Cutting through the noise on negative emissions

Abstract

What role should negative-emission technologies (NETs) play in supporting global climate change mitigation? This is a polarizing question in both academic circles and increasingly public discourse. A common argument is that NETs present a risky or high-stakes gamble to climate change mitigation. In this paper, we challenge this opposition to NETs. We show how this opposition is largely based on the results of integrated assessment models, which are the models that form the basis of IPCC reports. These models often show that a late-century, large-scale deployment of NETs is required to stabilize global warming to at or below 2°C this century. However, models are not real life, and such long-range forecasts are fraught with limitations. As a result, this is not a firm foundation for opposition. We make the case for place-based, bottom-up approaches for assessing the potential role for NETs in mitigation portfolios. Bottom-up approaches reveal the many ways in which NETs could (or could not) provide value to enhance economy-wide energy transition feasibility, such as through social and environmental co-benefits. For example, California has highly favorable attributes for NETs deployment. While applied to NETs, our findings more broadly suggest that more circumspect approaches are needed regarding the use of global models to inform mitigation pathways and strategies at jurisdictional scales. 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. 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. Negative emissions refers to the physical removal of carbon dioxide (CO2) from the atmosphere. Negative-emission technologies (NETs) range from natural climate solutions, including afforestation and soil carbon sequestration, to more human-driven interventions including bioenergy with carbon capture and storage (BECCS) and direct air capture with carbon storage (DACCS).1Minx J.C. Lamb W.F. Callaghan M.W. Fuss S. Hilaire J. Creutzig F. Amann T. Beringer T. de Oliveira Garcia W. Hartmann J. et al.Negative emissions—Part 1: research landscape and synthesis.Environ. Res. Lett. 2018; 13: 063001Crossref Scopus (305) Google Scholar,2Fuss S. Lamb W.F. Callaghan M.W. Hilaire J. Creutzig F. Amann T. Beringer T. de Oliveira Garcia W. Hartmann J. Khanna T. et al.Negative emissions—Part 2: costs, potentials and side effects.Environ. Res. Lett. 2018; 13: 063002Crossref Scopus (436) Google Scholar NETs provide climate change mitigation by directly drawing down and reducing the load of CO2 in the atmosphere, compared with actions that limit or prevent CO2 emissions into the atmosphere.3IPCCWorking group III mitigation of climate change.https://www.ipcc.ch/working-group/wg3/Date: 2020Google Scholar NETs have become a prominent research area in recent years, particularly following the publication of the IPCC’s fifth assessment report (AR 5) in 2014. The integrated assessment models (IAMs) related to AR 5 showed that, for almost all possible scenarios that limit global warming to 2°C this century, a late-century, large-scale deployment of NETs would be required.4IPCCClimate Change 2014: mitigation of climate change. Cambridge University Press, 2014https://doi.org/10.1017/CBO9781107415416Crossref Google Scholar This finding had a substantially polarizing impact on the research community, and battle lines were quickly drawn in relation to the prospect of NETs deployment. On the one hand, advocates argue that, due to the current trajectory of global greenhouse gas (GHG) emissions, NETs will be a necessary mitigation option, and therefore, investment in their research, development, and deployment is important.5Lomax G. Lenton T.M. Adeosun A. Workman M. Investing in negative emissions.Nature Clim. Change. 2015; 5: 498-500Crossref Scopus (53) Google Scholar, 6Lackner K.S. The promise of negative emissions.Science. 2016; 354: 714Crossref PubMed Scopus (19) Google Scholar, 7Deutch J. Is net zero carbon 2050 possible?.Joule. 2020; 4: 2237-2240Abstract Full Text Full Text PDF PubMed Scopus (31) Google Scholar On the other hand, opponents argue that NETs (notably BECCS) are unproven, may delay or deter near-term GHG emissions reductions, and present a risky and unethical bet for future generations8Fuss S. Canadell J.G. Peters G.P. Tavoni M. Andrew R.M. Ciais P. Jackson R.B. Jones C.D. Kraxner F. Nakicenovic N. et al.Betting on negative emissions.Nature Clim. Change. 2014; 4: 850-853Crossref Scopus (521) Google Scholar, 9Geden O. Policy: climate advisers must maintain integrity.Nature. 2015; 521: 27-28Crossref PubMed Scopus (69) Google Scholar, 10Lewis S. The dirty secret of the Paris climate deal.Foreign Policy. 2015; https://foreignpolicy.com/2015/12/17/the-dirty-secret-of-the-paris-climate-deal-carbon-capture-negative-emissions-global-warming/Google Scholar, 11Anderson K. Duality in climate science.Nat. Geosci. 2015; 8: 898-900Crossref Scopus (149) Google Scholar, 12Anderson K. Peters G. The trouble with negative emissions.Science. 2016; 354: 182-183Crossref PubMed Scopus (579) Google Scholar, 13Shue H. Climate dreaming: negative emissions, risk transfer, and irreversibility.J. Hum. Rights Environ. 2017; 8: 203-216Crossref Scopus (33) Google Scholar, 14Markusson N. McLaren D. Tyfield D. Towards a cultural political economy of mitigation deterrence by negative emissions technologies (NETs).Glob. Sust. 2018; 1: 1-9Google Scholar (we collectively refer to this set of arguments hereafter as the “risky backstop technology” opposition to NETs). As we explore further on, this opposition to NETs is based on a fairly fragile foundation; so is mostly serving to confuse the discourse on NETs and discourage progress on what could be an important mitigation option. Given our increasingly limited window to achieve a well below 2°C future, it is important that the scientific community can find common ground on NETs and provide a clear communication to policymakers. In this perspective, we aim to cut through the noise and provide a fresh take on NETs. Our argument is structured into three key parts. First, we show how much of the current opposition to NETs stems from IAMs. However, as IAMs are subject to several limitations, we argue that this opposition is not well founded. Second, we take a step back and provide a non-IAM-based perspective on NETs. We introduce a decision-making framework for subnational and national scales to determine the circumstances under which NETs could provide value as a mitigation option. We adopt a broad definition of value, which can accrue at two different levels, including the project-level (i.e., NETs provide direct environmental and social co-benefits) and energy transitions-level (i.e., NETs support the achievement of time-bound mitigation goals). Third, we apply this framework to case studies in California and New Mexico, highlighting how NETs could unlock environmental and social co-benefits and overcome socio-technical obstacles to energy transitions as part of helping jurisdictions achieve mitigation goals. We conclude with a discussion of the implications of this perspective and where further work is needed to help improve the understanding of how NETs can support effective global climate change mitigation. Much opposition to NETs is based on how NETs are represented in IAMs. In this section, we highlight how a combination of debatable and arbitrary input assumptions as well as scope limitations in AR 5-generation IAMs spawned the late-century, large-scale NETs scenario output. We explain how this has led to the perverse outcome of discouraging consideration of NETs as an available mitigation option. IAMs are complex computer models used to model how GHG emissions might change overtime,15Gambhir A. Butnar I. Li P.-H. Smith P. Strachan N. A review of criticisms of integrated assessment models and proposed approaches to address these, through the lens of BECCS.Energies. 2019; 12: 1747Crossref Scopus (60) Google Scholar most commonly to 2100,4IPCCClimate Change 2014: mitigation of climate change. Cambridge University Press, 2014https://doi.org/10.1017/CBO9781107415416Crossref Google Scholar in response to changes in manufactured (i.e., energy and industrial system) and natural (i.e., biosphere) capital and under a range of input assumptions, such as anticipated population and economic growth rates, resources availability, technology costs, and more. Multiple different discrete models, simulating energy, land, ocean, and atmospheric systems, are “integrated” to generate an estimate. Each of these models are complex in their own right but are also defined by feedback loops into each other (e.g., how changes to the energy system might impact the land system, which then impacts the energy system, and so forth). IAMs usually define a “ business-as-usual” scenario, which results in an expected GHG emissions outcome without any outside mitigation influence. Modelers then determine combinations of actions (e.g., shift to low-carbon energy resources across various sectors of the economy) that would be necessary to achieve a desired GHG emissions outcome, which corresponds to a global warming outcome at some measure of lowest cost. IAMs were first used in the mid-1980s, and although they have changed substantially since that time, the principle of modeling GHG emissions outcomes into the future to help guide action today remains the same. Although IAMs are one of the main analytical tools relied on to inform a variety of climate change policies, IAMs have several limitations.16Ackerman F. DeCanio S.J. Howarth R.B. Sheeran K. Limitations of integrated assessment models of climate change.Clim. Change. 2009; 95: 297-315Crossref Scopus (183) Google Scholar, 17Kopp R.E. Mignone B.K. 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Sust. 2018; 1: 1-10Google Scholar The modeling of NETs in AR 5-generation IAMs provides an example of the limitations of IAMs. As mentioned, AR 5 highlighted the important role of NETs to limit global warming to 2°C. However, it was the nature of the NETs deployment in modeled pathways—a late-century, large-scale deployment—that polarized the research community. This conceptualization of NETs gave rise to the dominant objections to NETs, including that NETs are unproven or speculative, may deter near-term GHG emissions reductions, and expose future generations to an unethical risk transfer. Although these may be valid arguments, we note that an important and often overlooked point is that these arguments depend on whether the modeling offers a realistic view of the future. If this is not the case, and given the absence of perfect foresight, it is certainly contestable, then these arguments are essentially moot. This shortcoming underlies the risky backstop technology opposition to NETs. In Table 1, we highlight some of the driving model assumptions and approaches to scenario design that served to generate the late-century, large-scale NETs “archetype” in AR 5-generation IAMs.23Obersteiner M. Bednar J. Wagner F. Gasser T. Ciais P. Forsell N. Frank S. Havlik P. et al.How to spend a dwindling greenhouse gas budget.Nat. Clim. Change. 2018; 8: 7-10Crossref Scopus (80) Google Scholar We observe how, notwithstanding that this modeling was based on the best available information at the time, model assumptions (e.g., cost of renewables) proved to be highly overestimated and that the approaches to scenario design (e.g., allowing for temperature overshoot; discount rate selection) are arbitrary and controversial. In addition, a lack of spatial, temporal, and social resolution in IAMs provided minimal limitations on the potential pace and scale of NETs (notably BECCS) deployment. Crucially, changes to one or more of these model inputs, either considered or applied in more recent analyses, undercuts the necessary creation of the late-century, large-scale NETs archetype.22Rogelj J. Huppmann D. Krey V. Riahi K. Clarke L. Gidden M. Nicholls Z. Meinshausen M. A new scenario logic for the Paris Agreement long-term temperature goal.Nature. 2019; 573: 357-363Crossref PubMed Scopus (164) Google Scholar,23Obersteiner M. Bednar J. Wagner F. Gasser T. Ciais P. Forsell N. Frank S. Havlik P. et al.How to spend a dwindling greenhouse gas budget.Nat. Clim. Change. 2018; 8: 7-10Crossref Scopus (80) Google Scholar,28Emmerling J. Drouet L. Wijst K.V.D. Vuuren D.V. Bosetti V. Tavoni M. The role of the discount rate for emission pathways and negative emissions.Environ. Res. 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Promises and perils of the Paris agreement.Science. 2019; 364: 829-830Crossref PubMed Scopus (19) Google Scholar This demonstrates the fragile basis of the risky backstop technology opposition to NETs and is an example of where our reliance on IAMs needs to be more circumspect. Meanwhile, this opposition to NETs is already engrained in both the literature and public discourse and continues to cloud the perception of NETs today.38Larkin A. Kuriakose J. Sharmina M. Anderson K. What if negative emission technologies fail at scale? Implications of the Paris Agreement for big emitting nations.Clim. Policy. 2018; 18: 690-714Crossref Scopus (69) Google Scholar, 39Carton W. “Fixing” climate change by mortgaging the future: negative emissions, spatiotemporal fixes, and the political economy of delay.Antipode. 2019; 51: 750-769Crossref Scopus (36) Google Scholar, 40McLaren D. Quantifying the potential scale of mitigation deterrence from greenhouse gas removal techniques.Clim. 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A review of criticisms of integrated assessment models and proposed approaches to address these, through the lens of BECCS.Energies. 2019; 12: 1747Crossref Scopus (60) Google Scholar,20Rosen R.A. Guenther E. The economics of mitigating climate change: what can we know?.Technological Forecasting and Social Change. 2015; 91: 93-106Crossref Scopus (74) Google Scholar,45Grubler A. Wilson C. Bento N. Boza-Kiss B. Krey V. McCollum D.L. Rao N.D. Riahi K. Rogelj J. De Stercke S. et al.A low energy demand scenario for meeting the 1.5°C target and sustainable development goals without negative emission technologies.Nat. Energy. 2018; 3: 515-527Crossref Scopus (437) Google Scholar,46van Vuuren D.P. Stehfest E. Gernaat D.E.H.J. van den Berg M. Bijl D.L. de Boer H.S. Daioglou V. Doelman J.C. Edelenbosch O.Y. Harmsen M. et al.Alternative pathways to the 1.5°C target reduce the need for negative emissions technologies.Nature Clim. Change. 2018; 8: 391-397Crossref Scopus (320) Google Scholar and low availability of some supply-side decarbonization options15Gambhir A. Butnar I. Li P.-H. Smith P. Strachan N. A review of criticisms of integrated assessment models and proposed approaches to address these, through the lens of BECCS.Energies. 2019; 12: 1747Crossref Scopus (60) Google Scholar•reduced capacity for decarbonization and increased reliance on NETs for 2°C targetTemperature target model design•allowed for 2°C temperature target overshoot compared with no or minimal overshoot23Obersteiner M. Bednar J. Wagner F. Gasser T. Ciais P. Forsell N. Frank S. Havlik P. et al.How to spend a dwindling greenhouse gas budget.Nat. Clim. Change. 2018; 8: 7-10Crossref Scopus (80) Google Scholar,34IPCCGlobal warming of 1.5°C: an IPCC special report on the impacts of global warming of 1.5°C Above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty. IPCC/WMO, 2018https://www.ipcc.ch/sr15/Google Scholar,37Lawrence M.G. Schäfer S. Promises and perils of the Paris agreement.Science. 2019; 364: 829-830Crossref PubMed Scopus (19) Google Scholar•allowed for optimal pathway of 2°C overshoot followed by late-century, large-scale NETsTime period model design•allowed for 2°C temperature target to be achieved by 2100 compared with an alternative time frame22Rogelj J. Huppmann D. Krey V. Riahi K. Clarke L. Gidden M. Nicholls Z. Meinshausen M. A new scenario logic for the Paris Agreement long-term temperature goal.Nature. 2019; 573: 357-363Crossref PubMed Scopus (164) Google Scholar•allowed for optimal pathway of 2°C overshoot followed by late-century, large-scale NETsLevel of spatial, temporal, and social resolution•low resolution in relation to NETs provided minimal limits as to the likely real-world boundaries for the pace and scale of NETs deployment36Fuhrman J. McJeon H. Doney S.C. Shobe W. Clarens A.F. From zero to hero?: why integrated assessment modeling of negative emissions technologies is hard and how we can do better.Front. Clim. 2019; 1: 11Crossref Scopus (27) Google Scholar•allowed for substantial NETs deployment for achieving 2°C targetSelection of discount rate•underlying IAMs typically used discount rates of 5% compared with lower (and equally justifiable) rates of 2%–3%28Emmerling J. Drouet L. Wijst K.V.D. Vuuren D.V. Bosetti V. Tavoni M. The role of the discount rate for emission pathways and negative emissions.Environ. Res. Lett. 2019; 14: 104008Crossref Scopus (41) Google Scholar•allowed for substantial 2°C overshoot, thereby requiring late-century, large-scale NETs Open table in a new tab We would like to clarify two important points. First, we note that it is not as if all previous works have failed to recognize the limitations of IAMs in modeling NETs.8Fuss S. Canadell J.G. Peters G.P. Tavoni M. Andrew R.M. Ciais P. Jackson R.B. Jones C.D. Kraxner F. Nakicenovic N. et al.Betting on negative emissions.Nature Clim. Change. 2014; 4: 850-853Crossref Scopus (521) Google Scholar,12Anderson K. Peters G. The trouble with negative emissions.Science. 2016; 354: 182-183Crossref PubMed Scopus (579) Google Scholar Yet, by the same token this has not prevented such works from making substantial claims against NETs (e.g., that NETs are “an unjust and high-stakes gamble” [Anderson and Peters [2016] go further, claiming that “there is a real risk that [NETs] will be unable to deliver on the scale of their promise.”12Anderson K. Peters G. The trouble with negative emissions.Science. 2016; 354: 182-183Crossref PubMed Scopus (579) Google Scholar We would pose the question: What promise—that NETs will be scaled according to model outputs?]) or otherwise characterizing NETs in deliberate ways (e.g., that NETs constitute a “bet” that we need to determine is “safe” to take)8Fuss S. Canadell J.G. Peters G.P. Tavoni M. Andrew R.M. Ciais P. Jackson R.B. Jones C.D. Kraxner F. Nakicenovic N. et al.Betting on negative emissions.Nature Clim. Change. 2014; 4: 850-853Crossref Scopus (521) Google Scholar based on model outputs. These claims unavoidably shape the perception of NETs (as do confusing efforts that frame NETs [notably engineered solutions such as BECCS] as some alternate “climate intervention” option, compared with simply [and correctly] being characterized as a relatively nascent, but valid, mitigation option14Markusson N. McLaren D. Tyfield D. Towards a cultural political economy of mitigation deterrence by negative emissions technologies (NETs).Glob. Sust. 2018; 1: 1-9Google Scholar,40McLaren D. Quantifying the potential scale of mitigation deterrence from greenhouse gas removal techniques.Clim. Change. 2020; 162: 2411-2428Crossref Scopus (22) Google Scholar). Second, and although this might appear to run counter to the central thesis we present here, we do think it is important that researchers continue to explore the risky backstop technology arguments against NETs, notably those relat