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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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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).
2
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 – impac