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opinion & comment
Investing in negative emissions
Guy Lomax, Timothy M. Lenton, Adepeju Adeosun and Mark Workman
Methods of removing CO2 from the atmosphere add vital flexibility to eorts to tackle climate change.
They must be brought into mainstream climate policy as soon as possible to open up the landscape for
innovation and development, and to discover which approaches work at scale.
To achieve the widely held policy target
of limiting average temperature change
to 2°C, integrated assessment models
(IAM) increasingly depend on massive-scale
‘negative emissions’ through biomass energy
with carbon capture and storage (BECCS),
deployed in the second half of this century1–6.
Yet this key technology is technically
immature today, and it is far from clear
whether such large-scale deployment several
decades in the future would be either feasible
or desirable1. Hence a recent Commentary by
Fuss etal. has branded BECCS a potentially
dangerous distraction1. But before anyone
dismisses what doesn’t yet exist, we argue
that the best way to determine “how safe
it is to bet on negative emissions in the
second half of this century”1 is to instigate
a policy framework for greenhouse-gas
removal (GGR) and invest in research and
development innovation now.
Two dimensions of flexibility
Scalable GGR approaches bring unique
exibility to the mitigation toolbox. BECCS,
along with other methods for removing
greenhouse gases from the atmosphere,
oer two key dimensions of exibility; they
decouple abatement opportunities from
emissions sources in both space and time7–9.
Decoupling in space allows GGR to
indirectly mitigate emissions from areas of
the energy system that are most dicult or
expensive to decarbonize. Such ‘project-
level’ negative emissions can in principle
bring many benets as a complement to
conventional mitigation eorts, depending
on the direction and ecacy of climate policy
and GGR deployment. Possibilities include
(i) buying time for the development of clean
technologies, the replacement of locked-in
sources, and changes in societal attitudes;
(ii) reducing the total costs of meeting climate
targets by displacing the most challenging
and expensive emissions sources; (iii) making
more aggressive emissions cuts feasible by
simply adding new mitigation options; or
(iv) allowing continuing use of fossil fuels in
certain key sectors such as aviation7,9.
Decoupling in time raises the idea that
GGR theoretically could be deployed at
massive scale to generate global ‘net-negative’
emissions later this century, allowing us to
recover from emitting too much earlier this
century and overshooting CO2 concentration
targets1,4,9. e negative emissions capacity
outlined in the IPCC’s Fih Assessment
Report 3 implies BECCS input of up to
10gigatonnes of CO2 abatement per year
with global net negative emissions from
around 2070.
It is this second dimension of
exibility —decoupling in time — that
Fuss etal. rightly caution against as “betting”
on negative emissions1. ey argue that our
ability to reach or even assess the feasibility of
a late-century, massive-scale BECCS scenario
is severely constrained by (at least) four
main groups of uncertainties surrounding:
(i) access to sucient biomass supply and
storage space for captured CO2; (ii) the
uncertain response of the global carbon cycle;
(iii) relative costs and viability of untested
technology; and (iv) social and political
factors. Similar uncertainties face other GGR
technologies. For example, estimates of the
cost of direct air capture range from below
US$100 to more than US$1,000 per tonne
of CO2 abated10, and approaches based on
interactions with natural systems such as
soils and ocean alkalinity raise concerns over
potential environmental impacts that are not
yet fully understood11–13.
Responses to uncertainty
In the face of such uncertainties it can seem
premature to commit to long-term policy
support. A natural, scientic response is
to call for a substantial interdisciplinary
research agenda to explore and try to
constrain the uncertainties1, so that we
can best assess the future potential of GGR
and guide policy through the remaining
uncertainties. But at such vast scales of global
deployment, and over such long timescales
of technological, political and societal
development, many of the uncertainties
are inherent, and can only ever be loosely
constrained by modelling and research9. is
is well illustrated by, for example, existing
estimates of global sustainable biomass
resource in 2050 to 2100, which range
from around 30×1018J (30EJ) per year
to over 600EJyr-1 depending on assumed
trends in diet, crop yields, land use and
population14. e call to try to constrain the
unconstrainable instead may lead to ‘analysis
paralysis’, losing valuable time and helping to
self-full the prophecy that GGR cannot be
realized at scale.
A central problem is the framing of GGR
as a large-scale, late-century approach that
would inevitably entail major environmental
and social consequences7. is presents
multiple issues for policy, and immediately
polarizes rather than nuances the debate.
It risks both over-emphasizing the need
10. Hegglin, M.I. etal. Nature Geosci. 7, 768–778 (2014).
11. Hansen, J. et al. Rev. Geophys. 48, RG4004 (2010).
12. Morgenstern, O. etal. J.Geophys. Res. 115, D00M02 (2010).
13. Eyring, V. etal. Atmos. Chem. Phys. 10, 9451–9472 (2010).
is Correspondence was inspired by discussions with
third-year Mathematics undergraduates at the University
of Exeter: C. Serjeant, R. Illingworth, G. Beresford,
D. Mehta, M. Stanton and A. Clements. We acknowledge
the modelling groups for making their simulations
available for this analysis, the Chemistry-Climate Model
Validation (CCMVal) Activity for WCRP’s (World
Climate Research Programme) SPARC (Stratospheric
Processes and their Role in Climate) project for
organizing and coordinating the model data analysis
activity, and the British Atmospheric Data Centre
(BADC) for collecting and archiving the CCMVal model
output. We thank T. Shepherd for helpful comments on
the manuscript. is research was supported by the NERC
PROBEC project NE/K016016/1.
A. J. Ferraro*, M. Collins and F. H. Lambert
College of Engineering, Mathematics and
Physical Sciences, University of Exeter, Exeter,
EX4 4QF, UK. *e-mail:
© 2015 Macmillan Publishers Limited. All rights reserved
opinion & comment
for precaution and regulation over the
possible advantages, and portraying
reliance on these highly uncertain scenarios
as an economically optimal policy. By
concentrating on events more than 50years
in the future, it takes the debate away
from current scientic knowledge, global
experience and policy planning horizons,
giving the false impression that eective
policy engagement with GGR in the near
term is of little value or urgency. is
distracts attention from the nearer-term
value that BECCS and other GGR could oer
in supplementing ongoing mitigation eorts
at more modest scales, and the urgency of
the early technical and policy groundwork
necessary to enable future scale-up7.
us, although research to try to constrain
long-term uncertainties is undoubtedly
important, these uncertainties should not
be used to justify inaction on more pressing
near-term technology development and
policy support needs. Indeed, an alternative
response to such uncertainty is to start
learning by doing. BECCS and its component
technologies are at a relatively early stage of
technical development, as are many other
GGR options (Fig.1). Individually, bioenergy
and CCS technology and industries
are themselves at an early stage, and
integrating them poses further challenges
to technical viability and achieving
attractive economics7,15,16.
Advancing from the current state
of technical readiness to maturity and
widespread deployment is a process that
takes many decades. For example, one o-
cited example of successful scale-up of a new
energy technology is the United Kingdoms
‘Dash for Gas, the development and
nationwide roll-out of combined-cycle gas
turbine power plants in the 1990s. Even with
heavy, sustained R&D programmes by both
industry and government, it took 30years to
move from the rst plants to a competitive
energy technology17. Given the widespread
remaining research and development
challenges, and the large-scale need for
GGR anticipated several decades from now,
timely research and demonstration of the
technologies are themselves priorities.
Roadblocks to policy engagement
To support this learning-by-doing approach,
early policy engagement is vital, but it is also
confronted by several potential roadblocks.
e task of accounting for the removed
greenhouse gases poses a considerable
challenge to practical policy integration.
Unlike emissions from fossil fuel combustion,
the ows of greenhouse gases involved with
GGR approaches are much more diverse
and less well understood. Especially with
approaches based on ecosystems, soils and
biomass, the greenhouse-gas storage varies
with time and external factors18, making it
dicult to accurately measure the amount
of carbon stored. Risks of emissions through
direct and indirect land-use change also
threaten the eectiveness of biomass-based
GGR, requiring eective ways of quantifying
or minimizing such eects through policy19.
Biomass combustion
Basic research
Aquatic crops (micro-/macro-algae)
Carbon monitoring
Land-based energy crops
Aorestation and
Lignocellulosic ethanol
Biochar soil
IGCC pre-combustion
Geological sequestration
and monitoring
Direct air capture:
supported amines
Direct air capture:
hydroxide solutions
CO2 utilization
Biomass production
and conversion
GGR technology components
Technology Readiness Level (TRL)
Applied research
with support
CO2 capture
CO2 sequestration
and utilization
CO2 transport
Figure 1 | Schematic diagram showing the Technology Readiness Levels (TRLs) of key science and technology components relevant to leading GGR approaches
of aorestation, BECCS, biochar (from biomass pyrolysis) and direct air capture, according to the authors’ assessment. IGCC, integrated gasification combined
cycle. TRLs are a method of characterizing technological maturity from the most basic research (TRL 1) through to full-scale real-world operation (TRL 9).
Many important elements of all GGR technologies are still in research and early demonstration. Technologies often take decades to advance from this stage to
commercial deployment (TRL 9) and widespread scale-up, even with continuous R&D support (see text).
© 2015 Macmillan Publishers Limited. All rights reserved
opinion & comment
Furthermore, GGR approaches do not
completely separate the greenhouse gases
from the natural carbon cycle, calling
into question the permanence of the
sequestration11,20. is problem ranges from
gradual decay of biochar in soils21, to diuse
leakage of CO2 from geological storage22,
to catastrophic release of forest carbon in a
wildre23. e risks or mechanisms of this
happening are oen poorly understood
and, as with storage itself, monitoring or
quantication of any loss is oen dicult.
ese issues create a challenge to
integrating GGR into international
accounting and accreditation schemes as
well as developing eective policy support
for them, and these challenges need to be
addressed early if the potential of BECCS
and other GGR is to be realized. Indeed,
developing eective and sustainable policy
is likely to require co-evolution and iterative
renement of policies as GGR eorts scale
up over decades, as is currently being seen in
the bioenergy sector24.
The risks of delaying policy engagement
Policy and technology development
undoubtedly take time, but delaying GGR
policy engagement also carries risks. First,
and most practically, it risks missing out
on the near-term and smaller-scale value
of some more mature and economically
attractive GGR options, potentially
including co-ring of biomass in fossil-
fuel CCS plants, sequestration through
biochar production, and carbonation of
mineral wastes15,25,26. Second, excluding
GGR from near-term policy attention
would reduce any incentives for businesses
and research organizations to expend
eort and investment on advancement
of GGR technology, and to engage with
policy to develop suitable support for
GGR-oriented businesses. Enabling such
innovation is essential to realizing the
long-term opportunity.
A nal risk arises from the fact that
policy decisions made today will dene
the context in which the high rates of GGR
deployment anticipated by modelling
will occur in several decades’ time7.
Infrastructure, assets and technology
choices in the energy system, in particular,
can have a lifetime of many decades, and
ongoing development of the bioenergy and
the CCS sectors now with no thought for
their future integration could make roll-out
of BECCS dicult and costly. An eective
policy approach must aim to strike a balance
between the urgent need for policy support
on these key issues and the high level of
current uncertainty, taking low- or no-
regrets steps towards integration of GGR
into policy and near-term development.
A way forward
A rst step forward can come from noting
that the practical and conceptual diculties
in accounting, and to some extent the
uncertainties, are shared to varying degrees
by several emissions reduction technologies
that are currently the focus of policy eorts.
Life-cycle assessment methodologies,
developing guidelines for carbon accounting
in forestry and land-use change, approaches
for reducing risks of indirect emissions
from bioenergy and accounting, monitoring
and liability mechanisms for geological
storage are all transferable to dierent GGR
methods, and these mechanisms can form the
basis for policy integration. ese ongoing
overhauls of emissions accounting across
all sectors represent a good opportunity to
incorporate GGR.
Based on the principles of integration with
mitigation policy and building exibility, we
therefore propose four principles for a high-
level strategy that can be applied in order to
begin to make progress towards successful
GGR integration7:
Fund research, development and
demonstration of GGR systems, focusing
on constraining uncertainties, developing
practical accounting methods and bridging
any other gaps between technology
maturity and policy needs. Given the
value of GGR in tackling the most dicult
emissions sources, diverting some funding
from more advanced and speculative clean
energy research may pay o.
Build up support for low-cost, early
opportunities through existing or new
bottom-up policy mechanisms. Examples
might include subsidies for electricity
generated from early BECCS opportunities
such as biomass co-ring in coal CCS
plants, or inclusion of soil carbon
enhancement or biochar in agricultural
policies. is will help to capture early
opportunities as well as stimulating
development and innovation.
Commit to full integration of GGR into
emissions accounting, accreditation and
overall policy strategy in the longer term,
including any carbon pricing mechanisms.
is process will undoubtedly be complex,
but the commitment will stimulate
investment, research and long-term
planning for GGR.
Develop steps to lay the broader
groundwork for future GGR and to
keep the GGR option open, avoiding
lock-out of valuable opportunities. e
rst three principles will go some way
towards achieving this, but there may be
further steps that can be taken that are
specic to each technology and must be
identied through close engagement with
stakeholders. An example of this might be
capture-ready’ requirements for bioenergy
plants to ensure that they can be retrotted
with CCS when this option becomes viable.
e challenge of meeting climate targets is
huge, and we will need to make use of every
tool at our disposal. GGR methods that can
extract CO2 from the atmosphere itself can
add vital exibility to the eorts and must
be brought into mainstream climate policy
as soon as possible to open up the landscape
for innovation and development. Eectively
integrating such diverse approaches into
policy will be challenging and complex, and
the principles proposed here only point to the
rst stages of the process. But they represent
essential steps that must be taken if we are not
to miss the opportunity that GGR provides.
Guy Lomax1*, Timothy M. Lenton2,
Adepeju Adeosun3 and Mark Workman4 are at
1Energy Futures Lab, Imperial College London,
Exhibition Road, London SW7 2AZ, UK. 2Earth
System Science Group, College of Life and
Environmental Sciences, University of Exeter,
North Park Road, Exeter EX4 4QE, UK. 3Virgin
Earth Challenge, Virgin Management, 179 Harrow
Road, London W2 6NB, UK. 4Grantham Institute
for Climate Change, Imperial College London,
Exhibition Road, London SW7 2AZ, UK.
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(Earthscan, 2009).
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(eds Metz, B. et al.) (Cambridge Univ. Press, 2005).
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Change, 2012).
25. Pratt, K. & Moran, D. Biomass Bioenergy 34, 1149–1158 (2010).
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© 2015 Macmillan Publishers Limited. All rights reserved
... NET proposals are heterogeneous, with large uncertainties around their risks and benefits. As a hedge against unforeseen risks, including the risk of technology failure, some technical experts advise that it would be wise to explore a diverse range of NETs alongside ambitious efforts to reduce emissions (Lomax et al., 2015, Nemet et al., 2018. The ocean has been posited by some as suitable for NETs because of its large available area, and the potential for CO 2 sequestration over extremely long timescales; yet the idea of intervening in complex marine ecosystems poses significant risks and societal concerns (GESAMP, 2019). ...
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In addition to mitigation and adaptation as strategies for governing climate futures, a third way of responding to climate change is now emerging: Intentional intervention into the global climate, often termed ‘climate engineering’ (CE). There is a growing awareness that formal governance of some types of CE is going to be needed in the coming years, and that informal governance is already being shaped by the discourses and practices of CE research and assessment. Increased attention is being paid to the types of scientific and societal discourses shaping the emergence of CE governance. Contributing to this literature, this thesis asks how the discursive construction of CE governance is taking place in science, industry, civil society, and politics. The project emphasises that, as discourse is the source code with which contested futures are written, ‘cracking the discursive code’ underpinning the CE governance debate can help critically anticipate the emergence of future governance practices and infrastructures. In this vein, the thesis peruses several interrelated aims: (1) Exploring a framework for shifting the analytical perspective on the role of discourse in (CE) governance development processes; (2) Anticipating and critically reflecting upon how given discursive structures may be making certain types of CE governance more/less thinkable and practicable, (3) emancipating those engaging in the CE governance debate to recognize and expand the bounds of the discursive structures they are reproducing, and (4) informing the design of participatory processes to further “open up” discursive diversity in CE governance development.
... Previous studies identified the competition for investment with cheap renewable energies -e.g. solar and wind-as one of the major barriers for BECCS implementation [49,120]. The modular approach could help overcome that barrier as it commonly involves smaller unit costs, quicker build schedules, and less risk to investors. ...
Bioenergy with carbon capture and storage (BECCS) technology is expected to support net-zero targets by supplying low carbon energy while providing carbon dioxide removal (CDR). BECCS is estimated to deliver 20 to 70 MtCO2 annual negative emissions by 2050 in the UK, despite there are currently no BECCS operating facility. This research is modelling and demonstrating the flexibility, scalability and attainable immediate application of BECCS. The CDR potential for two out of three BECCS pathways considered by the Intergovernmental Panel on Climate Change (IPCC) scenarios were quantified (i) modular-scale CHP process with post-combustion CCS utilising wheat straw and (ii) hydrogen production in a small-scale gasifier with pre-combustion CCS utilising locally sourced waste wood. Process modelling and lifecycle assessment were used, including a whole supply chain analysis. The investigated BECCS pathways could annually remove between −0.8 and −1.4 tCO2e tbiomass⁻¹ depending on operational decisions. Using all the available wheat straw and waste wood in the UK, a joint CDR capacity for both systems could reach about 23% of the UK's CDR minimum target set for BECCS. Policy frameworks prioritising carbon efficiencies can shape those operational decisions and strongly impact on the overall energy and CDR performance of a BECCS system, but not necessarily maximising the trade-offs between biomass use, energy performance and CDR. A combination of different BECCS pathways will be necessary to reach net-zero targets. Decentralised BECCS deployment could support flexible approaches allowing to maximise positive system trade-offs, enable regional biomass utilisation and provide local energy supply to remote areas.
... The European emissions trading system does not currently create incentives for removing CO 2 from the atmosphere (Daggash & Mac Dowell, 2019). A revision of the EU-wide CO 2 trading system to include CDR measures will be a challenge as CDR measures have different time spans of retention and bear diverse risks of unintended re-emission (Lomax et al., 2015) while the ETS creates a uniform price signal. Regulations not only need to reward the removal of CO 2 emissions but also to financially penalize their intended and unintended re-emission to generate an efficient market outcome. ...
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ere a net-zero-2050 Germany is envisioned by combining analysis from an energy-system model with insights into approaches that allow for a higher carbon circularity in the German system, and first results from assessments of national carbon dioxide removal potentials. A back-casting perspective is applied on how net-zero Germany could look like in 2050. We are looking back from 2050, and analyzing how Germany for the first time reached a balance between its sources of CO2 to the atmosphere and the anthropogenic sinks created. This would consider full decarbonization in the entire energy sector and being entirely emission-free by 2050 within three priorities identified as being the most useful strategies for achieving net-zero: (a) Avoiding- (b) Reducing- (c) Removing emissions. This work is a collaboration of interdisciplinary scientists with the Net-Zero-2050 cluster of the Helmholtz Climate Initiative HI-CAM
... The European emissions trading system (EU-ETS) does not currently create incentives for removing CO2 from the atmosphere [126]. A revision of the EU-wide CO2 trading system to include CDR measures will be a challenge as CDR measures have different time spans of retention and bear diverse risks of unintended re-emission [127] while the ETS creates a uniform price signal. Regulations not only need to reward the removal of CO2 emissions but also to financially penalise their intended and unintended re-emission to generate an efficient market outcome. ...
Full-text available
Plain Language Summary Here a net‐zero‐2050 Germany is envisioned by combining analysis from an energy‐system model with insights into approaches that allow for a higher carbon circularity in the German system, and first results from assessments of national carbon dioxide removal potentials. A back‐casting perspective is applied on how net‐zero Germany could look like in 2050. We are looking back from 2050, and analyzing how Germany for the first time reached a balance between its sources of CO2 to the atmosphere and the anthropogenic sinks created. This would consider full decarbonization in the entire energy sector and being entirely emission‐free by 2050 within three priorities identified as being the most useful strategies for achieving net‐zero: (a) Avoiding‐ (b) Reducing‐ (c) Removing emissions. This work is a collaboration of interdisciplinary scientists with the Net‐Zero‐2050 cluster of the Helmholtz Climate Initiative HI‐CAM.
... This is perhaps not surprising given that at 'large scale' these are still very much imagined and emerging technologies: BECCS does not yet exist at large scale whereas afforestation is very heterogenous and not currently available at a large scale in the UK. Pilotscale trials of these GGR technologies may assist in reducing some of the uncertainties observed in this study, by exploring options of how GGR supply chains could be enabled with specific incentives, regulations and policies (Lomax et al., 2015). By taking a more granular approach, this work has revealed some important details that may otherwise be overlooked or not considered if the foci of the study, such as BECCS and afforestation, are kept general and non-specific. ...
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Greenhouse gas removal (GGR) approaches are considered essential in several projections to meet the climate mitigation ambition of the Paris Agreement. Biomass Energy with Carbon Capture and Storage (BECCS) and afforestation are included extensively in mitigation scenarios but there are concerns about the feasibility of these approaches. This was explored with stakeholders from industry, non-governmental organisations (NGOs) and policy who were involved in interviews and a one-day participatory workshop. Multicriteria mapping (MCM) methodology was used to appraise the ‘real-world’ feasibility of four specific greenhouse gas removal supply chains at a granular level in the UK context. The MCM analysis shows that afforestation performs better in comparison to three BECCS supply chains, on criteria such as business model, social acceptability, and environmental sustainability. This innovative application of the MCM methodology enables the abstract representations of GGR in integrated assessment models to be explored at a more granular level through a supply chain analysis and thus gain a deeper understanding of the issues facing these approaches. The data gathered allows a wide range of technical, environmental, social and political criteria to be systematically applied in appraising the practical performance of different future implementation options for afforestation and BECCS. If these GGR supply chains are to become a reality on the scale required for 1.5 °C global warming, factors such as global cooperation, land availability, and the longevity of policies and incentives were found to be major challenges.
... Several negative emissions technologies and practices (NETPs) may be capable of delivering CDR and have been included in the Integrated Assessment Models (IAMs). Among all possible NETPs, bioenergy with carbon capture and storage (BECCS) emerges as particularly appealing [6,[8][9][10], as it delivers net negative emissions while providing sustainable and reliable energy. In essence, BECCS technologies involve a combination of bioenergy applications and carbon capture and storage (CCS). ...
Carbon dioxide removal options have been identified as key to achieving the climate change target laid out in the 2015 Paris Agreement. Bioenergy with carbon capture and storage (BECCS) is particularly attractive because it is capable of providing negative emissions and a reliable energy source. We here explore the complexity of the infrastructures involved in realizing a large-scale system and the sequestration potential of bioenergy in Europe. Starting from a minimum cost scenario, we develop cost-optimal solutions that minimize the environmental impact of the overall BECCS supply chain according to the life cycle impact assessment methodology. Our analysis is based on cooperation among the 28 countries of the European Union (as of 2018) to achieve a global carbon removal target. Given regional biomass and marginal land availability inputs and a carbon removal target of 0.61 GtCO2/year, the minimum-cost scenario provides negative emissions, with an overall cost of 140 Eur/MWh of bioelectricity generated or 117 Eur/tCO2 removed, without considering revenues from selling the electricity produced. On the other hand, minimizing environmental impacts increased costs by 45% relative to the first scenario, but further improved the environmental performance by 23%.
Despite recent efforts to reduce carbon dioxide emissions, scientists still project that we will not avoid dangerous climate change. Models that calculate that we can avoid this result almost exclusively rely upon carbon dioxide removal options to stay below this level of warming. Although a number of CDR technologies are theoretically possible, they all have limitations. More germane here, they all remain far from the level of development and installation required.
Grand hopes exist that carbon capture and storage can have a major decarbonization role at global, regional and sectoral scales. Those hopes rest on the narrative that an abundance of geological storage opportunity is available to meet all needs. In this Perspective, we present the contrasting view that deep uncertainty over the sustainable injection rate at any given location will constrain the pace and scale of carbon capture and storage deployment. Although such constraints will probably have implications in most world regions, they may be particularly relevant in major developing Asian economies. To minimize the risk that these constraints pose to the decarbonization imperative, we discuss steps that are urgently needed to evaluate, plan for and reduce the uncertainty over CO2 storage prospects.
In this chapter, a dynamic equilibrium strategy for integrated coal purchasing, blending, and distribution under an uncertain environment was proposed to reduce carbon dioxide emissions in large‐scale coal‐fired powered enterprises. The proposed method integrated all the key stages during the operation of large‐scale coal‐fired powered enterprises and tried to make a whole optimization framework. Different carbon emissions levels and satisfactory degrees were conducted to give insights into the conflict between economic development and environmental protection, and balance among short‐term and long‐term production plans. The real‐world application results indicated that the proposed method was able to achieve economic‐environmental coordination and sustainable development.
Negative-emission technologies (NETs) are widely viewed as a risky backstop technology for climate change mitigation. In this perspective, we challenge this limited view of NETs. We show how, notwithstanding their merit, integrated assessment models (IAMs) are largely responsible for establishing this opposition to NETs. This is because IAM-based assessments of NETs dominate the policy-facing literature, but as a result of model limitations, we are left with a deceptively shallow understanding of the role NETs could play to support long-term mitigation goals. Therefore, in the second part of this perspective, we provide a non-IAM-based fresh take on NETs. We explore NETs via a bottom-up analysis and introduce a decision-making framework to determine the circumstances under which NETs could provide value as a mitigation option at jurisdictional scales. We apply this framework to case studies in California and New Mexico, highlighting how NETs could overcome socio-technical obstacles and unlock a variety of environmental and social co-benefits as part of helping to achieve time-bound mitigation goals. Overall, this perspective aims to cut through what we see as a noisy discourse on NETs, which is wrapped-up in concerns that are dependent on scenario modeling and offer a plain evaluation of NETs as a potential climate change mitigation option.
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Greenhouse gas removal (GGR) methods such as direct air capture, bioenergy with carbon capture and storage, biochar and enhanced weathering have recently attracted attention as “geoengineering” options to reverse the build-up of greenhouse gases in the atmosphere. Contrary to this framing, however, we argue that GGR technologies can in fact form a valuable complement to emissions control within on-going mitigation efforts. Through decoupling abatement from emissions sources, they add much-needed flexibility to the mitigation toolbox, increasing feasibility and reducing costs of meeting climate targets. Integrating GGR effectively into policy raises significant challenges relating to uncertain costs, side effects, life-cycle effectiveness and accounting. Delaying policy action until these uncertainties are resolved, however, risks missing early opportunities, suffocating innovation and locking out the long-term potential of GGR. Based on an analysis of bioenergy with carbon capture and storage, we develop four policy principles to begin unlocking the potential of GGR: (i) support further research, development and demonstration; (ii) support near-term opportunities through modifying existing policy mechanisms; (iii) commit to full GGR integration in carbon accreditation and broader climate policy frameworks in future; (iv) develop sector-specific steps that lay the groundwork for future opportunities and avoid lock-out.
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The portfolio of approaches to respond to the challenges posed by anthropogenic climate change has broadened beyond mitigation and adaptation with the recent discussion of potential climate engineering options. How to define and categorize climate engineering options has been a recurring issue in both public and specialist discussions. We assert here that current definitions of mitigation, adaptation, and climate engineering are ambiguous, overlap with each other and thus contribute to confusing the discourse on how to tackle anthropogenic climate change. We propose a new and more inclusive categorization into five different classes: anthropogenic emissions reductions (AER), territorial or domestic removal of atmospheric CO2 and other greenhouse gases (D-GGR), trans-territorial removal of atmospheric CO2 and other greenhouse gases (T-GGR), regional to planetary targeted climate modification (TCM), and climate change adaptation measures (including local targeted climate and environmental modification, abbreviated CCAM). Thus, we suggest that techniques for domestic greenhouse gas removal might better be thought of as forming a separate category alongside more traditional mitigation techniques that consist of emissions reductions. Local targeted climate modification can be seen as an adaptation measure as long as there are no detectable remote environmental effects. In both cases, the scale and intensity of action are essential attributes from the technological, climatic, and political viewpoints. While some of the boundaries in this revised classification depend on policy and judgement, it offers a foundation for debating on how to define and categorize climate engineering options and differentiate them from both mitigation and adaptation measures to climate change. (C) 2013 John Wiley & Sons, Ltd.
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In order to meet stringent temperature targets, active removal of CO2 from the atmosphere may be required in the long run. Such negative emissions can be materialized when well-performing bioenergy systems are combined with carbon capture and storage (BECCS). Here, we develop an integrated global energy system and climate model to evaluate the role of BECCS in reaching ambitious temperature targets. We present emission, concentration and temperature pathways towards 1.5 and 2 ° C targets. Our model results demonstrate that BECCS makes it feasible to reach temperature targets that are otherwise out of reach, provided that a temporary overshoot of the target is accepted. Additionally, stringent temperature targets can be met at considerably lower cost if BECCS is available. However, the economic benefit of BECCS nearly vanishes if an overshoot of the temperature target is not allowed. Finally, the least-cost emission pathway over the next 50 years towards a 1.5 ° C overshoot target with BECCS is almost identical to a pathway leading to a 2 ° C ceiling target.
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The ability to directly remove carbon dioxide from the atmosphere allows the decoupling of emissions and emissions control in space and time. We ask the question whether this unique feature of carbon dioxide removal technologies fundamentally alters the dynamics of climate mitigation pathways. The analysis is performed in the coupled energy-economy-climate model ReMIND using the bioenergy with CCS route as an application of CDR technology. BECCS is arguably the least cost CDR option if biomass availability is not a strongly limiting factor. We compare mitigation pathways with and without BECCS to explore the impact of CDR technologies on the mitigation portfolio. Effects are most pronounced for stringent climate policies where BECCS is a key technology for the effectiveness of carbon pricing policies. The decoupling of emissions and emissions control allows prolonging the use of fossil fuels in sectors that are difficult to decarbonize, particularly in the transport sector. It also balances the distribution of mitigation costs across future generations. CDR is not a silver bullet technology. The largest part of emissions reductions continues to be provided by direct mitigation measures at the emissions source. The value of CDR lies in its flexibility to alleviate the most costly constraints on mitigating emissions.
Bioenergy with carbon capture and storage could be used to remove carbon dioxide from the atmosphere. However, its credibility as a climate change mitigation option is unproven and its widespread deployment in climate stabilization scenarios might become a dangerous distraction.
Using biomass to provide energy services is a strategically important option for increasing the global uptake of renewable energy. Yet the practicalities of accelerating deployment are mired in controversy over the potential resource conflicts that might occur, particularly over land, water and biodiversity conservation. This calls into question whether policies to promote bioenergy are justified. Here we examine the assumptions on which global bioenergy resource estimates are predicated. We find that there is a disjunct between the evidence that global bioenergy studies can provide and policymakers' desire for estimates that can straightforwardly guide policy targets. We highlight the need for bottom-up assessments informed by empirical studies, experimentation and cross-disciplinary learning to better inform the policy debate.
[1] Chemical weathering is an integral part of both the rock and carbon cycles and is being affected by changes in land use, particularly as a result of agricultural practices such as tilling, mineral fertilization, or liming to adjust soil pH. These human activities have already altered the terrestrial chemical cycles and land-ocean flux of major elements, although the extent remains difficult to quantify. When deployed on a grand scale, Enhanced Weathering (a form of mineral fertilization), the application of finely ground minerals over the land surface, could be used to remove CO2 from the atmosphere. The release of cations during the dissolution of such silicate minerals would convert dissolved CO2 to bicarbonate, increasing the alkalinity and pH of natural waters. Some products of mineral dissolution would precipitate in soils or be taken up by ecosystems, but a significant portion would be transported to the coastal zone and the open ocean, where the increase in alkalinity would partially counteract “ocean acidification” associated with the current marked increase in atmospheric CO2. Other elements released during this mineral dissolution, like Si, P, or K, could stimulate biological productivity, further helping to remove CO2 from the atmosphere. On land, the terrestrial carbon pool would likely increase in response to Enhanced Weathering in areas where ecosystem growth rates are currently limited by one of the nutrients that would be released during mineral dissolution. In the ocean, the biological carbon pumps (which export organic matter and CaCO3 to the deep ocean) may be altered by the resulting influx of nutrients and alkalinity to the ocean. This review merges current interdisciplinary knowledge about Enhanced Weathering, the processes involved, and the applicability as well as some of the consequences and risks of applying the method.
Carbon capture and storage (CCS) technologies are often highlighted as a crucial component of future low carbon energy systems in the UK and internationally. Whilst these technologies are now in the demonstration phase world-wide, they are still characterised by a range of technical, economic, policy, social and legal uncertainties. This paper applies a framework for the analysis of these uncertainties that was previously developed by the authors to a historical evidence base. This evidence base comprises nine case studies, each of which focuses on a technology that is partly analogous to CCS. The paper's analysis of these case studies examines the conditions under which the uncertainties concerned have been at least partly resolved, and what lessons can be drawn for CCS. The paper then uses the case study evidence to discuss linkages between the uncertainties in the analysis framework, and how these linkages differ from those that were originally expected. Finally, the paper draws conclusions for the methodological approach that has been used and for strategies to develop and deploy CCS technologies.
The relationship between the level of atmospheric CO2 (carbon dioxide) and the impacts of climate change is uncertain, but a safe concentration may be surpassed this century. Therefore, it is necessary to develop technologies that can accelerate CO2 removal from the atmosphere. This paper explores the engineering challenges of a technology that manipulates the carbonate system in seawater by the addition of calcium oxide powder (CaO; lime), resulting in a net sequestration of atmospheric CO2 into the ocean (ocean liming; OL). Every tonne of CO2 sequestered requires between 1.4 and 1.7 t of limestone to be crushed, calcined, and distributed. Approximately 1 t of CO2 would be created from this activity, of which >80% is a high purity gas (pCO2 > 98%) amenable to geological storage. It is estimated that the thermal and electrical energy requirements for OL would be 0.6–5.6 and 0.1–1.2 GJ tCO2−1 captured respectively. A preliminary economic assessment suggests that OL could cost approximately US$72–159 t−1 of CO2. The additional CO2 burden of OL makes it a poor alternative to point source mitigation. However, it may provide a means to mitigate some diffuse emissions and reduce atmospheric concentrations.