when performed with sustainabil-
ity issues under consideration (Strong et al 2009a),
particularly when compared to alternative portfolio
options.
24

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
0 0
10
20
30
100
200
300
2008 2010 2012 2014 2016 2006 2008 2010 2012 2014 2016
Cost [US$(2011)/tCO2]
% of Studies
Publication Year Publication Year
100 75 50 25 0
Sequestration Potential [Gt CO2/year]
Biochar - Costs Biochar - Potentials
Figure 12. Costs and potentials for biochar. The heat bar distribution of literature estimates in each panel are calculated as in figure
6, with individual publication cost ranges represented by lines (costs panel); and maximum estimates of negative emissions potential
plotted by publicationyear (potentials panel). All estimatesare global. Estimates and ranges at the top and bottom end of the distribution
are labelled; the data can be further explored in our online supporting material available at https://mcc-apsis.github.io/NETs-review/.
3.6. Biochar
Biochar is obtained from pyrolysis, i.e. the thermal
degradation of organic material in the absence of oxy-
gen. Added to soils, biochar is a means to increase soil
carbon stocks as well as improve soil fertility and other
ecosystem properties.
Potentials and costs. Recent assessments esti-
mate that the use of biochar could sequester between
0.6 GtCO2yr−1 and 11.9 GtCO2yr−1 , largely depend-
ing on the availability of biomass for biochar
production (see figure 12). Lenton (2010)calculated
CO2removal rates of 2.8–3.3 GtCO2yr−1 if all felling
losses from forestry, 50% of currently unused crop
residues, and burned biomass from shifting cultivation
fires were used to produce biochar. Lee et al (2010)
finds an even higher potential of 11.9 GtCO2yr−1 by
assuming that more than 80% of all currently harvested
biomass is converted into biochar. The author revised
this estimate to 6.1 GtCO2yr−1 in a later publication
(Lee and Day 2013) based on the assumption that the
worlds annual unused waste biomass contains only
about 12.1 GtCO2yr−1. Accounting only for the use
of late stover as a feedstock for biochar, Roberts et al
(2010) estimate achievable annual carbon sequestration
around 0.7 GtCO2yr−1.
Higher GHG mitigation potentials are generally
found in studies of future biochar applications rising
from 1–1.8 GtCO2yr−1 in 2030 (Lomax et al 2015,
Paustian et al 2016, Pratt and Moran 2010,Griscom
et al 2017), 1.8–4.8 GtCO2yr−1 in 2050 (Powell and
Lenton 2012,Smith2016, Moore et al 2010), and 2.6–
4.8 GtCO2yr−1 in 2100 (Woolf et al 2010). Abatement
cost estimates vary significantly. While some stud-
ies suggest that CO2prices between less than US$30
and 50/tCO2are sufficient for economically viable
biochar application (Lomax et al 2015,Robertset al
2010), other estimates reach US$60–120/tCO2(Shack-
ley et al 2011,McGlashanet al 2012,Smith2016),
especially for dedicated feedstocks, highlighting the
potential importance of waste feedstock for commer-
cially viable biochar projects. Currently, high biochar
prices prevents its large-scale application (Vochozka
et al 2016,Dickinsonet al 2015).
Side effects. A meta-analysis by Jeffery et al (2011)
indicates that crop productivity increases by 10% on
average following biochar soil amendment, but yield
effects ranged between positive and negative with dif-
ferent soil types, environmental, and management
conditions. Further effects of biochar amendments
include lower emissions of N2OandCH
4,wherelower
CH4emissions were measured especially on flooded
soils (Kammann et al 2017). Biochar can also have a
positive effect on the soil’s water balance. On temperate
soils, a 16% reduction in water losses was measured,
which at the same time reduced the negative effects
of soil dryness on microbial abundances by up to
80% (Bamminger et al 2016). However, the effects
of high biochar application rates which change the
microbial composition of the soil are still unknown
(Jiang et al 2016). Furthermore, increased plant growth
due to biochar may lead to a lower defense effec-
tiveness of the genes related to the defense of the
plants, thus increasing the vulnerability against insects,
pathogens and drought (Viger et al 2015). Large-scale
25

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
biochar application can also darken the soil sur-
face, decrease surface albedo and hence change the
land surface radiation balance, although application
rates would need to be extreme for such an effect
to occur. A recent study in Mediterranean agricul-
tural landscapes found that the albedo effect can
reduce biochar’s mitigation potential by up to ∼30%
during periods of high solar irradiance (Bozzi et al
2015). Fine biochar particles may also be released into
the atmosphere during production, transportation and
distribution by wind. These black carbon aerosols can
reduce air quality and cause a positive direct and indi-
rect radiative forcing which would further reduces the
net mitigation effect of biochar application (Ravi et al
2016, Genesio et al 2016).
Permanence and saturation. The most important
property of biochar with regard to climate protection
is its stability in the soil. In order to achieve effective
and long-term carbon storage, biochar should remain
in the soil for as long as possible. Laboratory tests and
other observations indicate centennial scale turnover
of biochar (Wang et al 2016). However, depending on
soil type and biochar production temperature, results
may vary between a few decades and several centuries
(Fang et al 2014). Lower residence times occur under
higher temperature typical for tropical and sub-tropical
regions (Zimmermann et al 2012) and acidic soils
(Sheng et al 2016).
Authors’assessment. Large-scale trials of biochar
addition to agricultural soils under field conditions
are still missing. Feasibility, long-term mitigation
potentials, side-effects, and trade-offs therefore remain
largely unknown. Furthermore, available global esti-
mates of biochar CO2sequestration potentials do not
yet account for the complex, site-dependent effects
of biochar applications that differ on with biochar
types, soil types, environmental, and management con-
ditions highlighted by recent laboratory analysis. In
our opinion, a lower range of 0.3–2 GtCO2yr−1 by
2050 seems plausible given the limited availability of
biomass realistically available for the production of
biochar. For comparison, the World’s total biomass
harvest on cropland and in forests in 2000 amounted
to 0.4 GtCO2yr−1 (Haberl et al 2007). The wide range
of cost estimates reflects the underlying uncertain-
ties regarding feedstock availability as well as biochar
production technologies and application strategies.
Economic benefits from higher yields may offset
some costs of biochar application. Because there is
no experience with large-scale production and use
of biochar, cost estimates remain inherently uncer-
tain. Against this background, mean ranges of biochar
costs between US$90/tCO2and US$120/tCO2based
on literature reviews (see for example (Smith 2016))
should be regarded as first rough estimates. Introduc-
ing biochar carbon offset methods to carbon trading
markets to further offset costs may also be com-
plicated because soil carbon is difficult to measure,
especially over large areas.
3.7. Soil carbon sequestration
Soil carbon sequestration (SCS) occurs when land
management change increases the soil organic carbon
content, resulting in a net removal of CO2from the
atmosphere. Since the level of carbon in the soil is
a balance of carbon inputs (e.g. from litter, residues,
roots, manure) and carbon losses (mostly through res-
piration, increased by soil disturbance), practices that
either increase inputs, or reduce losses can promote
SCS.
Potentials and costs. Of the 22 articles (Batjes 1998,
Benbi 2013,Conant2011,Hendersonet al 2015,Lal
2003b,2003a,2004a,2004c,2004b,2010,2011,2013,
Lassaletta and Aguilera 2015,LorenzandLal2014,
Powlson et al 2014, Salati et al 2010,Smith2012,2016,
Sommer and Bossio 2014,Minasnyet al 2017,Met-
ting et al 2001,Smithet al 2008) that included global
technical potentials for SCS, 16 provided minimum-
maximum ranges, and six provided best estimates
without a range. All estimates are ‘bottom-up’and
are calculated by multiplying a per-area sequestration
potential for each practice with an area over which the
practice could be applied.
Most of the variation contributing to the large vari-
ation in estimates arises from the area assumed to be
available, with the estimates at the high end of the
ranges assuming that e.g. all cropland and grassland
area are amenable to SCS. Lower estimates assume con-
straints (e.g. not all grassland is managed, degraded la nd
excluded etc (Smith et al 2008)).
Among the high estimates of maximum potential
(six articles, with around or above 7 GtCO2yr−1 ), five
are the top end of wide ranges (Lal 2003b,2010,2013,
2011,Minasnyet al 2017), though the mean of the
ranges are also above 7 GtCO2yr−1. The other high
estimate (Batjes 1998) was not estimated in the paper,
but was aggregating estimates from other pap ers (figure
13).
Ten of the global estimates of potential areat the low
end of the range as they consider individual practices.
For individual practices applied globally, the techni-
cal potentials are 1.47–2.93 GtCO2yr−1 for croplands,
0.73–1.47 GtCO2yr−1 for desertification control (Lal
2004b), 3.6 GtCO2yr−1 in dryland ecosystems (Lal
2004a), 1.47–3.67 GtCO2yr−1 for reclamation of agri-
cultural soils (Benbi 2013), 0.4–0.6 GtCO2yr−1 for
no tillage in croplands (Powlson et al 2014), 0.51–
1.25 GtCO2yr−1 for degraded land restoration (Salati
et al 2010), 4–8 GtCO2yr−1 for agro-forestry (Lorenz
and Lal 2014), 1.1–2.5 GtCO2yr−1 through forestry
and agriculture (Conant 2011), 3.3–6.7 GtCO2yr−1 in
croplands (Zomer et al 2017), 1.36–2.71 GtCO2yr−1
for croplands and pastures (Sommer and Bossio
2014) and 0.15 and 0.20 GtCO2yr−1 for grazing opti-
mization and planting of legumes in grazing land,
respectively (Henderson et al 2015).
The remainder of the estimates of maximum
potential (seven articles: (Lal 2003a,2004c, Lassaletta
and Aguilera 2015, Metting et al 2001,Smith2012,
26

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Figure 13. Costs and potentials for soil carbon sequestration. The heat bar distribution of literature estimates in each panel are
calculated as in figure 6, with individual publication cost ranges represented by lines (costs panel); and maximum estimates of negative
emissions potential plotted by publication year (potentials panel). Estimatesand ranges at the top and bottomend of the distribution
are labelled; the data can be further explored in our online supporting material available at https://mcc-apsis.github.io/NETs-review/.
Smith et al 2016b,2008)) are in the range of around
3–5 GtCO2yr−1, consistent with mean or median of
all estimates. Using the mid-point of the range for the
seventeen studies quoting ranges, and the best esti-
mate for the six articles not giving ranges, the mean
and median global technical potential for SCS were
4.28 and 3.677 GtCO2yr−1 , respectively (n= 23), with
a range of 1.1–1.37 GtCO2yr−1 using absolute min-
imum and maximum range values, or 2.91–5.65 or
2.28–5.34 GtCO2yr−1 using the mean and median
of the minimum range values, respectively (n= 17),
with this range considered feasible as a technical
potential.
There are few papers providing estimates of cost
per tonne of CO2equivalent (tCO2eq) removed by
SCS since this is very practice- and context-specific,
and depends greatly on, for example, labor costs and
degree of mechanization (Smith 2016). Only three
papers (Smith 2012,2016, Smith et al 2008)provided
estimates of economic potential for SCS, at US$20,
US$50 and 100/tCO2e, all of which are derived from the
same analysis. The SCS potential at US$20/tCO2ewas
1.38 (1.34–1.42) GtCO2yr−1 ,atUS$50/tCO2e was 2.32
(2.23–2.44) GtCO2yr−1 and at US$100/tCO2ewas3.7
(3.56–3.83) GtCO2yr−1,though(Smith2016)pro-
vides a lower estimate for global SCS at US$100/tCO2e
of 1.47–2.57 GtCO2yr−1 sinceitexcludessomeprac-
tices. The higher estimates of potential are associated
with higher carbon prices, as expected, since carbon
price is an indicator of level of climate change mit-
igation ambition. Using practices and costs listed in
(Smith et al 2008), (Smith 2016) note that about 20%
of the mitigation from SCS is realized at negative
cost (−45–0 US$/tCO2eq.) and about 80% realized
is between US$0andUS$10/tCO2eq. giving esti-
mates of global costs for implementation of SCS
globally as −7.7 B$(comprising 16.9 B$of savings,
and 9.2 B$of positive costs).
Side effects. Side effects are noted in a num-
ber of articles, featuring for example, improved soil
quality and health (Lal 2004b), improved and more
stable crop yield (Pan et al 2009), increased methane
emissions when SCS is encouraged in rice paddies
through addition of farmyard manure (Nayak et al
2015), or increased emissions of nitrous oxide if SCS is
encouraged by increasing plant productivity with nitro-
gen fertilizer (Liao et al 2015). Nonetheless, many
practices can be used with no adverse side effects.
Side effects were assessed and summarized in Smith
(2016). Though SCS is applied on large land areas, it
canbedonewithoutchanginglanduse,sotheland
footprint is zero. The water footprint is also negligible,
as is energy use and impact on albedo (Smith 2016).
Increased SCS results in more organic nitrogen in the
soil, which could be mineralized to become a substrate
for nitrous oxide (N2O) production, although the eff ect
is difficult to quantify (Smith 2016). The stoichiome-
try of the organic matter means that for every t C/ha
of soil organic matter added, nutrients, that is nitro-
gen, phosphorous and potassium, would increase by
80 kg ha−1,20kgha
−1 and 15 kg ha−1 , respectively (Lal
2004b,Smith2016). This could be derived from the
organic matter added, though if it requires external
nutrient addition, the increased nutrient level could
27

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
have knock-on effects on pollution if those nutrients
were lost to water courses.
Permanence and saturation. AdrawbackofSCSis
sink saturation. Though SCS negative emission poten-
tials are often expressed as per-year values, the potential
is time limited. SCS potential is large at the outset, but
decreases as soils approach a new, higher equilibrium
value (Smith 2012), such that the potential decreases
to zero when the new equilibrium is reached. This sink
saturation occurs after 10–100 years, depending on the
SCS option, soil type and climate zone (slower in colder
regions), with IPCC using a default saturation time of
20 years (Smith 2016). As sinks derived from SCS are
also reversible (Smith 2012), practices need to be main-
tained, even when the sink is saturated so any yearly
costs will persist even after the emission potential has
reduced to zero at sink saturation. Sink saturation also
means that SCS implemented in 2020 will no longer
be effective as a NET after 2040 (assuming 20 years for
sink saturation; Smith 2016).
Authors’assessment. The mean and median
global technical potentials for SCS of 4.28 and
3.677 GtCO2yr−1 (n= 23) represent good global esti-
mates of the technical global potential for SCS, with
ranges of 2.91–5.65 (using mean values of range mini-
mums/maximums) or 2.28–5.34 (using median values
of range minimums/maximums) GtCO2yr−1 (n= 17),
providing a good estimate of the spread of literature
ranges. Values below these ranges mostly consider only
single practices (e.g. no tillage, agro-forestry, restora-
tion of degraded land, grazing management), so do not
provide estimates for full global potential for SCS, while
values above these ranges (>7GtCO
2yr−1)arechar-
acterized by unconstrained estimates (e.g. by assuming
that high per-area estimates could be applied to all
cropland/ grassland areas globally with the same effec-
tiveness), so provide the very maximum, unconstrained
theoretical potential that would never be achievable in
reality. Based on this analysis, the best estimate (with
range) of realistic technical potential is considered to
be close to the median of the minimums of the ranges
provided, which for SCS is 3.8 (2.3–5.3) GtCO2yr−1 .
Costs are low, estimated here in the range of US$0–
100/tCO2,and the side effects are likely to be less of an
issue than for many other NETS, though sink satura-
tion and reversibility (non-permanence) are significant
drawbacks for SCS. As with the other technology esti-
mates, these ranges are for 2050, but once achieved,
cannot be maintained indefinitely due to sink satura-
tion. Since soils have been managed for millennia, there
is a high level of knowledge of practices and readiness
for adoption. Soil carbon sequestration is immediately
deployable since the agricultural and land manage-
ment practices required (e.g. improved rotations with
reduced fallow that increase carbon inputs to the soil
and addition of organic materials, such as manure or
compost, and other aspects of improved cropland and
grazing land management), are generally well known
by farmers and land managers (UNEP 2017).
3.8. Other and emerging NETs
There is a plethora of new ideas on how to extract
carbon from the atmosphere, some of which are yet to
be exposed in the literature. We discuss here some of
the newer literature based on a broad search of the Web
of Science and Scopus, and expert advice, focusing on
three avenues that have gained traction in the debate
around negative emissions.
Firstly, a recent strand of literature examines the
removal of non-CO2GHGs (GGR) such as methane
from the atmosphere. Such a process would be
valuable—per unit mass, methane is a more potent
GHG than CO2(Montzka et al 2011)—and could
compensate for emissions in the food sector and out-
gassing from lakes, wetlands, and oceans (Stolaroff
et al 2012). (Boucher and Folberth 2010) review several
existing technologies for methane removal (cryogenic
separation, molecular sieves or gates, and adsorption
filters based on zeolite minerals) and find low con-
fidence that any of these are currently economically
or energetically suitable for large-scale air capture,
however. More recent research (e.g. by de Richter
et al 2017) examines other technologies that also
consider non-CO2GHGs like N2O.
Secondly, there is a growing branch of literature on
Blue Carbon, i.e. the management of sea grasses, man-
groves, and salt marshes along coasts in order to expand
their carbon sinks. (Macreadie et al 2017) assess the lit-
erature for three different routes of Blue Carbon. They
find that reducing nutrient inputs, avoiding unnaturally
high levels of bioturbation (i.e. the turning of soils and
sediments by animals or plants), and restoring natu-
ral hydrology will maximize carbon sequestration and
minimize carbon losses. However, there are to date
no robust quantifications of a global negative emis-
sions potential from Blue Carbon. Still, most of the
options to enhance Blue Carbon also reduce human
and environmental impacts on coastal ecosystems—
an important co-benefit. (Johannessen and Macdonald
2016) report the Blue Carbon sink at 0.4%–0.8% of
global human-made emissions.
Thirdly, CO2could be used as synthetic feed-
stock for chemical materials because of its apparent
abundance, non-toxicity, and low cost. Potential prod-
ucts include Poly Propylene Carbonate (Qin et al
2015), carbon mineralization, Enhanced Oil Recovery
(EOR), biodiesel and synfuel production (Abanades
et al 2017) and other chemical applications providing
economic incentives and opportunities for techno-
logical learning for carbon capture. Note however
that current Life Cycle Analyses suffer from at least
one of the three following pitfalls that raise doubts
as to whether carbon capture and utilisation can
really contribute much to achieving large-scale neg-
ative emissions: (i) utilized CO2might intuitively be
considered as carbon-negative without actually being
so; (ii) accounting problems exist with respect to the
allocation of emissions to individual products; and
(iii) there may be negligence of CO2storage duration
28

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
(von der Assen et al 2013). Furthermore, MacDowell
et al (2017) voice serious concern about scale issues,
concluding that it is highly improbable that the chemi-
cal conversion of CO2will contribute more than 1% to
the mitigation needed to achieve the Paris Agreement’s
long-term temperature goal.
4. Synthesis
In this section, we synthesize the findings from the dif-
ferent NETs assessments in section 3and situate them
in the scenario evidence from section 2.Figure14 and
table 2show the ranges for the global potentials and
costs in 2050 and distill the main side effects, subcate-
gorized as either positive, or at risk of being negative.
We condition the cost and potential ranges with the
authors’assessments (summarized in the central plot
in figure 14). These assessments should be interpreted
as deployment ranges that are feasible in the context
of generally favorable conditions, i.e. long-term pol-
icy support, with key decisions made in the technology
cycle and deployment phase to generate demand pull,
and few social, economic or environmental shocks in
the relevant agricultural and land use sectors33 .One
aim of this review is to comprehensively cover the rele-
vant literature based on a transparent literature search
and selection process (see SI). We therefore also com-
pare our results with those from existing, previous
assessments.
4.1. Potentials
The deployment potentials in previous NETs assess-
ments vary considerably across studies. This study
spans the entire ranges of estimates for all individual
NETs reported in these previous assessments (Royal
Society 2009,McLaren2012, Friends of the Earth
2011, Vaughan and Lenton 2011,McGlashanet al
2012, National Academy of Sciences 2015,Calde-
cott et al 2015,Fusset al 2016,Smithet al 2016a,
Rubin et al 2015,Ciaiset al 2013,Lenton2010).
This overlap, in principle, confirms the ambition of
this review to provide comprehensiveness, yet it does
not ensure that estimates are weighted according to
the distribution of evidence. In the absence of com-
parable efforts, this can only be judged in terms of
the review procedure: our general approach is sum-
marized in Minx et al (2017) and outlined in detail
in the SI of this review.
Land-based mitigation options including biochar,
AR, EW as well as SCS each have a potent ial in the range
of 1–4 GtCO2yr−1 in 2050—noting that achieving the
higher end of the ranges gets increasingly demanding
and will require higher carbon prices. There is con-
33 They do not represent technical potentials, however, and do take
constraints into account, while the larger ranges we find in the
literature are partially due to the fact that different potentials are
considered, e.g. economic potentials and technical potentials.
siderable disagreement about reasonable deployment
potentials for BECCS and skepticism that deployment
ranges as seen in many scenarios can be reached. To
our judgment, due to constraints on the availability
of sustainable biomass, it will be extremely difficult
to achieve annual carbon removal rates of 5 GtCO2
with this technology by mid-century; however, end-of
the-century potentials might be considerably higher,
assuming that population peaks and reduces pressure
on land, alongside further yield improvements. DACCS
deployment will heavily depend on suitable energy
sources and cost developments. Given its nascent stage
of development, it will be an option with limited
potential in 2050. Yet, if DACCS becomes competitive,
potential deployment will be driven by cost support
and rates of upscaling, with no obvious upper biophys-
ical limit, barring storage, material and thermodynamic
constraints.
Whether these deployment potentials are well-
aligned with requirements identified in long-term
mitigationscenarios consistent with the 1.5 ◦Cand2◦C
scenarios, respectively, is questionable. This review
shares the wide-spread concern that reaching annual
deployment scales of 10–20 GtCO2yr−1 via BECCS at
the end of the 21st century, as is the case in many
scenarios, is not possible without severe adverse side
effects. Deployment scales reached in 2 ◦C scenarios
with limited BECCS deployment (corresponding to
about 100EJ of bioenergy) appear to be more realistic.
Opportunities for reaching larger deployment scales
emerge when NET portfolios composed of the various
technologies—rather than a single technology—are
developed and scaled-up over time. A discussion and
synthesis of development and upscaling bottlenecks are
provided in Nemet et al (2018) and Minx et al (2017),
respectively. Such a discussion of NET portfolios with
a variety of technologies contributing potentially at
more modest scales is important, but almost com-
pletely absent from the discussion and the reviewed
body of literature. Exceptions such as in (Strefler et al
2018a) confirm that adding other NETs (in this case
terrestrial enhanced weathering) to BECCS can sub-
stantially reduce side effects (in this case reduce the
land footprint, while still reaching considerable nega-
tive emissions potentials).
However, the deployment scales of individual tech-
nologies cannot be simply added up: first, some
technologies or practices compete with one another
for resources, e.g. land in the case of afforestation,
reforestation and BECCS; and competit ion for biomass
in the case of soil carbon sequestration, biochar and
BECCS (high biomass extraction rates for BECCS
will undermine the build-up and retention of soil
carbon via sustainable management practices, or via
biochar production). Second, scenarios that deploy
small portfolios of two or three NETs show that
adding another technology raises deployment, but at
a decreasing rate, i.e. NETs deployment is lower for
each technology when two or more rather than a
29

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Figure 14. Evidence and authors’assessments on negative emissions costs, deployment potentials, key side-effects, and cost/potential
trends beyond 2050. Note that risks of negative side effects are often contingent on implementation, e.g. large-scale afforestation with
mono-cultures versus ag roforestry projects, or biochar from dedicated crops versus residues. Panels A-G con trast authors’assessments
(hatched box, also reproduced in the central overview figure) with literature estimates (represented by a distribution function and
density plot). Density functions are computed using a Gaussian smoothing kernel density estimator; grey areas are defined by taking
the range of costs (potentials) that include the maximum density and that yield a bounded integral value of 0.5. Reference year for all
estimates provided is 2050. As annual deployments of soil carbon sequestration and afforestation cannot be sustained as long as other
technologies (due to sink saturation) we represent these technologies as dashed boxes in the central figure with an asterisk (∗).
30

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Table 2. Summary of assessment results (potentials and cost authors’assessment with the full range across the literature in square brackets, rounded numbers).
NET Potentials Cost Positive impacts Negative impacts Permanence/
Saturation
GtCO2yr−1 US$/tCO2Socio-economic Environmental Biophysical Socio-economic Environ-mental Biogeo-physical
BECCS 0.5–5 [1–85] 100–200 [15–400] Market
opportunities,
economic
diversification,
energy
independence,
technology
development and
transfer
GHG emissions
substitution
Food security,
health impacts
Biodiversity losses,
deforestation and
forest degradation,
through air
pollution CO2
leakage, impacts of
fertilizer use on
soil and water
Albedo change,
direct and indirect
LUC GHG
emissions (N2O,
CO2under
leakage)
High permanency
for adequate
geological storage,
long-term
governance of
storage, limits on
rates of bioenergy
production and
carbon
sequestration
DACCS 0.5–5 [limited by
upscaling and
costs]
100–300
[25–1000]
Business
opportunities,
subject to a
predictable CO2
price
Specific
applications could
improve indoor air
quality
CO2penalty if
high (thermal)
energy demand
satisfied by fossil
fuels; currently
high front-up
capital costs.
Material/waste
implications not
known but cannot
be excluded
Some spatial
requirements
High permanency
for adequate
geological storage
Afforestation and
re-forestation
0.5–3.6 [0.5–7] 5–50 [0–240] Employment
(caveat: low-paid
seasonal jobs),
local livelihoods
Biodiversity if
native and diverse
species are used (in
spite of lower CO2
storage)
Improved soil
carbon, nutrient
and water cycling
impacts
Less agricultural
exports, higher
food prices
Biodiversity losses
for high-carbon
monocultures and
under
displacement
Direct and indirect
LUC, albedo
change (boreal:
offsetting impact;
temperate:
neutralized)
Saturation of
forests; vulnerable
to disturbance;
post-AR forest
management
essential
31

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Table 2. Continued.
NET Potentials Cost Positive impacts Negative impacts Permanence/
Saturation
GtCO2yr−1 US$/tCO2Socio-economic Environmental Biophysical Socio-economic Environ-mental Biogeo-physical
Enhanced
weathering
2–4 [0–100] 50–200 [15–3460] Increase in crop
yields
Improved plant
nutrition
Improved soil
fertility, nutrient
and moisture,
increase in soil pH,
increasing cation
exchange capacity
in depleted soils
Human health
impacts associated
to fine grained
material
Ecological impacts
of mineral
extraction and
transport on a
massive scale
Direct and indirect
land use change if
biomass sourced
from dedicated
crops, potentially
heavy metal release
depending on the
soil characteristics,
risks of fine
grained material,
changes in soil
hydraulic
properties
Saturation of soil;
Residence time
from months to
geological time
scale
Ocean fertilization extremely limited
[0.5–44]
No authors’
assessment due to
limited potential
[0–460]
Potential increase
in fish catches
Enhanced
biological
production
None Unknown impacts
on marine biology
and food web
structure, changes
to nutrient balance
Anoxia in surface
ocean, probable
enhanced
production of
N2OandCH
4
Fragile, Saturation
of oceans;
Permanence from
millennia to
months/days
Biochar 0.5–2 [1–35] 30–120 [10–345] Increased crop
yields and reduced
drought
Reduced CH4 and
N2O emissions
from soils
Improved soil
carbon, nutrient
and water cycling
impacts
Competition for
biomass resources
Down-regulation
of plant defence
genes may increase
plant vulnerability
against insects,
pathogens, and
drought
Albedo change
partly offsetting
mitigation effect,
even though
likelihood low, as
biochar would be
buried.
Residence times of
biochars between
decades to
centuries
depending on soil
type, management
&environmental
conditions
Soil carbon
sequestration
2–5 [0.5–11] 0–100 [−45–100] Improved soil
resilience and
improved
agricultural
production,
Negative cost
options
Mostly reduced
pollution and
improved soil
quality
Mostly positive
impacts on soil,
water and air
quality
None Possible increase
in N2O emissions
andNandPlosses
to water due to
more N and P
substrate for
mineralisation
Need for addition
of N and P to
maintain
stoichiometry of
soil organic matter
Soil sinks saturate
and are reversible
when the
management
practice
promoting SCS
ceases
32

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
single technology is deployed (even if technologies are
of very different types such as BECCS and DACCS (or
EW)) (Marcucci et al 2017,Humpen
¨
oder et al 2014,
Chen and Tavoni 2013).
Beyond 2050, long term cumulative potentials are
a function of scalability and sink saturation. We sum-
marise these in figure 14 by indicating the expected
post-2050 trend in costs and potentials for each tech-
nology (qualitative arrows in the top-right of each
sub-figure). In the case of land-based options, these
constraints are severe. For instance, although SCS has a
very high mid-century potential (4–7 GtCO2yr−1)—
and can be quickly realised through changes in
farming and land management practices—after con-
sistent application the sink will saturate within ∼20
years and will require on-going maintenance. Biochar
is similarly constrained in terms of saturation, although
few studies yet point to the total feasible sink poten-
tial. Cumulative afforestation potential is constrained
by available land, with newly afforested sites saturating
within ∼100 years. We might then consider these three
options ‘21st century NETs’: promising stop-gaps, but
limited in long-term potential.
While our review highlights the limitations of
BECCS, unlike other land-based options it does not
saturate as quickly over time. The cycle of biomass
production and sequestration could conceivably con-
tinue up to the point that geological storage potential
is maximised, thereby sequestering a large cumula-
tive amount of CO2. However, BECCS is constrained
to maximum yearly potentials, as determined by a
sustainable scale of biomass production on land
(though as mentioned previously, technological
progress and a population peak could ease this pressure,
allowing for more annual CO2uptake). Lastly, DACCS
emerges as a relatively promising long-term option
beyond 2050, being limited in potential only by the
economic (and energetic) feasibility of scale-up.
4.2. Costs
Costs impose further economic limits to NETs deploy-
ment (see Smith et al 2016a). Across technologies,
costs vary significantly (figure 14). Particularly, land
management options like soil carbon sequestration,
biochar, afforestation and reforestation have a small-
scale availability at low, zero, or even negative costs in
places. Yet, despite technology cost reductions from
learning, the marginal costs of abatement tend to
increase with deployment, particularly for land man-
agement options such as afforestation and refores tation
(due to opportunity costs for land) and soil carbon
management (due to the exhaustion of cost-efficient
‘low-hanging’management options). Hence we see
these options increasing in costs beyond 2050 (fig-
ure 14). On the other hand, biochar may offer some
prospects for modest cost decreases as pyrolysis tech-
niques are still in their infancy and may yet benefit from
scale and learning dynamics.
Enhanced weathering is a relatively expensive
option due to the high energy requirements for grind-
ing the minerals to sufficiently small size. Hence,
carbon prices of US$50 and more are required if larger
deployments are to be reached, with prices progres-
sively increasing as proximate mining and deployment
locations are exhausted. On the other hand, less devel-
oped technologies like BECCS and DACCS (Nemet
et al 2018) are comparatively costly (US$100–200
and US$100–300/tCO2, respectively), but once avail-
able can be more easily scaled up—particularly in
thecaseofDACCS.Thelong-termcosttrendsof
BECCS are a matter of significant uncertainty—in
the literature and within the authors’assessment—
as they are shaped by multiple dynamics. Principally
these include the opportunity costs for land and
biomass, prospects for biomass yield increases and
alternative sources (e.g. algae), and the prospects for
bringing down plant costs via scaling and technolog-
ical learning. However, beyond a deployment level of
5GtCO2yr−1, we judge costs to increase as pressures
on land and biomass progressively grow, albeit with a
heavy caveat of uncertainty. With DACCS, however,
the literature is strongly suggestive of long-term cost
decreases, albeit starting from a high level.
Overall, cost and potential considerations could
suggest a natural order for phasing in different
NETs—a discussion we will further elaborate in Minx
et al (2017) by infusing development and upscaling
considerations from Nemet et al (2018). Interestingly,
these clusters of technologies also differ in terms of how
securely they store carbon. While DACCS and BECCS
store the carbon relatively safely mainly in geological
reservoirs (Bui et al 2018, de Coninck and Benson
2014), soil and biomass-sequestered carbon are per-
manently at risk of rapid release, should a reversal in
management decisions take place.
4.3. Side-effects
An often neglected aspect of NETs is constituted by
theco-benefitstheymayyield.Theliteratureshows
evidence that afforestation, soil carbon management,
enhanced weathering (on land), and biochar may
all contribute to soil quality, nutrient retention and
water cycling under appropriate management regimes.
Where these changes result in enhanced crop yields, the
socio-economic benefits to local and regional liveli-
hoods may be considerable. These benefits partially
explain the negative costs associated with soil carbon
management, as well as its maturity and existing imple-
mentation, alongside that of afforestation. Nonetheless,
there is a clear literature bias towards developed
countries concerning the land-based NETs, raising an
obvious need for research into the generally poorer
initial site conditions and more fragile social institu-
tions that are prevalent in developing nations. Another
non-trivial consideration is whether trace-GHGs will b e
mitigated (potentially biochar) or intensified (BECCS)
by changes to land management practices—an issue
33

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
that will require concerted long-term studies to track
fertilizer inputs, management practices, and resulting
land-use emissions.
For most NETs, whether co-benefits or nega-
tive impacts are realized depends on implementation
strategy and scale, at least in principle. For instance,
monocrop plantations of eucalyptus may be an effi-
cient means to draw down carbon, but are inferior to
agroforestry initiatives when considering a broader set
of social and environmental goals. More problemat-
ically, large-scale BECCS and afforestation programs
will drive up demand for land, posing risks for
food production, biodiversity, and land set aside for
other purposes (living space, nature reserves, and
other cultural, aesthetic or productive uses) (Newbold
et al 2015,Creutzig2017). As these effects play
out in global markets, the broader success of large-
scale land-based NETs will hence crucially depend on
global governance of land (Creutzig 2017). Cultivating
marginal land offsets this risk, as would exploiting waste
biomass feedstocks as inputs for biochar and BECCS.
However, with increasing scale, the opportunities
for careful implementation decline, forcing trade-offs
among valued land uses, and indeed between land
use NETs themselves. Another important considera-
tion here is the direct warming effect from a changing
surface albedo in Northern latitudes. This issue is
principally relevant for afforestation (planting trees in
in high latitudes is effectively counterproductive)—
but also for biochar and BECCS, both of which
will change prevailing soil and crop appearances. In
addition, it is not always clear what marginal and
degraded land really is and where it is, as defini-
tions and mappings diverge widely, so the potential
of biomass from marginal and degraded land is
unclear34.
Given the nascent stage of direct air capture,
enhanced weathering and ocean fertilization options,
some side-effects are probably not yet anticipated—
or have already been anticipated, but not subjected
to sufficient research. Research on the side effects of
direct air capture is basically non-existent. Conc eivably,
a large scale DACCS program will require extensive
amounts of materials, and therefore mineral ext raction,
refining, transportation and waste disposal infrastruc-
tures. The ecological impacts of these infrastructures
could be even more problematic for enhanced weath-
ering and ocean fertilization, which would require an
extensive mobilization of materials at a regional or
global scale. Further issues have been anticipated for
enhanced weathering (local air pollution, heavy metal
pollution in soils) and for ocean fertilization (surface
34 Scientists do not agree how much land is actually unused and
at the same time available for cultivation (Coelho et al 2012,Erb
et al 2007,Haberlet al 2010,FritzandSee2008), and the datasets
and definitions used for degraded and marginal land are ambiguous
(Sonneveld and Dent 2009). Even the terminology of marginal or
degraded lands is usually not clear.
ocean anoxia, nutrient balance shifts, potential large-
scale ecosystem changes), but remain under-examined
in practice.
4.4. Knowledge gaps
The systematic review of the NETs literature conducted
here does not only provide us with a comprehensive
assessment of their potentials, costs and side effects,
but has also unveiled areas of uncertainty.
For the study of the individual options to remove
carbon, all of them still need further work on esti-
mating the economic costs (and benefits) of real
world deployment and a quantification of environ-
mental, economic and social externalities associated
with deployment. This will then also enable more com-
prehensive modelling. In addition, there is a need to
better understand the barriers to implementation of
NETs and how these can be overcome. This includes
research on policies, incentive schemes and finance,
public acceptance, governance and actual demonstra-
tion projects. For enhanced weathering and ocean
fertilisation, for instance, the largest research gap iden-
tified in this assessment is the missing existence of
real field experiments. Also, potentials need to be
adjusted for new insights with respect to biophysi-
cal impacts of NETs deployment (e.g. on albedo),
changes in the carbon cycle caused by large-scale
negative emissions (Jones et al 2016,Tokarskaand
Zickfeld 2015) and changes in land cover due to cli-
mate change. Finally, moving from research needs to
gaps in practical knowledge, more actual pilot projects
are necessary.
Other research gaps are specific to certain NETs.
For example, in the case of NETs with high land
requirements such as BECCS, it will be important
to improve the mapping of available land, especially
marginal and degraded land. To this end, harmonized
definitions need to be developed and operationalized.
Based on this, geographically explicit regional studies
on potentials are needed. These bottom-up potentials
furthermore have to be matched to the global, top-
down ones. For afforestation and reforestation, for
example, there are too few studies explicitly cov ering the
tropics, yet this is the biome that global models associate
with the largest carbon removal potentials. Similarly,
only few studies examine the practical issues of imple-
menting soil carbon sequestration in the developing
countries, where biophysical as well as socio-economic
challenges may diverge substantially from the exist-
ing knowledge base. Also in the case of enhanced
weathering, proper management at the global scale
would demand databases of possible application sce-
narios combining rock products, soil conditions, local
climates and targeted plant systems.
Furthermore, there are many emerging ideas for
removing greenhouse gases, some of which have been
discussed in this review (e.g. methane removal), but
not assessed due to smaller or more fragmented bod-
ies of literature. As the corresponding knowledge
34

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
matures and more studies on their global potentials and
costs becomes available, they need to be systematically
assessed as well.
Finally, for the IAMs, the following research gaps
can be identified: (i) the need for integrated portfolios
of NETs in IAMs, which should include an evalua-
tion of interactions with other mitigation options and
effects of NETs on non-climate sustainable develop-
ment goals; (ii) a better understanding of geo-physical
constraints of negative emissions and implementation
in IAMs; (iii) an analysis of NETs deployment dynam-
ics in a risk management framework, acknowledging
that many decisions in climate change mitigation will
have to be made in the short term and therefore
under uncertainty.
5. Outlook
The assessment conducted in this paper has shown
that the state of the literature is at very different
stages for each of the various NETs considered: there
is a rapidly growing literature on BECCS, which is
closely interlinked with the scenario literature on
low-stabilization pathways. On the other hand, some
NETs look back at a much longer history in terms
of literature—even if carbon removal from the atmo-
sphere has not historically been the main motivation.
Examples of these are afforestation and reforesta-
tion, biochar and soil carbon storage. DACCS, ocean
fertilization and terrestrial and marine enhanced weat h-
ering still have to find their way into the scenario
literature, though this is starting to happen for some
of the options. Beyond that, more ideas are emerg-
ing to withdraw CO2and also other GHGs from the
atmosphere, as has been briefly discussed.
Our review highlights some more general confu-
sion around the role of negative emissions in climate
change mitigation: while 1.5 ◦C scenarios stronlgy
depend on NETs, 2 ◦C scenarios may rely on only
limited or zero deployment of NETs. This result is
in contrast to some of the claims that have been
made in the literature (Williamson 2016, Gasser et al
2015,Parson2017). Yet, right-sizing negative emis-
sions towards what seems possible today (Field and
Mach 2017) would require rapid and sustained emis-
sion reductions in the short-term. The window of
opportunity for limiting NETs dependence is clos-
ing rapidly due the cumulative warming effect of
CO2in the atmosphere and the lock-in of large-scale
carbon-intensive infrastructure. An emission trajec-
tory as suggested by the current nationally determined
contributions (NDCs) would already lock remaining
2◦C pathways deeply into NETs dependence—
similar to the NETs dependence for 1.5 ◦C pathways
today (Riahi et al 2015).
From a risk management perspective, the uncer-
tainties and risks around large-scale NETs deployment
suggest a need for swiftly ratcheting up emissions
reductions over the next decade in order to limit
our dependence on NETs for keeping temperature
rise below 2 ◦C. Based on our assessment, large-scale
deployment of NETs, as implied by some of the cur-
rent literature on 1.5 ◦C scenarios, appears unrealistic
given the biophysical and economic limits that are
suggested by the available, yet still patchy, science
today.Thesameconcernsof realism apply equally
to 2 ◦C scenarios that delay action until 2030. The
direct policy implications of this NETs review are thus
that, given the assessed uncertainties, strategies should
aim at limiting warming to below 2 ◦Cwiththeleast
possible assumed dependence on NETs. Simultane-
ously, NETs should be further researched, but until
they are demonstrably available at the global scale
there should be no delay in a global peak and decline
of CO2emissions. Whether global temperature can
be limited to 1.5 ◦C as part of the Paris Agreements
long-term temperature goal will depend on the pace
of technological learning and would require positive
surprises compared to the current state of knowledge.
If NETs become available at scale over the course of
the next 50 years, they will still play a fundamental
role in the context of 1.5 ◦C by enabling to revert
global mean temperature rise from possibly higher
peak levels.
Acknowledgments
The authors acknowledge scientific assistance from
Nicolas Koch and Sebastian L ¨
ubbers at the MCC.
SF has conducted the work for this article in the
frame of the project ‘Comparative assessment and
region-specific optimisation of GGR’under grant
reference NE/P019900/1 funded by the Natural Envi-
ronment Research Council of the UK and led by
Imperial College. This work furthermore has ben-
efitted from her activities in the Global Carbon
Project (Managing Global Negative Emission Tech-
nologies). JM and JH have contributed to this article
under the Project ‘Pathways and Entry Points to limit
global warming to 1.5 ◦C’funded by the German
Ministry of Research and Education (Grant refer-
ence: 01LS1610B). The input of PS contributes to
the UKERC-funded Assess-BECCS (UKERC/FFR2/3)
project and the NERC-funded Soils-R-GGREAT
(NE/P019455/1) project. TA, JHa, and WOG were
funded by the German Research Foundation’s priority
program DFG SPP 1689 on ‘Climate Engineering—
Risks, Challenges and Opportunities?’and specifically
the CEMICS2 project. GL and JLVV have con-
tributed to this manuscript under the Project ‘Strategic
Scenario Analysis’funded by the German Min-
istry of Research and Education (Grant reference:
03EK3046B).
35

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Glossary
Term Acronym Definition
Afforestation and
reforestation
AR Afforestation refers to planting of new forests on lands that historically have not
contained forests, while reforestation refers to planting of forests on lands which have,
historically, previously contained forests but which have been converted to some other
use. (IPCC Special Report on Land Use, Land-Use Change, and Forestry (IPCC 2000))
Albedo The fraction of solar radiation reflected by a surface or object, often expressed as a
percentage (IPCC 2014a, AR5 WGIII Glossary).
Alkalinity A measure of the capacity of an aqueous solution to neutralize acids. (AR5 WGI
Glossary): HCO3−+2∗CO32− +OH−–H++minor chemical species
Aragonite saturation
state
ASS Is obtained by the product of dissolved calcium and carbonate ions in seawater divided
by the aragonite solubility in seawater. If ASS is higher or lower than one sea water is
respectively over- or undersaturated with respect to aragonite. If ASS is equal one, the
sea water solution is saturated.
Bicarbonate A chemical compound: HCO3−
Biochar Biochar is charcoal used as a soil amendment produced through pyrosysis or gasification.
Biochar turnover time Average lifetime biochar in a soil after application.
Bioenergy carbon
capture and storage
BECCS The application of carbon dioxide capture and storage (CCS) technology to bioenergy
conversion processes. Depending on the total lifecycle emissions, including total
marginal consequential effects (from indirect land-use change (iLUC) and other
processes), BECCS has the potential for net carbon dioxide (CO2) removal from the
atmosphere.
Biological synthetic
natural gas
Bio-SNG Bio-gas made by chemical synthesis
Biomass gasification Gasification combines pyrolysis with further processing of generated gases to produce
syngas with energetic properties
Black liquor A liquid residue containinglignin compounds and inorganic chemicals, formed when
pulpwood is heated in alkaline solution in the kraft papermaking process (Oxford
Dictionaries).
Carbon budget The cumulative amount of net carbon dioxide emissions that can be released while still
limiting warming with a specific minimum probability to below a given temperature
threshold.
Carbon capture and
storage
CCS A process in which a relatively pure stream of carbon dioxide (CO2) from industrial and
energy-related sources is separated (captured), conditioned, compressed and
transported to a storage location for longterm isolation from the atmosphere.
Carbon dioxide
emission
The release of carbon dioxide to the atmosphere from various anthropogenic activities
(e.g. fossil fuel combustion, cement production, land-use changes). (AR5 WGIII
Glossary). In this review, we distinguish: gross positive CO2emissions (the amount of
CO2that is released into the atmosphere via anthropogenic activities), gross negative
CO2emissions (the amount of CO2that is removed from the atmosphere via negative
emission technologies), net positive CO2emissions (net CO2emissions that are
positive), and net negative emissions (net CO2emissions that are negative). Note that
the adjectives ‘positive’and ‘negative’refer only to the sign of CO2emissions. See also
definition of net emissions.
Carbon dioxide
removal
CDR See NETs definition.
Cogeneration Cogeneration (also referred to as combined heat and power, or CHP) is the
simultaneous generation and useful application of electricity and useful heat. (IPCC
2014a, AR5 WGIII Glossary)
Computable general
equilibrium model
CGE A class of economic models that use actual economic data (i.e. input/output data),
simplify the characterization of economic behaviour, and solve the whole system
numerically. CGE models specify all economic relationships in mathematical terms and
predict the changes in variables such as prices, output and economic welfare resulting
from a change in economic policies, given information about technologies and
consumer preferences. (Hertel 1997)(IPCC2014a, AR5 WGIII Glossary)
Cost effectiveness A policy is more cost-effective if it achieves a goal, such as a given pollution abatement
level, at lower cost. (IPCC 2014a, AR5 WGIII Glossary)
Cost-benefit analysis CBA Monetary measurement of all negative and positive impacts associated with a given
action. Costs and benefits are compared in terms of their difference and/or ratio as an
indicator of how a given investment or other policy effort pays off seen from the society’s
point of view. (IPCC 2014a, AR5 WGIII Glossary)
Direct air carbon
capture and storage
DACCS Chemical process by which dilute CO2is removed from the surrounding atmosphere.
36

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Discounting A mathematical operation making monetary (or other) amounts received or expended at
different times (years) comparable across time. The discounter uses a fixed or possibly
time-varying discount rate (>0) from year to year that makes future value worth less
today. (IPCC 2014a, AR5 WGIII Glossary)
Enhanced oil recovery EOR Enhanced oil recovery: the recovery of oil additional to that produced naturally by fluid
injection or other means. (IPCC 2005, Glossary)
Enhanced weathering EW Artificial stimulation of the natural process of rock decomposition while increasing the
cation release to produce alkalinity and geogenic nutrients.
Evaporative cooling The process of water transpiration from vegetation, which results in a local cooling effect.
Fermentation The biochemical process by which organic substances, particularly carbohydrates, are
decomposed by the action of enzymes to provide chemical energy, as in the production
of alcohol (Oxford Reference).
Fischer-Tropsch FT A process that transforms a gas mixture of CO and H2 into liquid hydrocarbons and
water. (IPCC 2005, Glossary)
Integrated assessment IA A method of analysis that combines results and models from the physical, biological,
economic, and social sciences, and the interactions among these components in a
consistent framework to evaluate the status and the consequences of environmental
change and the policy responses to it. (IPCC 2014a, AR5 WGIII Glossary)
Integrated
gasification combined
cycle
IGCC Power generation in which hydrocarbons or coal are gasified (q.v.) and the gas is used as
afueltodrivebothagasandasteamturbine.
Integrated model Integrated models explore the interactions between multiple sectors of the economy or
components of particular systems, such as the energy system. They may also include
representations of the full economy, land use and land use change (LUC), and the
climate system (based on IPCC 2014a, AR5 WGIII glossary).
Macronutrient nitrogen, phosphorus, potassium, calcium, sulfur, magnesium, carbon, oxygen, and
hydrogen are macronutrients for plants.
Mafic A rock with relative high magnesium, calcium and iron silicate content.
Micronutrient Iron, boron,manganese, zinc, copper, molybdenum or nickel are micronutrients for
plants.
Natural sink Process or mechanism of the Earth System that removes CO2from the atmosphere after
an initial perturbation.
Negative emission
technology
NET A technology or management option referring to a set of techniques that aim to remove
CO2directly from the atmosphere by either (1) increasing natural sinks for carbon or (2)
using chemical engineering to remove the CO2, with the intent of reducing the
atmospheric CO2concentration (based on CDR definition in IPCC 2014a: Annex II:
Glossary).
Net emissions The sum of gross positive and gross negative CO2emissions. See also definition of
carbon dioxide emission.
Ocean CO2
permanence
Correspond to the time, in which CO2would remain within the different ocean layers as
dissolved inorganic/organic carbon.
Overshoot
pathways/scenarios
Emissions, concentration, or temperature pathways/scenarios in which the metric of
interest temporarily exceeds, or ‘overshoots’, the long-term goal. (IPCC 2014a,AR5
WGIII Glossary)
Oxyfuel combustion Combustion of a fuel with pure oxygen or a mixture of oxygen, water and carbon dioxide
Pyrolysis Heating of biomass in the absence of oxygen. In this process, the chemical compounds
of biomass decompose into charcoal and combustible gases, and some of them can be
condensed into bio-oil.
Representative
Concentration
Pathway
RCP RCPs are scenarios that include time series of emissions and concentrations of the full
suite of greenhouse gases (GHGs) and aerosols and chemically active gases, as well as
land use/land cover. The word representative signifies that each RCP provides only one
of many possible scenarios that would lead to the specific radiative forcing
characteristics. The term pathway emphasizes that not only the long-term concentration
levels are of interest, but also the trajectory taken over time to reach that outcome (Moss
et al 2010). (IPCC 2014a, AR5 WGIII Glossary)
Shared
socioeconomic
pathway
SSP The SSPs are new emission and socio-economic scenarios (Riahi et al 2017)thathave
been developed to supersede the SRES scenarios (IPCC 2000). An SSP is one of a
collection of pathways that describe alternative futures of socio-economic development
in the absence of climate policy intervention.The combination of SSP-based
socio-economic scenarios and Representative Concentration Pathway (RCP)—based
climate projections provide a useful integrative frame for climate impact and policy
analysis. (IPCC 2014a, AR5 WGIII Glossary)
Soil amendment Material worked into the soil or applied on the surface to enhance plant growth.
Structural trap A geological structure capable of retaining hydrocarbons, sealed structurally by a fault or
fold. (IPCC 2005, Glossary)
Ultramafic A rock with very high magnesium silicate, as well as high calcium and iron silicate
content. The high cation content of magnesium and calcium is the cause for high
dissolved inorganic carbon binding capacity in the form of alkalinity.
37

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
ORCID iDs
Sabine Fuss https://orcid.org/0000-0002-8681-9839
William F Lamb https://orcid.org/0000-0003-3273-
7878
J´
erˆ
ome Hilaire https://orcid.org/0000-0002-9879-
6339
Gregory F Nemet https://orcid.org/0000-0001-7859-
4580
Joeri Rogelj https://orcid.org/0000-0003-2056-9061
Jan C Minx https://orcid.org/0000-0002-2862-0178
Thorben Amann https://orcid.org/0000-0001-9347-
0615
Wagner de Oliveira Garcia https://orcid.org/0000-
0001-9559-0629
Jens Hartmann https://orcid.org/0000-0003-1878-
9321
Pete Smith https://orcid.org/0000-0002-3784-1124
References
Abanades J C, Alonso M and Rodr´
ıguez N 2011 Biomass
combustion with in situ CO2capture with CaO. I. process
description and economics Ind. Eng. Chem. Res. 50 6972–81
Abanades J C, Rubin E S, Mazzotti M and Herzog H J 2017 On the
climate change mitigation potential of CO2conversion to fuels
Energy Environ. Sci. 10 2491–9
Akgul O, MacDowell N, Papageorgiou L G and Shah N 2014 A
mixed integer nonlinear programming (MINLP) supply chain
optimisation framework for carbon negative electricity
generation using biomass to energy with CCS (BECCS) in the
UK Int. J. Greenh. Gas Control 28 189–202
Akimoto K, Sano F and Tomoda T 2017 GHG emission pathways
until 2300 for t he 1.5 ◦C temperature rise target and the
mitigation costs achieving the pathways Mitig. Adapt. Strateg.
Glob. Change 1–14
Al-Qayim K, Nimmo W and Pourkashanian M 2015 Comparative
techno-economic assessment of biomass and coal with CCS
technologies in a pulverized combustion power plant in the
United Kingdom Int. J. Greenh. Gas Control 43 82–92
Anda M, Mulyani A, Suparto S and Suparto S 2012 Mineralogical
characterization and chemical properties of soils as a
consideration for establishing sustainable soil management
strategies Indones. J. Agric. Sci. 13 54
Anda M, Shamshuddin J and Fauziah C I 2015 Improving chemical
properties of a highly weathered soil using finely ground basalt
rocks CATENA 124 147–61
Anderson K 2015 Duality in climate science Nat. Geosci 8
898–900
Anderson K and Peters G 2016 The trouble with negative emissions
Science 354 182–3
Anderson R G et al 2011 Biophysical considerations in forestry for
climate protection Front. Ecol. Environ. 9174–82
Arasto A, Onarheim K, Tsupari E and K¨
arki J 2014 Bio-CCS:
feasibility comparison of large scale carbon-negative solutions
Energy Procedia 63 6756–69
Arora V K and Montenegro A 2011 Small temperature benefits
provided by realistic afforestation efforts Nat. Geosci. 4
514–8
von der Assen N, Jung J and Bardow A 2013 Life-cycle assessment
of carbon dioxide capture and utilization: avoiding the pitfalls
Energy Environ. Sci. 62721–34
Aumont O and Bopp L 2006 Globalizing results from ocean in situ
iron fertilization studies Glob. Biogeochem. Cycles 20
Azar C, Johansson D J A and Mattsson N 2013 Meeting global
temperature targets—the role of bioenergy with carbon
capture and storage Environ. Res. Lett. 81–8
Azar C, Lindgren K, Larson E and M¨
ollersten K 2006 Carbon
capture and storage from fossil fuels and biomass—costs and
potential role in stabilizing the atmosphere Clim. Change 74
47–79
Azar C, Lindgren K, Obersteiner M, Riahi K, van Vuuren D P, den
Elzen K M G J, M¨
ollersten K and Larson E D 2010 The
feasibility of low CO2concentration targets and the role of
bio-energy with carbon capture and storage (BECCS) Clim.
Change 100 195–202
de Baar H J W, Gerringa L J A, Laan P and Timmermans K R 2008
Efficiency of carbon removal per added iron in ocean iron
fertilization Mar. Ecol. Prog. Ser. 364 269–82
Bachu S, Bonijoly D, Bradshaw J, Burruss R, Holloway S,
Christensen N P and Mathiassen O M 2007 CO2storage
capacity estimation: methodology and gaps Int. J. Greenh. Gas
Control 1430–43
Bakker D C E, Bozec Y, Nightingale P D, Goldson L, Messias M-J,
de Baar H J W, Liddicoat M, Skjelvan I, Strass V and Watson A
J 2005 Iron and mixing affect biological carbon uptake in
SOIREE and EisenEx, two Southern Ocean iron fertilisation
experiments Deep Sea Res. Part I Oceanogr. Res. Pap. 52
1001–19
Bakker D C E, Watson A J and Law C S 2001 Southern Ocean iron
enrichment promotes inorganic carbon drawdown Deep Sea
Res. Part II Top. Stud. Oceanogr. 48 2483–507
Bamminger C, Poll C, Sixt C, H¨
ogy P, W¨
ust D, Kandeler E and
Marhan S 2016 Short-term response of soil microorganisms
to biochar addition in a temperate agroecosystem under soil
warming Agric. Ecosyst. Environ. 233 308–17
Barbarossa V, Vanga G, Viscardi R and Gattia D M 2014 CO2as
carbon source for fuel synthesis Energy Procedia 45 1325–9
Barkakaty B, Sumpter B G, Ivanov I N, Potter M E, Jones C W and
Lokitz B S 2017 Emerging materials for lowering atmospheric
carbon Environ. Technol. Innov. 730–43
Batjes N H 1998 Mitigation of atmospheric CO2concentrations by
increased carbon sequestration in the soil Biol. Fertil. Soils 27
230–5
Bauer N et al 2017 Shared socio-economic pathways of the energy
sector—quantifying the narratives Glob. Environ. Change 42
316–30
Beal C M, Archibald I, Huntley M E, Greene C H and Johnson Z I
2018 Integrating algae with bioenergy carbon capture and
storage (ABECCS) increases sustainability Earth’s Futur 6
524–42
Benbi D K 2013 Greenhouse gas emissions from agricultural soils:
sources and mitigation potential J. Crop Improv. 27 752–72
Ben´
ıtez P C, McCallum I, Obersteiner M and Yamagata Y 2007
Global potential for carbon sequestration: geographical
distribution, country risk and policy implications Ecol. Econ.
60 572–83
Ben´
ıtez P C and Obersteiner M 2006 Site identification for carbon
sequestration in Latin America: a grid-based economic
approach Forest Policy Econ. 8636–51
ten Berge H F, van der Meer H G, Steenhuizen J W, Goedhart P W,
Knops P and Verhagen J 2012 Olivine weathering in soil and
its effects on growth and nutrient uptake in Ryegrass (Lolium
perenne L.): a pot experiment PLoS ONE 7e42098
Beringer T, Lucht W and Schaphoff S 2011 Bioenergy production
potential of global biomass plantations under environmental
and agricultural constraints GCB Bioenergy 3299–312
Bertram C 2010 Ocean iron fertilization in the context of the Kyoto
protocol and the post-Kyoto process Energy Policy 38 1130–9
Betts R A, Falloon P D, Goldewijk K K and Ramankutty N 2007
Biogeophysical effects of land use on climate: model
simulations of radiative forcing and large-scale temperature
change Agric. Forest Meteorol. 142 216–33
Blanford G J, Kriegler E and Tavoni M 2014 Harmonization
vsfragmentation: overview of climate policy scenarios in
EMF27 Clim. Change 123 383–96
Borras S and Franco J 2010 Towards a broader view of the politics
of global land grab: rethinking land issues, reframing
resistance Initiatives in Critical Agrarian Studies Working
Paper Series 11–39
38

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Boucher O and Folberth G A 2010 New directions: atmospheric
methane removal as a way to mitigate climate change? Atmos.
Environ. 44 3343–5
Boyd P W and Denman K L 2008 Implications of large-scale iron
fertilization of the oceans Mar. Ecol. Prog. Ser. 364 213–8
Boyd P W et al 2004 The decline and fate of an iron-induced
subarctic phytoplankton bloom Nature 428 549
Boysen L R, Lucht W and Gerten D 2017 Trade-offs for food
production, nature conservation and climate limit the
terrestrial carbon dioxide removal potential Glob. Change Biol.
23 4303–17
Bozec Y, Bakker D C E, Hartmann C, Thomas H, Bellerby R G J,
Nightingale P D, Riebesell U, Watson A J and de Baar H J W
2005 The CO2system in a Redfield context during an iron
enrichment experiment in the Southern Ocean Mar. Ch em. 95
89–105
Bozzi E, Genesio L, Toscano P, Pieri M and Miglietta F 2015
Mimicking biochar-albedo feedback in complex
Mediterranean agricultural landscapes Environ. Res. Lett. 10
84014
Bradshaw J, Bachu S, Bonijoly D, Burruss R, Holloway S,
Christensen N P and Mathiassen O M 2007 CO2storage
capacity estimation: issues and development of standards Int.
J. Greenh. Gas Control 162–8
Brennan L and Owende P 2010 Biofuels from microalgae—A
review of technologies for production, processing, and
extractions of biofuels and co-products Renew. Sustain.
Energy Rev. 14 557–77
Bright R M, Zhao K, Jackson R B and Cherubini F 2015
Quantifying surface albedo and other direct biogeophysical
climate forcings of forestry activities Glob. Change Biol. 21
3246–66
Bringezu S 2014 Carbon recycling for renewable materials and
energy supply J. Ind. Ecol. 18 327–40
Brown S, Sathaye J, Cannell M and Kauppi P E 1995 Management
of forests for mitigation of greenhouse gas emissions Climate
Change 1995: impacts, adaptions and mitigation of climate
change: scientific-technical analyses (Cambridge: Cambridge
University Press) pp 773–98
Bruckner T et al 2014 Energy systems climate change 2014:
mitigation of climate change Contribution of Working Group
III to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change ed E Edenhofer et al (Cambridge:
Cambridge University Press)
Buck H J 2016 Rapid scale-up of negative emissions technologies:
social barriers and social implications Clim. Change 139
155–67
Bui M et al 2018 Carbon capture and storage (CCS): the way
forward Energy Environ. Sci. (https://doi.org/10.1039/
C7EE02342A)
Caldecott B, Lomax G and Workman M 2015 Stranded Carbon
Assets and Negative Emissions Technologies (Oxford: Smith
School of Enterprise and the Environment)
Calvin K, Edmonds J, Bond-Lamberty B, Clarke L, Kim S H, Kyle P,
Smith S J, Thomson A and Wise M 2009 2.6: Limiting climate
change to 450 ppm CO2equivalent in the 21st century Energy
Econ. 31 S107–20
Calvin K, Wise M, Kyle P, Patel P, Clarke L and Edmonds J 2014
Trade-offs of different land and bioenergy policies on the path
to achieving climate targets Clim. Change 123 691–704
Canadell J G and Raupach M R 2008 Managing forests for climate
change mitigation Science 320 1456–7
Cannell M G R 2002 Carbon sequestration and biomass energy
offset: theoretical, potential and achievable capacities globally,
in Europe and the UK Biomass Bioenergy 24 97–116
Cao L and Caldeira K 2010 Can ocean iron fertilization mitigate
ocean acidification? Clim. Change 99 303–11
Carbo M C, Smit R, Van Der Drift B and Jansen D 2011 Bio energy
with CCS (BECCS): Large potential for BioSNG at low CO2
avoidance cost Energy Procedia 42950–4
Chen C and Tavoni M 2013 Direct air capture of CO2and climate
stabilization: A model based assessment Clim. Change 118
59–72
Ciais P et al 2013 Carbon and other biogeochemical cycles Climate
Change 2013: The Physical Science Basis. Contribution of
Working Group I to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change ed T F Stocker, D
Qin, G-K Plattner, M Tignor, S K Allen, J Boschung, A Nauels,
Y Xia, V Bex and P M Midgley (Cambridge: Cambridge
University Press)
Clarke L E et al 2014 Assessing transformation pathways Clim.
Change 2014 Mitig. Clim. Change Contrib. Work. Gr. III to
Fifth Assess. Rep. Intergov. Panel Clim. Change ed O Edenhofer
(Cambridge: Cambridge University Press) pp 413–510
Coelho S T, Agbenyega O, Agostini A, Erb K-H, Haberl H,
Hoogwijk M, Lal R, Lucon O, Masera O and Moreira J R 2012
Chapter 20—land and water: linkages to bioenergy
(Cambridge: Cambridge University Press) pp 1459–526
Conant R T 2011 Sequestration through forestry and agriculture
Wiley Interdiscip. Rev. Clim. Change 2238–54
de Coninck H and Benson S M 2014 Carbon dioxide capture and
storage: issues and prospects Annu. Rev. Environ. Resour. 39
243–70
Corathers L A 2015 Mineral resource of the month: lime Earth
(https://www.earthmagazine.org/article/mineral-resource-
month-lime)
Cornelissen S, Koper M and Deng Y Y 2012 The role of bioenergy
in a fully sustainable global energy system Biomass Bioenergy
41 21–33
Creutzig F 2016 Economic and ecological views on climate change
mitigation with bioenergy and negative emissions GCB
Bioenergy 84–10
Creutzig F 2017 Govern land as a global comm ons Nature 546 28–9
Creutzig F, Agoston P, Goldschmidt J C, Luderer G, Nemet G and
Pietzcker R C 2017 The underestimated potential of solar
energy to mitigate climate change Nat. Energy 217140
Creutzig F, Corbera E, Bolwig S and Hunsberger C 2013
Integrating place-specific livelihood and equity outcomes into
global assessments of bioenergy deployment Environ. Res.
Lett. 835047
Creutzig F et al 2015 Bioenergy and climate change mitigation: an
assessment GCB Bioenergy 7916–44
Cullen J J and Bo yd P W 2008 Predictin g and verifying the intended
and unintended consequences of large-scale ocean iron
fertilization Mar. Ecol. Prog. Ser. 364 295–302
d’Hotman D V and Villiers O 1961 Soil rejuvenation with crushed
basalt in Mauritius Int. Sugar J. 63 363–4
Dale V H, Kline K L, Wiens J and Fargione J 2010 Biofuels:
implications for Land Use and Biodiversity (Washington, DC:
Ecological Society of America)
Deng Q, McMahon D E, Xiang Y, Yu C-L, Jackson R B and Hui D
2017 A global meta-analysis of soil phosphorus dynamics after
afforestation New Phytol. 213 181–92
Denman K L 2008 Climate change, ocean processes and ocean iron
fertilization Mar. Ecol. Prog. Ser. 364 219–25
D´
ıaz S, Wardle D A and Hector A 2009 Incorporating biodiversity
in climate change mitigation initiatives Biodiversity, Ecosystem
Functioning, and Human Wellbeing (Oxford: Oxford
University Press)
Dickinson D, Balduccio L, Buysse J, Ronsse F, van Huylenbroeck G
and Prins W 2015 Cost-benefit analysis of using biochar to
improve cereals agriculture GCB Bioenergy 7850–64
Dixon R K, Winjum J K, Andrasko K J, Lee J J and Schroeder P E
1994 Integrated land-use systems: assessment of promising
agroforest and alternative land-use practices to enhance
carbon conservation and sequestration Clim. Change 27
71–92
Dooley J J J 2013 Estimating the supply and demand for deep
geologic CO2storage capacity over the course of the 21st
century: a meta-analysis of the literature Energy Procedia 37
5141–50
Dooley J J J, Dahowski R T, Davidson C L, Bachu S, Gupta N and
Gale J 2005 A CO2-storage supply curve for North America
and its implications for the deployment of carbon dioxide
capture and storage systems Greenh. Gas Control Technol. 7
593–601
39

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Dornburg V et al 2010 Bioenergy revisited: key factors in global
potentials of bioenergy Energy Environ. Sci. 3258
MacDowell N, Fennell P S, Shah N and Maitland G C 2017 The role
of CO2capture and utilization in mitigating climate change
Nat. Clim. Change 7243–9
Edenhofer O and Kowarsch M 2015 Cartography of pathways: a
new model for environmental policy assessments Environ. Sci.
Policy 51 56–64
Edenhofer O, Seyboth K, Creutzig F and Schl¨
omer S 2013 On the
sustainability of renewable energy sources Annu. Rev. Environ.
Resour. 38 169–200
Edmonds J A, Dooley J J, Kim S H, Friedman S J and Wise M A
2007 Technology in an integrated assessment model: the
potential regional deployment of carbon capture and storage
in the context of global CO2stabilization Human-Induced
Climate Change: An Interdisciplinary Assessment ed M E
Echlesinger, H S Kheshgi, J Smith, F D La Chesnaye, J M
Reilly, T Wilson and C Kolstad (Cambridge: Cambridge
University Press)
Edmonds J, Luckow P, Calvin K, Wise M, Dooley J, Kyle P, Kim S
H, Patel P and Clarke L 2013 Can radiative forcing be limited
to 2.6 Wm–2 without negative emissions from bioenergy and
CO2capture and storage? Clim. Change 118 29–43
Eom J, Edmonds J, Krey V, Johnson N, Longden T, Luderer G,
Riahi K and Van Vuuren D P 2015 The impact of near-term
climate policy choices on technology and emission transition
pathways Technol. Forecast. Soc. Change 90 73–88
Erb K-H, Gaube V, Krausmann F, Plutzar C, Bondeau A and
Haberl H 2007 A comprehensive global 5 min resolution
land-use data set for the year 2000 consistent with national
census data J. Land Use Sci. 2191–224
Erb K-H, Haberl H and Plutzar C 2012a Dependency of global
primary bioenergy crop potentials in 2050 on food systems,
yields, biodiversity conservation and political stability Energy
Policy 47 260–9
Erb K-H K-H, Mayer A, Krausmann F, Lauk C, Plutzar C,
Steinberger J and Haberl H 2012b The interrelations of future
global bioenergy potentials, food demand, and agricultural
technology Socioecon. Environ. Impacts Biofuels Evid. from
Dev. Nations (Cambridge: Cambridge Unviersity Press)
pp 27–52
Fabbri A et al 2011 From geology to economics: technico-economic
feasibility of a biofuel-CCS system Energy Procedia 42901–8
Fang Y, Singh B, Singh B P and Krull E 2014 Biochar carbon
stability in four contrasting soils Eur. J. Soil Sci. 65 60–71
Field C B, Campbell J E and Lobell D B 2008 Biomass energy: the
scale of the potential resource Trends Ecol. Evol. 23 65–72
Field C B and Mach K J 2017 Rightsizing carbon dioxide removal
Science 356 706–7
Fischer G and Schrattenholzer L 2001 Global bioenergy potentials
through 2050 Biomass Bioenergy 20 151–9
Fornell R, Berntsson T and Åsblad A 2013 Techno-economic
analysis of a kraft pulp-mill-based biorefinery producing both
ethanol and dimethyl ether Energy 50 83–92
Friedlingstein P et al 2006 Climate–Carbon cycle feedback analysis:
results from the C4MIP model intercomparison J. Clim. 19
3337–53
Friends of the Earth 2011 Negatonnes—An Initial Assessment of
The Potential For Negative Emission Techniques To
Contribute Safely And Fairly To Meeting Carbon Budgets In
The 21st Century
Fritz S and See L 2008 Identifying and quantifying uncertainty and
spatial disagreement in the comparison of global land cover
for different applications Glob. Change Biol. 14 1057–75
Fuss S 2017 The 1.5 ◦C target, political implications, and the role of
BECCS Oxf. Res. Encycl. Clim. Sci. pp 1–29
Fuss S et al 2014 Betting on negative emissions Nat. Clim. Change 4
850–3
Fuss S et al 2016 Research priorities for negative emissions Environ.
Res. Lett. 11 115007
Gale J and Freund P 2001 Coal-bed methane enhancement with
CO2sequestration worldwide potential Environ. Geosci. 8
210–7
Gale J J 2004 Using coal seams for Co2sequestration Geol. Belgica 7
99–103
de Garcia O W, Amann T and Hartmann J 2018 Biomass demand
can affect forests nutrient budgets in wood exportation regions
Sci. Rep. 85280
Gasser T, Guivarch C, Tachiiri K, Jones C D and Ciais P 2015
Negative emissions physically needed to keep global warming
below 2 ◦CNat. Commun. 67958
GEA 2012 Global Energy Assessment—Toward a Sustainable
Future (Cambridge: Cambridge University Press)
Geden O 2015 Climate advisers must maintain integrity Nature 521
27–8
Genesio L, Vaccari F P and Miglietta F 2016 Black carbon aerosol
from biochar threats its negative emission potential Glob.
Change Biol. 22 2313–4
Gernaat D E H J, Calvin K, Lucas P L, Luderer G, Otto S A C, Rao S,
Strefler J and van Vuuren D P 2015 Understanding the
contribution of non-carbon dioxide gases in deep mitigation
scenarios Glob. Environ. Change 33 142–53
Gnanadesikan A, Sarmiento J L and Slater R D 2003 Effects of
patchy ocean fertilization on atmospheric carbon dioxide and
biological production Glob. Biogeochem. Cycles 17
Godec M, Koperna G and Gale J 2014 CO2-ECBM: a review of its
status and global potential Energy Procedia 63 5858–69
Goldberg D S, Lackner K S, Han P, Slagle A L and Wang T 2013
Co-location of air capture, subseafloor CO2sequestration,
and energy production on the Kerguelen plateau Environ. Sci.
Technol. 47 7521–9
Gonz´
alez M F and Ilyina T 2016 Impacts of artificial ocean
alkalinization on the carbon cycle and climate in Earth system
simulations Geophys. Res. Lett. 43 6493–502
Gough C and Upham P 2011 Biomass energy with carbon capture
and storage (BECCS or Bio-CCS) Greenh. Gases Sci. Technol.
1324–34
Greve M, Reyers B, Mette Lykke A and Svenning J-C 2013 Spatial
optimization of carbon-stocking projects across Africa
integrating stocking potential with co-benefits and feasibility
Nat. Commun. 42975
Griscom B W et al 2017 Natural climate solutions Proc. Natl Acad.
Sci. USA 114 11645–50
Haberl H 2015 The growing role of biomass for future resource
supply-prospects and pitfalls Sustainability Assessment of
Renewables-Based Products ed J Dewulf, S De Meester and R
Alvarenga (Chichester: Wiley) pp 1–18
Haberl H, Beringer T, Bhattacharya S C, Erb K-H and Hoogwijk M
2010 The global technical potential of bio-energy in 2050
considering sustainability constraints Curr. Opin. Environ.
Sustain. 2394–403
Haberl H, Erb K H, Krausmann F, Bondeau A, Lauk C, M¨
uller C,
Plutzar C and Steinberger J K 2011 Global bioenergy
potentials from agricultural land in 2050: Sensitivity to climate
change, diets and yields Biomass Bioenergy 35 4753–69
Haberl H, Erb K H, Krausmann F, Gaube V, Bondeau A, Plutzar C,
Gingrich S, Lucht W and Fischer-Kowalski M 2007
Quantifying and mapping the human appropriation of net
primary production in earth’s terrestrial ecosystems Proc. Natl
Acad.Sci.USA104 12942–7
Hakala K, Kontturi M and Pahkala K 2008 Field biomass as global
energy source Agric. Food Sci. 18 347–65
Hall J M, Van Holt T, Daniels A E, Balthazar V and Lambin E F
2012 Trade-offs between tree cover, carbon storage and
floristic biodiversity in reforesting landscapes Landsc. Ecol. 27
1135–47
Hallegatte S et al 2016 Mapping the climate change challenge Nat.
Clim. Change 6663–8
Hangx S J T and Spiers C J 2009 Coastal spreading of olivine to
control atmospheric CO2concentrations: a critical analysis of
viability Int. J. Greenh. Gas Control 3757–67
Harrison D P 2013 A method for estimating the cost to sequester
carbon dioxide by delivering iron to the ocean Int. J. Glob.
Warming 5231–54
Harrison D P 2017 Global negative emissions capacity of ocean
macronutrient fertilization Environ. Res. Lett. 12 35001
40

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Hartmann J and Kempe S 2008 What is the maximum potential for
CO2sequestration by ‘stimulated’weathering on the global
scale? Naturwissenschaften 95 1159–64
Hartmann J, West A J, Renforth P, K ¨
ohler P, De La Rocha C L,
Wolf-Gladrow D A, D ¨
urr H H and Scheffran J 2013 Enhanced
chemical weathering as a geoengineering strategy to reduce
atmospheric carbon dioxide, supply nutrients, and mitigate
ocean acidification Rev. Geophys. 51 113–49
Hassani N, Sedaeesola B, Jalali F, Mirazimi S and Karami M 2016
Simulation of CO2injection in Asmari reservoir for EOR and
sequestration, and investigation of effective operational
parameters: case study Pet. Res. 25 4–14
Hauck J, K¨
ohler P, Wolf-Gladrow D and V¨
olker C 2016 Iron
fertilisation and century-scale effects of open ocean
dissolution of olivine in a simulated CO2removal experiment
Environ. Res. Lett. 11
Henderson B B, Gerber P J, Hilinski T E, Falcucci A, Ojima D S,
Salvatore M and Conant R T 2015 Agriculture, ecosystems
and environment greenhouse gas mitigation potential of the
world’s grazing lands: modeling soil carbon and nitrogen fl
uxes of mitigation practices Agric. Ecosyst. Environ. 207
91–100
Hendriks C A and Blok K 1995 Underground storage of carbon
dioxide Energy Convers. Manage. 36 539–42
Hertel T W 1997 Global Trade Analysis: Modeling and Applications
(Cambridge: Cambridge University Press)
Holloway S 2009 Storage capacity and containment issues for
carbon dioxide capture and geological storage on the UK
continental shelf Proc. Inst. Mech. Eng. Part A J. Power Energy
223 239–48
Holmes G and Keith D W 2012 An air–liquid contactor for
large-scale capture of CO2from air Phil.Trans.R.Soc.A370
4380–403
Holmes G, Nold K, Walsh T, Heidel K, Henderson M A, Ritchie J,
Klavins P, Singh A and Keith D W 2013 Outdoor prototype
results for direct atmospheric capture of carbon dioxide
Energy Procedia 37 6079–95
Holz C, Siegel L, Johnston E B, Jones A D and Sterman J 2017
Ratcheting ambition to limit warming to 1.5 ◦C –trade-offs
between emission reductions and carbon dioxide removal
Hoogwijk M, Faaij A, Eickhout B, de Vries B and Turkenburg W
2005 Potential of biomass energy out to 2100, for four IPCC
SRES land-use scenarios Biomass Bioenergy 29 225–57
Hoogwijk M, Faaij A, de Vries B and Turkenburg W 2009
Exploration of regional and global cost-supply curves of
biomass energy from short-rotation crops at abandoned
cropland and rest land under four IPCC SRES land-use
scenarios Biomass Bioenergy 33 26–43
Houghton R A, Byers B and Nassikas A A 2015 A role for tropical
forests in stabilizing atmospheric CO2Nat. Clim. Change 5
1022–3
House K Z, House C H, Schrag D P and Aziz M J 2007
Electrochemical acceleration of chemical weathering as an
energetically feasible approach to mitigating anthropogenic
climate change Environ. Sci. Technol. 41 8464–70
HouseKZ,BacligAC,RanjanM,vanNieropEA,WilcoxJand
Herzog H J 2011 Economic and energetic analysis of
capturing CO2from ambient air Proc. Natl Acad. Sci. 108
20428–33
Huang Z, Jiang D, Lu L and Ren Z J 2016 Ambient CO2capture
and storage in bioelectrochemically mediated wastewater
treatment Bioresour. Technol. 215 380–5
Hulme M 2016 1.5 ◦C and climate research after the Paris
Agreement Nat. Clim. Change 6222–4
Humpen¨
oder F, Popp A, Dietrich J P, Klein D, Lotze-Campen H,
Bonsch M, Bodirsky B L, Weindl I, Stevanovic M and M¨
uller
C 2014 Investigating afforestation and bioenergy CCS as
climate change mitigation strategies Environ. Res. Lett. 9
64029
Hunsberger C, Bolwig S, Corbera E and Creutzig F 2014 Livelihood
impacts of biofuel crop production: implications for
governance Geoforum 54 248–60
IEA GHG 2005 Building the Cost Curves for CO2Storage: European
Sector 162 (Cheltenham: International Energy Agency
Greenhouse Gas R&D Programme)
IEA GHG 1998 Enhanced Coal Bed Methane Recovery with CO2
Sequestration PH3/3
IEA Greenhouse Gas R&D Programme 2000 Barriers to Overcome
in Implementation of CO2CaptureandStorage(1)Storagein
Disused Oil and Gas Fields. Report Number PH3/22
IPCC 2005 Carbon Dioxide Capture and Storage (SRCCS) ed B
Metz, O Davidson, H de Coninck, M Loos and L Meyer
(Cambridge: Cambridge University Press)
IPCC 2013 Climate change 2013: the physical science basis
Contribution of Working Group I to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change ed D
Qin,GKPlattner,MTignor,SKAllen,JBoschung,ANauels,
Y Xia, V Bex and P M Midgley (Cambridge: Cambridge
University Press)
IPCC 2014a Climate change 2014: mitigation of climate change
Contribution of Working Group III to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change ed O
Edenhofer et al (Cambridge: Cambridge University Press)
IPCC 2014b Climate change 2014: synthesis report Contribution of
Working Groups I, II and III to the Fifth Assessment Report of
the Intergovernmental Panel on Climate Change ed R K
Pachauri and L A Meyer (Geneva: IPCC)
IPCC 2000 Land Use, Land-Use Change, and Forestry ed R T
Watson, I R Noble, B Bolin, N H Ravindranath, D J Verardo
and D J Dokken (Cambridge: Cambridge University Press)
Ishimoto Y, Sugiyama M, Kato E, Moriyama R, Tsuzuki K and
Kurosawa A 2017 Putting Costs of Direct Air Capture in
Context
Jackson R B, Jobbagy E G, Avissar R, Roy S B, Barrett D J, Cook C
W, Farley K A, le Maitre D C, McCarl B A and Murray B C
2005 Trading water for carbon with biological carbon
sequestration Science 310 1944–7
Jackson R B et al 2008 Protecting climate with forests Environ. Res.
Lett. 344006
Jaju M M, Nader F H, Roure F and Matenco L 2016 Optimal
aquifers and reservoirs for CCS and EOR in the Kingdom of
Saudi Arabia: an overview Arab. J. Geosci. 9604
Jeffery S, Verheijen F G A, van der Velde M and Bastos A C 2011 A
quantitative review of the effects of biochar application to soils
on crop productivity using meta-analysis Agric. Ecosyst.
Environ. 144 175–87
Jiang X, Denef K, Stewart C E and Cotrufo M F 2016 Controls and
dynamics of biochar decomposition and soil microbial
abundance, composition, and carbon use efficiency during
long-term biochar-amended soil incubations Biol. Fertil. Soils
52 1–14
Jin X, Gruber N, Frenzel H, Doney S C and McWilliams J C 2008
The impact on atmospheric CO2of iron fertilization induced
changes in the ocean’sbiologicalpumpBiogeosciences 5
385–406
Johannessen S C and Macdonald R W 2016 Geoengineering with
seagrasses: is credit due where credit is given? Environ. Res.
Lett. 11 113001
Johnson N, Parker N and Ogden J 2014 How negative can biofuels
with CCS take us and at what cost? Refining the economic
potential of biofuel production with CCS using
spatially-explicit modeling Energy Procedia 63 6770–91
Jones A D, Calvin K V, Collins W D and Edmonds J 2015
Accounting for radiative forcing from albedo change in future
global land-use scenarios Clim. Change 131 691–703
Jones C D et al 2016 Simulating the Earth system response to
negative emissions Environ. Res. Lett. 11 95012
Jones I S F 2014 The cost of carbon management using ocean
nourishment Int. J. Clim. Change Strateg. Manage. 6391–400
Joos F, Siegenthaler U and Sarmiento J L 1991 Possible Effects of
Iron Fertilization in the Southern Ocean on atmospheric CO2
concentration Glob. Biogeochem. Cycles 5135–50
Kaiser J 2000 Panel estimates possible carbon Sinks Science 288
942LP–943
41

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Kammann C et al 2017 Biochar as a tool to reduce the agricultural
greenhouse-gas burden—knowns, unknowns and future
research needs J. Environ. Eng. Landsc. Manage. 25 114–39
K¨
arki J, Tsupari E and Arasto A 2013 CCS feasibility improvement
in industrial and municipal applications by heat utilization
Energy Procedia 37 2611–21
Karnosky D F, Sharma P, Thakur R C, Kinouchi M, King J, Kubiske
M E and Birdsey R A 2003 Changing atmospheric carbon
dioxide: a threat or benefit? Dev. Environ. Sci. 357–84
Keith D W, Ha-Duong M and Stolaroff J K 2006 Climate strategy
with CO2capture from the air Clim. Change 74 17–45
Keith D W 2009 Why capture CO2from the atmosphere? Science
325 1654–5
Keller D P, Lenton A, Scott V, Vaughan N E, Bauer N, Ji D, Jones C
D, Kravitz B, Muri H and Zickfeld K 2018 The carbon
dioxide removal model intercomparison project (CDR-MIP):
Rationale and experimental protocol for CMIP6 Geosci. Model
Dev. 11 1133–60
Kittner N, Lill F and Kammen D M 2017 Energy storage
deployment and innovation for the clean energy transition
Nat. Energy 217125
Klein D, Humpen¨
oder F, Bauer N, Dietrich J P, Popp A, Leon
Bodirsky B, Bonsch M and Lotze-Campen H 2014 The global
economic long-term potential of modern biomass in a
climate-constrained world Environ. Res. Lett. 974017
K¨
ohler P, Abrams J F, V¨
olker C, Hauck J and Wolf-Gladrow D A
2013 Geoengineering impact of open ocean dissolution of
olivine on atmospheric CO2, surface ocean pH and marine
biology Environ. Res. Lett. 8
K¨
ohler P, Hartmann J and Wolf-Gladrow D A 2010
Geoengineering potential of artificially enhanced silicate
weathering of olivine Proc. Natl Acad Sci. USA 107 20228–33
Koide H, Tazaki Y, Noguchi Y, Iijima M, Ito K and Shindo Y 1993
Underground storage of carbon dioxide in depleted natural
gas reservoirs and in useless aquifers Eng. Geol. 34 175–9
Koornneef J, van Breevoort P, Hamelinck C, Hendriks C, Hoogwijk
M, Koop K, Koper M, Dixon T and Camps A 2012 Global
potential for biomass and carbon dioxide capture, transport
and storage up to 2050 Int. J. Greenh. Gas Control 11 117–32
Kothandaraman J, Goeppert A, Czaun M, Olah G A and Prakash G
K S 2016 Conversion of CO2from air into methanol using a
polyamine and a homogeneous ruthenium catalyst J. Am.
Chem. Soc. 138 778–81
Krausmann F, Schandl H, Eisenmenger N, Giljum S and Jackson T
2017 Material flow accounting: measuring global material use
for sustainable development Annu. Rev. Environ. Resour. 42
647–75
Kraxner F and Nordstr ¨
om E-M 2015 Bioenergy Futures: A Global
Outlook on the Implications of Land Use for Forest-Based
Feedstock Production The Future Use of Nordic Forests (Cham:
Springer) pp 63–81
Kraxner F et al 2013 Global bioenergy scenarios—future forest
development, land-use implications, and trade-offs Biomass
Bioenergy 57
Kreidenweis U, Humpen ¨
oder F, Stevanovi ´
cM,BodirskyBL,
Kriegler E, Lotze-Campen H and Popp A 2016 Afforestation
to mitigate climate change: impacts on food prices under
consideration of albedo effects Environ. Res. Lett. 11 85001
Krey V, Luderer G, Clarke L and Kriegler E 2014a Getting from
here to there—energy technology transformation pathways in
the EMF27 scenarios Clim. Change 123 369–82
Krey V et al 2014b Annex II: metrics &methodology Climate
Change 2014: Mitigation of Climate Change. Contribution of
Working Group III to the Fifth Assessment Report of the
Intergovernmental Panel on Climate Change ed O Edenhofer,
R Pichs-Madruga, Y Sokona, E Farahani, S Kadner, K Seyboth
and A Adler (Cambridge: Cambridge University Press)
Kriegler E, Edenhofer O, Reuster L, Luderer G and Klein D 2013a Is
atmospheric carbon dioxide removal a game changer for
climate change mitigation? Clim. Change 118 45–57
Kriegler E et al 2013b What does the 2 ◦C target imply for a global
climate agreement in 2020? The LIMITS study on Durban
Platform scenarios Clim. Change Econ. 4134
Kriegler E et al 2014 The role of technology for achieving climate
policy objectives: overview of the EMF 27 study on global
technology and climate policy strategies Clim. Change 123
353–67
Kriegler E et al 2016a Will economic growth and fossil fuel scarcity
help or hinder climate stabilization?Clim. Change 136
7–22
Kriegler E, Mouratiadou I, Luderer G, Edmonds J and Edenhofer O
2016b Introduction to the RoSE special issue on the impact of
economic growth and fossil fuel availability on climate
protection Clim. Change 136 1–6
Kriegler E, Riahi K, Bosetti V, Capros P, Petermann N, van Vuuren
D P, Weyant J P and Edenhofer O 2015 Introduction to the
AMPERE model intercomparison studies on the economics of
climate stabilization Technol. Forecast. Soc. Change 90 1–7
Kulkarni A R and Sholl D S 2012 Analysis of equilibrium-based
TSA processes for direct capture of CO2from air Ind. Eng.
Chem. Res. 51 8631–45
Kuuskraa V A, Boyer C M and Kelafant J A 1992 Coalbed Gas-1
hunt for quality basins goes abroad Oil Gas J. 90
Lackner K S 2009 Capture of carbon dioxide from ambient air Eur.
Phys.J.Spec.Top.176 93–106
Lackner K S, Brennan S, Matter J M, Park A H A,Wright A and Van
Der Zwaan B 2012 The urgency of the development of CO2
capture from ambient air Proc. Natl Acad. Sci. 109 13156–62
Lackner K, Ziock H-J and Grimes P 1999 Carbon Dioxide
Extraction from Air: Is it an Option? (NM: Los Alamos
National Lab)
Laganiere J, Angers D A and Pare D 2010 Carbon accumulation in
agricultural soils after afforestation: a meta-analysis Glob.
Change Biol. 16 439–53
Lal R 2010 Beyond Copenhagen: mitigating climate change and
achieving food security through soil carbon sequestration
Food Secur. 2169–77
Lal R 2003a Global potential of soil carb on sequestration to mitigate
the greenhouse effect CRC. Crit. Rev. Plant Sci. 22 151–84
Lal R 2004a Carbon sequestration in dryland ecosystems Environ.
Manage. 33 528–44
Lal R 2004b Soil carbon sequestration impacts on global climate
change and food security Science 304 1623–7
Lal R 2004c Soil carbon sequestration to mitigate climate change
Geoderma 123 1–22
Lal R 2013 Intensive agriculture and the soil carbon pool J. Crop
Improv. 27 735–51
Lal R 2003b Offsetting global CO2emissions by restoration of
degraded soils and intensification of world agriculture and
forestry L. Degrad. Dev. 14 309–22
Lal R 2011 Sequestering carbon in soils of agro-ecosystems Food
Policy 36 S33–9
Lassaletta L and Aguilera E 2015 Soil carbon sequestration is a
climate stabilization wedge: comments on Sommer and Bossio
(2014) J. Environ. Manage. 153 48–9
Laude A, Ricci O, Bureau G, Royer-Adnot J and Fabbri A 2011
CO2capture and storage from a bioethanol plant: carbon and
energy footprint and economic assessment Int. J. Greenh. Gas
Control 51220–31
Lauri P, Havl´
ık P, Kindermann G, Forsell N, B¨
ottcher H and
Obersteiner M 2014 Woody biomass energy potential in 2050
Energy Policy 66 19–31
Lee J W and Day D M 2013 Smokeless biomass pyrolysis for
producing biofuels and biochar as a possible arsenal to control
climate change Advanced Biofuels and Bioproducts (New York:
Springer) pp 23–34
Lee J W, Hawkins B, Day D M and Reicosky D C 2010
Sustainability: the capacity of smokeless biomass pyrolysis for
energy production, global carbon capture and sequestration
Energy Environ. Sci. 31695
Lemoine D M, Fuss S, Szolgayova J, Obersteiner M and Kammen D
M 2012 The influence of negative emission technologies and
technology policies on the optimal climate mitigation
portfolio Clim. Change 113 141–62
Lemus R and Lal R 2005 Bioenergy crops and carbon sequestration
CRC. Crit. Rev. Plant Sci. 24 1–21
42

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Lenton T M 2014 CHAPTER 3 the global potential for carbon
dioxide removal Geoengineering of the Climate System
(Cambridge: The Royal Society of Chemistry) pp 52–79
Lenton T M 2010 The potential for land-based biological CO2
removal to lower future atmospheric CO2concentration
Carbon Manage. 1145–60
Leonardos O H, Fyfe W S and Kronberg B I 1987 The use of
ground rocks in laterite systems: an improvement to the use of
conventional soluble fertilizers? Chem. Geol. 60 361–70
Li X, Wei N, Liu Y, Fang Z, Dahowski R T and Davidson C L 2009
CO2point emission and geological storage capacity in China
Energy Procedia 12793–800
Liao Y, Wu W L, Meng F Q, Smith P and Lal R 2015 Increase in soil
organic carbon by agricultural intensification in northern
China Biogeosciences 12 1403
Liu W W, Wang X, Lu F and Ouyang Z 2016 Influence of
afforestation, reforestation, forest logging, climate change,
CO2concentration rise, fire, and insects on the carbon
sequestration capacity of the forest ecosystem Acta Ecol. Sin.
36 2113–22
Locatelli B, Catterall C P, Imbach P, Kumar C, Lasco R,
Mar´
ın-Spiotta E, Mercer B, Powers J S, Schwartz N and
Uriarte M 2015 Tropical reforestation and climate change:
beyond carbon Restor. Ecol. 23 337–43
Lomax G, Workman M, Lenton T and Shah N 2015 Reframing the
policy approach to greenhouse gas removal technologies
Energy Policy 78 125–36
Lopes A S 1996 Soils under Cerrado: a success story in soil
management Better Crop. Int. 10 10
Lorenz K and Lal R 2014 Soil organic carbon sequestration in
agroforestry systems. A review Agron. Sustain. Dev. 34
443–54
Luckow P, Wise M A, Dooley J J and Kim S H 2010 Large-scale
utilization of biomass energy and carbon dioxide capture and
storage in the transport and electricity sectors under stringent
CO2concentration limit scenarios Int. J. Greenh. Gas Control
4865–77
Luderer G, Krey V, Calvin K, Merrick J, Mima S, Pietzcker R, Van
Vliet J and Wada K 2014 The role of renewable energy in
climate stabilization: results from the EMF27 scenarios Clim.
Change 123 427–41
Luderer G, Pietzcker R C, Bertram C, Kriegler E, Meinshausen M
and Edenhofer O 2013 Economic mitigation challenges: how
further delay closes the door for achieving climate targets
Environ. Res. Lett. 834033
Macreadie P I, Nielsen D A, Kelleway J J, Atwood T B, Seymour J R,
Petrou K, Connolly R M, Thomson A C G, Trevathan-Tackett
S M and Ralph P J 2017 Can we manage coastal ecosystems to
sequester more blue carbon? Front. Ecol. Environ. 15 206–13
Magill B 2017 World’s first commercial CO2capture plant goes live
|climatecentralClim. Cent. (www.climatecentral.org/
news/first-commercial-co2-capture-plant-live-21494)
Manning D A 2015 How will minerals feed the world in 2050? Proc.
Geol. Assoc. 126 14–7
Manning D A and Renforth P 2013 Passive sequestration of
atmospheric CO2through coupled plant-mineral reactions in
urban soils Environ. Sci. Technol. 47 135–41
Marcucci A, Kypreos S and Panos E 2017 The road to achieving the
long-term Paris targets: energy transition and the role of direct
air capture Clim. Change 144 181–93
Markels M and Barber R 2001 Sequestration of CO2by ocean
fertilization Poster Presentation for NETL Conference on
Carbon Sequestration (Washington, DC: Conference
Secretariat) pp 14–7
Matear R J 2004 Enhancement of oceanic uptake of anthropogenic
CO2by macronutrient fertilization J. Geophys. Res. 109
Mazzotti M, Baciocchi R, Desmond M J and Socolow R H 2013
Direct air capture of CO2with chemicals: optimization of a
two-loop hydroxide carbonate system using a countercurrent
air-liquid contactor Clim. Change 118 119–35
McGlashan N, Shah N, Caldecott B and Workman M 2012
High-level techno-economic assessment of negative emissions
technologies Process Saf. Environ. Prot. 90 501–10
McKinley D C et al 2011 A synthesis of current knowledge on
forests and carbon storage in the United States Ecol. Appl. 21
1902–24
McLaren D 2012 A comparative global assessment of potential
negative emissions technologies Spec. Issue Negat. Emiss.
Technol. 90 489–500
Meinshausen M, Raper S C B and Wigley T M L 2011 Emulating
coupled atmosphere-ocean and carbon cycle models with a
simpler model, MAGICC6—Part 1: Model description and
calibration Atmos. Chem. Phys. 11 1417–56
Metting F B, Smith J L, Amthor J S and Izaurralde R C 2001 Science
needs and new technology for increasing soil carbon
sequestration Clim. Change 51 11–34
Meylan F D, Moreau V and Erkman S 2015 CO2utilization in the
perspective of industrial ecology, an overview J. CO2Util. 12
101–8
Millar R J, Fuglestvedt J S, Friedlingstein P, Rogelj J, Grubb M J,
Matthews H D, Skeie R B, Forster P M, Frame D J and Allen M
R 2017 Emission budgets and pathways consistent with
limiting warming to 1.5 ◦CNat. Geosci. 10 741–7
Minasny B et al 2017 Soil carbon 4 per mille Geoderma 292 59–86
van Minnen J G, Strengers B J, Eickhout B, Swart R J and Leemans
R 2008 Quantifying the effectiveness of climate change
mitigation through forest plantations and carbon
sequestration with an integrated land-use model Carbon
Balance Manage. 33
Minx J C et al 2018 Negative emissions: Part 1—research
landscape, ethics and synthesis Environ. Res. Lett. 13 063001
Minx J, Lamb W F, Callaghan M W, Bornmann L and Fuss S 2017
Fast growing research on negative emissions Environ. Res.
Lett. 12
M¨
ollersten K, Gao L and Yan J 2006 CO2capture in pulp and paper
mills: CO2balances and preliminary cost assessment Mitig.
Adapt. Strateg. Glob. Change 11 1129–50
M¨
ollersten K, Gao L, Yan J and Obersteiner M 2004 Efficient
energy systems with CO2capture and storage from renewable
biomass in pulp and paper mills Renew. Energy 29 1583–98
Montserrat F, et al 2017 Olivine dissolution in seawater:
implications for CO2sequestration through enhanced
weathering in coastal environments Env. Sci. Technol. 51
3960–72
Montzka S A, Dlugokencky E J and Butler J H 2011 Non-CO2
greenhouse gases and climate change Nature 476 43–50
Moore J C, Jevrejeva S and Grinsted A 2010 Efficacy of
geoengineering to limit 21st century sea-level rise Proc. Natl
Acad. Sci. 107 15699–703
Morales-Florez V, Santos A, Lemus A and Esquivias L 2011
Artificial weathering pools of calcium-rich industrial waste for
CO2sequestration Chem. Eng. J. 166 132–7
Moreira D and Pires J C M 2016 Atmospheric CO2capture by
algae: negative carbon dioxide emission path Bioresour.
Technol. 215 371–9
Mortensen G M, Bergmo P E S and Emmel B U 2016
Characterization and estimation of CO2storage capacity for
the most prospective aquifers in Sweden Energy Procedia 86
352–60
Moss R H and et al 2010 The next generation of scenarios for
climate change research and assessment Nature 463 747–56
(https://www.ncbi.nlm.nih.gov/pubmed/20148028)
Movagharnejad K, Emamgholivand A, Mousavi H and Kordkheili
M S 2012 Preliminary country-scale assessment of carbon
dioxide storage potential in Iran Greenh. Gases Sci. Technol. 2
151–61
M¨
uller A, Schmidhuber J, Hoogeveen J and Steduto P 2008 Some
insights in the effect of growing bio-energy demand on global
food security and natural resources Water Policy 10 83
Mutopo P, Haaland H, Boamah F and Wideng ˚
ard M 2011 Biofuels,
land grabbing and food security in Africa (Uppsala: Zed
Books)
Napier-Munn T 2015 Is progress in energy-efficient comminution
doomed? Miner. Eng. 73 1–6
National Academy of Sciences 2015 Climate Intervention
(Washington, DC: National Academies Press)
43

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Nayak D et al 2015 Management opportunities to mitigate
greenhouse gas emissions from Chinese agriculture Agric.
Ecosyst. Environ. 209 108–24
Nemet G F and Brandt A R 2012 Willingness to pay for a climate
backstop: liquid fuel producers and direct CO2air capture
Energy J. 33 53–81
Nemet G F et al 2018 Negative emissions - Part 3: Innovation and
upscaling Environ. Res. Lett. 13 063003
Newbold T et al 2015 Global effects of land use on local terrestrial
biodiversity Nature 520 45–50
Nijsen M, Smeets E, Stehfest E and van Vuuren D P 2012 An
evaluation of the global potential of bioenergy production on
degraded lands GCB Bioenergy 4130–47
Nilsson S and Schopfhauser W 1995 The carbon-sequestration
potential of a global afforestation program Clim. Change 30
267–93
Nkouathio D G, Wandji P, Bardintzeff J M, Tematio P, Dongmo A
K and Tchoua F 2008 Utilisation des roches volcaniques pour
la remineralisation des sols ferrallitiques des regions tropicales.
Cas des pyroclastites basaltiques du graben de Tombel (Ligne
volcanique du Cameroun) Bull. Soc. Vaudoise Sci. Nat. 91
1–14
Nordhaus W D 1991 The cost of slowing climate change: a survey
Energy J. 12 37–65
O’Neill B C et al 2017 The roads ahead: Narratives for shared
socioeconomic pathways describing world futures in the 21st
century Glob. Environ. Change 42 169–80
Obersteiner M, Alexandrov G, Ben´
ıtezPC,McCallumI,KraxnerF,
Riahi K, Rokityanskiy D and Yamagata Y 2006 Global supply
of biomass for energy and carbon sequestration from
afforestation/reforestation activities Mitig. Adapt. Strateg.
Glob. Change 11 1003–21
OECD/IEA 2015 updated to: IEA (2017) Market Report Series Coal
2017 (Paris: OECD/IEA)
Onarheim K, Mathisen A and Arasto A 2015 Barriers and
opportunities for application of CCS in Nordic industry—A
sectorial approach Int. J. Greenh. Gas Control 36 93–105
Ormerod W G, Webster I C, Audus H and Riemer P W F 1993 An
overview of large scale CO2disposal options Energy Convers.
Manage. 34 833–40
Oschlies A 2009 Impact of atmospheric and terrestrial CO2
feedbacks on fertilization-induced marine carbon uptake
Biogeosciences 61603–13
Oschlies A, Pahlow M, Yool A and Matear R J 2010 Climate
engineering by artificial ocean upwelling: channelling the
sorcerer’s apprentice Geophys. Res. Lett. 37
Pan G, Smith P and Pan W 2009 The role of soil organic matter in
maintaining the productivity and yield stability of cereals in
China Agric. Ecosyst. Environ. 129 344–8
Parson E A 2017 Opinion: climate policymakers and assessments
must get serious about climate engineering Proc. Natl Acad.
Sci. 114 9227–30
Paustian K, Lehmann J, Ogle S, Reay D, Robertson G P and Smith P
2016 Climate-smart soils Nature 532 49–57
Peters G P 2016 The ‘best available science’to inform 1.5 ◦Cpolicy
choices Nat. Clim. Change 6646
Peters G P and Geden O 2017 Catalysing a political shift from low
to negative carbon Nat. Clim. Change 7619
Plevin R J, Delucchi M A and Creutzig F 2014 Using attributional
life cycle assessment to estimate climate-change mitigation
benefits misleads policy makers J. Ind. Ecol. 18
Plevin R J, Jones A D, Torn M S, Gibbs H K and Gibbs H K 2010
Greenhouse gas emissions from biofuels’indirect land use
change are uncertain but may be much greater than previously
estimated Environ. Sci. Technol. 44 8015–21
Popp A et al 2017 Land-use futures in the shared socio-economic
pathways Glob. Environ. Change 42 331–45
Popp A, Dietrich J P, Lotze-Campen H, Klein D, Bauer N, Krause
M, Beringer T, Gerten D and Edenhofer O 2011 The
economic potential of bioenergy for climate change mitigation
with special attention given to implications for the land system
Environ. Res. Lett. 634017
Popp A et al 2014 Land-use transition for bioenergy and climate
stabilization: model comparison of drivers, impacts and
interactions with other land use based mitigation options
Clim. Change 123 495–509
Powell T W R and Lenton T M 2012 Future carbon dioxide
removal via biomass energy constrained by agricultural
efficiency and dietary trends Energy Environ. Sci. 5
8116
Power I M, Harrison A L, Dipple G M, Wilson S A, Kelemen P B,
Hitch M and Southam G 2013 Carbon mineralization: from
natural analogues to engineered systems Rev. Mineral.
Geochem. 77 305–60
Powlson D S, Stirling C M, Jat M L, Gerard B G, Palm C A, Sanchez
P A and Cassman K G 2014 Limited potential of no-till
agriculture for climate change mitigation Nat. Clim. Change 4
678
Pratt K and Moran D 2010 Evaluating the cost-effectiveness of
global biochar mitigation potential Biomass Bioenergy 34
1149–58
Pritchard C, Yang A, Holmes P and Wilkinson M 2015
Thermodynamics, economics and systems thinking: what role
for air capture of CO2?Process Saf. Environ. Prot. 94 188–95
Qin Y, Sheng X, Liu S, Ren G, Wang X and Wang F 2015 Recent
advances in carbon dioxide based copolymers J. CO2Util. 11
3–9
Ranjan M and Herzog H J 2011 Feasibility of air capture Energy
Procedia 42869–76
Rao S and Riahi K 2006 The role of non-CO3greenhouse gases in
climate change mitigation: long-term scenarios for the 21st
century Energy J. 27 177–200
Raven J A and Falkowski P G 1999 Oceanic sinks for atmospheric
CO2Plant Cell Environ. 22 741–55
Ravi S, Sharratt B S, Li J, Olshevski S, Meng Z and Zhang J 2016
Particulate matter emissions from biochar-amended soils as a
potential tradeoff to the negative emission potential Sci. Rep. 6
35984
Reilly J, Melillo J, Cai Y, Kicklighter D, Gurgel A, Paltsev S, Cronin
T, Sokolov A and Schlosser A 2012 Using land to mitigate
climate change : hitting the target, recognizing the trade-offs
Environ. Sci. Technol. 46 5672–9
Rembauville M, Salter I, Dehairs F, Miquel J-C and Blain S 2018
Annual particulate matter and diatom export in a high
nutrient, low chlorophyll area of the Southern Ocean Polar
Bio. 41 25–40
Renforth P 2012 The potential of enhanced weathering in the UK
Int. J. Greenh. Gas Control 10 229–43
Renforth P and Henderson G 2017 Assessing ocean alkalinity for
carbon sequestration Rev. Geophys. 55 636–74
Renforth P, Washbourne C L, Taylder J and Manning D A 2011
Silicate production and availability for mineral carbonation
Environ. Sci. Technol. 45 2035–41
Renner M, Sweeney S and Kubit J 2008 Green Jobs: Working for
People and the Environment (WORLDWATCH REPORT)
Rhodes J S and Keith D W 2005 Engineering economic analysis of
biomass IGCC with carbon capture and storage Biomass
Bioenergy 29 440–50
Riahi K et al 2015 Locked into Copenhagen pledges—implications
of short-term emission targets for the cost and feasibility of
long-term climate goals Technol. Forecast. Soc. Change 90
8–23
Riahi K et al 2017 The shared socioeconomic pathways and their
energy, land use, and greenhouse gas emissions implications:
an overview Glob. Environ. Change 42 153–68
Richards K R and Stokes C 2004 A review of forest carbon
sequestration cost studies: a dozen years of research Clim.
Change 63 1–48
de Richter R, Ming T, Davies P, Liu W and Caillol S 2017 Removal
of non-CO2greenhouse gases by large-scale atmospheric solar
photocatalysis Prog. Energy Combust. Sci. 60 68–96
Rist L, Feintrenie L and Levang P 2010 The livelihood impacts of oil
palm: smallholders in Indonesia Biodivers. Conserv. 19
1009–24
44

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Roberts K G, Gloy B A, Joseph S, Scott N R and Lehmann J 2010
Life cycle assessment of biochar systems: estimating the
energetic, economic, and climate change potential Environ.
Sci. Technol. 44 827–33
Roberts M J and Schlenker W 2010 The US biofuel mandate and
world food prices: an econometric analysis of the demand and
supply of calories NBER Working Paper, 15921
Robledo-Abad C 2017 Bioenergy production and sustainable
development: science base for policymaking remains limited
GCB Bioenergy 9541–56
Rochedo P R R, Costa I V L, Imp´
erio M, Hoffmann B S,
MerschmannPRDC,OliveiraCCN,SzkloAandSchaeffer
R 2016 Carbon capture potential and costs in Brazil J. Clean.
Prod. 131 280–95
Rogelj J, Hare W, Lowe J, van Vuuren D P, Riahi K, Matthews B,
Hanaoka T, Jiang K and Meinshausen M 2011 Emission
pathways consistent with a 2 ◦C global temperature limit Nat.
Clim. Change 1413–8
Rogelj J and Knutti R 2016 Geosciences after Paris Nat. Geosci 9
187–9
Rogelj J, Luderer G, Pietzcker R C, Kriegler E, Schaeffer M, Krey V
and Riahi K 2015a Energy system transformations for limiting
end-of-century warmin g to below 1.5 ◦CNat. Clim. Change 5
519–27
Rogelj J, Schaeffer M, Meinshausen M, Knutti R, Alcamo J, Riahi K
and Hare W 2015b Zero emission targets as long-term global
goals for climate protection Environ. Res. Lett. 10 105007
Rogelj J, McCollum D L, O’Neill B C and Riahi K 2013a 2020
emissions levels required to limit warming to below 2 ◦CNat.
Clim. Change 3405–12
Rogelj J, McCollum D L, Reisinger A, Meinshausen M and Riahi K
2013b Probabilistic cost estimates for climate change
mitigation Nature 493 79–83
Rogelj J, Meinshausen M and Knutti R 2012 Global warming under
old and new scenarios using IPCC climate sensitivity range
estimates Nat. Clim. Change 2248–53
Rogelj J et al 2018 Scenarios towards limiting climate change below
1.5 ◦CNat. Clim. Change 8in press
Rogelj J, Schaeffer M, Friedlingstein P, Gillett N P, van Vuuren D P,
Riahi K, Allen M and Knutti R 2016 Differences between
carbon budget estimates unravelled Nat. Clim. Change 6
245–52
Rogner H-H et al 2012 Energy resources and potentials Global
Energy Assessment—Toward a Sustainable Future
(Cambridge: Cambridge University Press) pp 423–512
R¨
o¨
os E, Bajˇ
zelj B, Smith P, Patel M, Little D and Garnett T 2017
Protein futures for Western Europe: potential land use and
climate impacts in 2050 Reg. Environ. Change 17 367–77
RoseSK,AhammadH,EickhoutB,FisherB,KurosawaA,RaoS,
Riahi K and van Vuuren D P 2012 Land-based mitigation in
climate stabilization Energy Econ. 34 365–80
Royal Society 2009 Geoengineering the Climate: Science,
Governance and Uncertainty (London: The Royal Society)
Rubin E S, Davison J E and Herzog H J 2015 The cost of CO2
capture and storage Int. J. Greenh. Gas Control 40 378–400
Russell L M et al 2012 Ecosystem impacts of geoengineering: a
review for developing a science plan Ambio 41 350–69
Sakwa-Novak M A, Yoo C-J, Tan S, Rashidi F and Jones C W 2016
Poly (ethylenimine)-functionalized monolithic alumina
honeycomb adsorbents for CO2capture from air
ChemSusChem 91859–68
Salati S, Spagnol M and Adani F 2010 The impact of crop plant
residues on carbon sequestration in soil: a useful strategy to
balance the atmospheric CO2
Samir K and Lutz W 2017 The human core of the shared
socioeconomic pathways: population scenarios by age, sex and
level of education for all countries to 2100 Glob. Environ.
Change 42 181–92
Sanchez D L and Callaway D S 2016 Optimal scale of
carbon-negative energy facilities Appl. Energy 170 437–44
Sanz-P´
erez E S, Murdock C R, Didas S A and Jones C W 2016
Direct capture of CO2from ambient air Chem. Rev. 116
11840–76
Sarmiento J L and Orr J C 1991 Three-dimensional simulations of
the impact of Southern Ocean nutrient depletion on
atmospheric CO2and ocean chemistry Limnol. Oceanogr. 36
1928–50
Sato T, Qadir M, Yamamoto S, Endo T and Zahoor A 2013 Global,
regional, and country level need for data on wastewater
generation, treatment, and use Agric. Water Manage. 130
1–13
Schaeffer M, Gohar L, Kriegler E, Lowe J, Riahi K and van Vuuren
D 2015 Mid- and long-term climate projections for
fragmented and delayed-action scenarios Technol. Forecast.
Soc. Change 90 257–68
Schoneveld G, German L, Andrade R, Chin M, Caroko W and
Romero-Hern´
andez O 2010 The role of national governance
systems in biofuel development: a comparative analysis of
lessons learned CIFOR
Schuiling R D and Krijgsman P 2006 Enhanced weathering: an
effective and cheap tool to sequester CO2Clim. Change 74
349–54
Searle S and Malins C 2015 A reassessment of global bioenergy
potential in 2050 GCB Bioenergy 7328–36
Sedjo R A and Solomon A M 1991 Climate and forests Greenhouse
Warming: Abatement and Adaptation (Resources for the
Future)
Selosse S and Ricci O 2017 Carbon capture and storage: lessons
from a storage potential and localization analysis Appl. Energy
188 32–44
Shackley S, Hammond J, Gaunt J and Ibarrola R 2011 The
feasibility and costs of biochar deployment in the UK Carbon
Manage. 2335–56
Sheng Y, Zhan Y and Zhu L 2016 Reduced carbon sequestration
potential of biochar in acidic soil Sci. Total Environ. 572
129–37
Simon A J, Kaahaaina N B, Friedmann S J and Aines R D 2011
Systems analysis and cost estimates for large scale capture of
carbon dioxide from air Ene. Procedia 42893–2900
Sinha A, Darunte L A, Jones C W, Realff M J and Kawajiri Y 2017
Systems design and economic analysis of direct air capture of
CO2through temperature vacuum swing adsorption using
MIL-101(Cr)-PEI-800 and mmen-Mg2(dobpdc) MOF
adsorbents Ind. Eng. Chem. Res. 56 750–764
Smeets E M W and Faaij A P C 2007 Bioenergy potentials from
forestry in 2050: an assessment of the drivers that determine
the potentials Clim. Change 81 353–90
Smeets E M W, Faaij A P C, Lewandowski I M and Turkenburg W C
2007 A bottom-up assessment and review of global bio-energy
potentials to 2050 Prog. Energy Combust. Sci. 33 56–106
Smith L and Torn M S 2013 Ecological limits to terrestrial
biological carbon dioxide removal Clim. Change 118
89–103
Smith P 2012 Agricultural greenhouse gas mitigation potential
globally, in Europe and in the UK: what have we learnt in the
last 20 years? Glob. Change Biol. 18 35–43
Smith P 2016 Soil carbon sequestration and biochar as negative
emission technologies Glob. Change Biol. 22 1315–24
Smith P et al 2014 Agriculture, forestry and other land use
(AFOLU) Climate Change 2014: Mitigation of Climate
Change. Contribution of Working Group III to the Fifth
Assessment Report of the Intergovernmental Panel on Climate
Change ed O Edenhofer et al (Cambridge: Cambridge
University Press)
Smith P et al 2016a Biophysical and economic limits to negative
CO2emissions Nat. Clim. Change 642–50
Smith P, Haszeldine R S and Smith S M 2016b Preliminary
assessment of the potential for, and limitations to, terrestrial
negative emission technologies in the UK Environ. Sci. Process.
Impacts 18 1400–5
Smith P et al 2013 How much land-based greenhouse gas
mitigation can be achieved without compromising food
security and environmental goals? Glob. Change Biol. 19
2285–302
Smith P et al 2008 Greenhouse gas mitigation in agriculture Phil.
Trans. R. Soc. London B Biol. Sci. 363 789–813
45

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Smith W K, Zhao M and Running S W 2012 Global bioenergy
capacity as constrained by observed biospheric productivity
rates Bioscience 62 911–22
Socolow R, Desmond M, Aines R, Blackstock J, Bolland O,
Kaarsberg T, Lewis N, Mazzotti M, Pfeffer A and Sawyer K
2011 Direct air capture of CO2with chemicals ATechnol.
Assess. APS Panel Public Aff. (www.aps.org/policy/reports/
assessments/upload/dac2011.pdf)
Sohngen B and Mendelsohn R 2003 An optimal control model of
forest carbon sequestration Am.J.Agric.Econ.85 448–57
Solomon S, Plattner G-K, Knutti R and Friedlingstein P 2009
Irreversible climate change due to carbon dioxide emissions
Proc. Natl Acad. Sci. 106 1704–9
Sommer R and Bossio D 2014 Dynamics and climate change
mitigation potential of soil organic carbon sequestration J.
Environ. Manage. 144 83–7
Sonneveld B G J S and Dent D L 2009 How good is GLASOD? J.
Environ. Manage. 90 274–83
Sonntag S, Pongratz J, Reick C H and Schmidt H 2016
Reforestation in a high-CO2world—Higher mitigation
potential than expected, lower adaptation potential than
hoped for Geophys. Res. Lett. 43 6546–53
Stolaroff J K, Bhattacharyya S, Smith C A, Bourcier W L,
Cameron-smith P J and Aines R D 2012 Review of methane
mitigation technologies with application to rapid release of
methane from the Arctic Environ. Sci. Technol. 46 6455–69
van Straaten P 2006 Farming with rocks and minerals: challenges
and opportunities An. Acad. Bras. Cienc. 78 731–47
Strefler J, Amann T, Bauer N, Kriegler E and Hartmann J 2018a
Potential and costs of carbon dioxide removal by Enhanced
weathering of rocks Environ. Res. Lett. 13
Strefler J, Bauer N, Kriegler E, Popp A, Giannousakis A and
Edenhofer O 2018b Between scylla and charybdis: delayed
mitigation narrows the passage between large-scale CDR and
high costs Environ. Res. Lett. 13 44015
Strengers B J, van Minnen J G and Eickhout B 2008 The role of
carbon plantations in mitigating climate change: potentials
and costs Clim. Change 88 343–66
Strong A, Chisholm S, Miller C and Cullen J 2009a Ocean
fertilization: time to move on Nature 461 347–8
Strong A L, Cullen J J and Chisholm S W 2009b Ocean fertilization:
science, policy, and commerce Oceanography 22 236–61
Tandon P and Jin Q 2017 Microalgae culture enhancement
through key microbial approaches Renew. Sustain. Energy
Rev. 80 1089–99
Tavoni M and Socolow R 2013 Modeling meets science and
technology:anintroductiontoaspecialissueonnegative
emissions Clim. Change 118 1–14
Tavoni M, Sohngen B and Bosetti V 2007 Forestry and the carbon
market response to stabilize climate Energy Policy 35 5346–53
Taylor L L, Quirk J, Thorley R M S, Kharecha P A, Hansen J,
Ridgwell A, Lomas M R, Banwart S A and Beerling D J 2016
Enhanced weathering strategies for stabilizing climate and
averting ocean acidification Nat. Clim. Change 6402–6
Thomson A M, C ´
esar Izaurralde R, Smith S J and Clarke L 2008
Integrated estimates of global terrestrial carbon sequestration
Glob. Environ. Change 18 192–203
Thr¨
an D, Seidenberger T, Zeddies J and Offermann R 2010 Global
biomass potentials—resources, drivers and scenario results
Energy Sustain. Dev. 14 200–5
Tilman D, Hill J and Lehman C 2006 Carbon-negative biofuels
from low-input high-diversity grassland biomass Science 314
1598–600
Timilsina G R, Beghin J C, van der Mensbrugghe D and Mevel S
2012 The impacts of biofuels targets on land-use change and
food supply: a global CGE assessment Agric. Econ. 43 315–32
Tokarska K B and Zickfeld K 2015 The effectiveness of net negative
carbon dioxide emissions in reversing anthropogenic climate
change Environ. Res. Lett. 10 94013
Tokimatsu K, Yasuoka R and Nishio M 2017 Global zero emissions
scenarios: The role of biomass energy with carbon capture and
storage by forested land use Appl. Energy 185 1899–906
Torres A B, Marchant R, Lovett J C, Smart J C R and Tipper R 2010
Analysis of the carbon sequestration costs of afforestation and
reforestation agroforestry practices and the use of cost curves
to evaluate their potential for implementation of climate
change mitigation Ecol. Econ. 69 469–77
Trick C G, Bill B D, Cochlan W P, Wells M L, Trainer V L and
Pickell L D 2010 Iron enrichment stimulates toxic diatom
production in high-nitrate, low-chlorophyll areas Proc. Natl
Acad.Sci.USA107 5887–92
UNEP 2017 The Emissions Gap Report 2017 (Nairobi: United
Nations Environment Program)
UNFCCC 2015 Adoption of the Paris Agreement (Geneva: United
Nations Framework Convention on Climate Change) (http://
unfccc.int/resource/docs/2015/cop21/eng/l09r01.pdf)
USDOE 2014 FE/NETL CO2saline storage cost model: model
description and baseline results Report No
DOE/NETL-2014-1659 (Pittsburgh, PA: US Dept of Energy,
National Energy Technology Laboratory)
Vangkilde-Pedersen T et al 2009 Assessing European capacity for
geological storage of carbon dioxide-the EU GeoCapacity
project Energy Procedia 12663–70
Vangkilde-Pedersen T, Kirk K, Vincke O and Neele F 2008 Le
Nindre Y-M Project no. SES6–518318 EU GeoCapacity
Assessing European Capacity for Geological Storage of Carbon
Dioxide 63
Vaughan N E and Lenton T M 2011 A review of climate
geoengineering proposals Clim. Change 109 745–90
Venter M, Venter O, Laurance S and Bird M 2012 Recarbonization
of the Humid Tropics BT—Recarbonization of the Biosphere:
Ecosystems and the Global Carbon Cycle edRLal,KLorenz,R
FH
¨
uttl, B U Schneider and J von Braun (Dordrecht:
Springer) pp 229–52
Viger M, Hancock R D, Miglietta F and Taylor G 2015 More plant
growth but less plant defence? First global gene expression
data for plants grown in soil amended with biochar GCB
Bioenergy 7658–72
de Visser E, Hendriks C, Hamelinck C, van de Brug E, Jung M,
Meyer S, Harmelink M, Knopf S and Gerling P 2011
PlantaCap: A ligno-cellulose bio-ethanol plant with CCS
Energy Procedia 42941–9
Vochozka M, Marou˘
skov´
aA,V
´
achal J and Strakov´
a J 2016 Biochar
pricing hampers biochar farming Clean Technol. Environ.
Policy 18 1225–31
van Vuuren D P, Deetman S, van Vliet J, van den Berg M, van
Ruijven B J and Koelbl B 2013 The role of negative CO2
emissions for reaching 2 ◦C—insights from integrated
assessment modelling Clim. Change 118 15–27
vanVuuren DP,denElzenMGJ,LucasPL,EickhoutB,Strengers
B J, van Ruijven B, Wonink S and van Houdt R 2007
Stabilizing greenhouse gas concentrations at low levels: an
assessment of reduction strategies and costs Clim. Change 81
119–59
van Vuuren D P and Riahi K 2011 The relationship between
short-term emissions and long-term concentration targets
Clim. Change 104 793–801
van Vuuren D P, Riahi K, Calvin K, Dellink R, Emmerling J,
Fujimori S, Kc S, Kriegler E and O’Neill B 2017a The Shared
Socio-economic Pathways: trajectories for human
development and global environmental change Glob. Environ.
Change 42 148–52
van Vuuren D P et al 2017b Energy, land-use and greenhouse gas
emissions trajectories under a green growth paradigm Glob.
Environ. Change 42 237–50
Wang J, Xiong Z and Kuzyakov Y 2016 Biochar stability in soil:
meta-analysis of decomposition and priming effects GCB
Bioenergy 8512–23
46

Environ. Res. Lett. 13 (2018) 063002 Sabine Fuss et al
Wang Y, Yan X and Wang Z 2014 The biogeophysical effects of
extreme afforestation in modeling future climate Theor. Appl.
Climatol. 118 511–21
Washbourne C-L, Renforth P and Manning D A C 2012
Investigating carbonate formation in urban soils as a method
for capture and storage of atmospheric carbon Sci. Total
Environ. 431 166–75
WickeB,SmeetsE,DornburgV,VashevB,GaiserT,Turkenburg
W and Faaij A 2011 The global technical and economic
potential of bioenergy from salt-affected soils Energy Environ.
Sci. 42669
Wiens J, Fargione J and Hill J 2011 Biofuels Biodiversity Ecol. Appl.
21 1085–95
Williams P M and Druffel E R M 1987 Radiocarbon in dissolved
organic matter in the central North Pacific Ocean Nature 330
246–8
Williamson P 2016 Emissions reduction: scrutinize CO2removal
methods Nature 530 5–7
Wilson S A, Dipple G M, Power I M, Thom J M, Anderson R G,
Raudsepp M, Gabites J E and Southam G 2009 Carbon
dioxide fixation within mine wastes of ultramafic-hosted ore
deposits: examples from the clinton creek and cassiar
chrysotile deposits, Canada Econ. Geol. 104 95–112
Woolf D, Amonette J E, Street-Perrott F A, Lehmann J and Joseph S
2010 Sustainable biochar to mitigate global climate change
Nat. Commun. 156
Wright R, Mourits F, Rodr´
ıguez L B and Serrano M D 2013 The
first North American carbon storage atlas Energy Procedia 37
5280–9
Yamamoto H, Yamaji K and Fujino J 2000 Scenario analysis of
bioenergy resources and CO2emissions with a global land use
and energy model Appl. Energy 66 325–37
Yamasaki A, Fujii M, Kakizawa M and Yanagisawa Y 2002
Reduction process of CO2emissions by treating with waste
concrete via an artificial weathering process Environmental
Challenges and Greenhouse Gas Control for Fossil Fuel
Utilization in the 21st Century ed M M Maroto-Valer, C Song
and Y Soong (Boston, MA: Springer) pp 189–201
Zahariev K, Christian J R and Denman K L 2008 Preindustrial,
historical, and fertilization simulations using a global ocean
carbon model with new parameterizations of iron limitation,
calcification, and N2 fixation Prog. Oceanogr. 77 56–82
Zeebe R E 2005 Feasibility of ocean fertilization and its impact on
future atmospheric CO2levels Geophys. Res. Lett. 32
Zeman F 2014 Reducing the cost of Ca-based direct air capture of
CO2Environ. Sci. Technol. 48 11730–5
Zickfeld K, MacDougall A H and Matthews H D 2016 On the
proportionality between global temperature change and
cumulative CO2emissions during periods of net negative CO2
emissions Environ. Res. Lett. 11 55006
Zilberman D, Hochman G, Rajagopal D, Sexton S and Timilsina G
2013 The impact of biofuels on commodity food prices:
assessment of findings Am.J.Agric.Econ.95 275–81
Zimmermann M, Bird M I, Wurster C, Saiz G, Goodrick I, Barta J,
Capek P, Santruckova H and Smernik R 2012 Rapid
degradation of pyrogenic carbon Glob. Change Biol. 18
3306–16
Zomer R J, Bossio D A, Sommer R and Verchot L V 2017 Global
sequestration potential of increased organic carbon in
cropland soils Sci. Rep. 71–8
Zomer R J, Trabucco A, Bossio D A and Verchot L V 2008 Climate
change mitigation: a spatial analysis of global land suitability
for clean development mechanism afforestation and
reforestation Agric. Ecosyst. Environ. 126 67–80
47