Carbon dioxide removal (CDR) is necessary for reaching net zero emissions, with studies showing potential deployment at multi-GtCO(2) scale by 2050. However, excessive reliance on future CDR entails serious risks, including delayed emissions cuts, lock-in of fossil infrastructure, and threats to sustainability from increased resource competition. This study highlights an alternative pathway-prioritizing near-term non-CDR mitigation and minimizing CDR dependence. We impose a 1 GtCO(2) limit on global novel CDR deployment by 2050, forcing aggressive early emissions reductions compared to 8-22 GtCO(2) in higher CDR scenarios. Our results reveal that this low CDR pathway significantly decreases fossil fuel use, greenhouse gas (GHG) emissions, and air pollutants compared to higher CDR pathways. Driving rapid energy transitions eases pressures on land (including food cropland), water, and fertilizer resources required for energy and negative emissions. However, these sustainability gains come with higher mitigation costs from greater near-term low/zero-carbon technology deployment for decarbonization. Overall, this work provides strong evidence for maximizing non-CDR strategies such as renewables, electrification, carbon neutral/negative fuels, and efficiency now rather than betting on uncertain future CDR scaling. Ambitious near-term mitigation in this decade is essential to prevent lock-in and offer the best chance of successful deep decarbonization. Our constrained CDR scenario offers a robust pathway to achieving net zero emissions with limited sustainability impacts.

Carbon dioxide removal (CDR) is a necessary measure to achieve global climate targets. However, each CDR approach has potential trade-offs and spillover effects on the global energy-land-water system. To minimize these negative impacts and achieve climate targets, it has become increasingly important to assess different CDR deployment plans and how they interact with the Earth's system. The existing literature on CDR deployment in China is yet to uncover the potential synergies and trade-offs associated with varying CDR deployment strategies for the country's pathway to carbon neutrality. Here, we model five different CDR approaches categorized under four CDR deployment plans for China's climate aspirations using a variant of the Global Change Assessment Model, GCAM-CDR. Our goal here is not to definitively conclude the superiority of any specific CDR deployment plan for China's climate ambitions but instead to primarily explore and emphasize the potential implications associated with each group of CDR strategies. We show that China's pursuit of carbon neutrality solely through land-based biological CDR methods could result in reduced fossil fuel consumption, greenhouse gas emissions, and total air pollutants. However, an integrated approach that combines both land-based biological and geo/chemical CDR strategies could enhance the economic feasibility of achieving China's climate objectives. Moreover, it could reduce land and water requirements for energy generation and negative emissions, thereby mitigating the pressure on food croplands and other sustainability issues. Compared with an integrated approach, relying solely on land-based biological CDR processes for China's carbon neutrality goal could lead to a cumulative reduction in residual GHG emissions by at least 2 GtCO2e over the next four decades (in 5-year intervals) as China progresses towards carbon neutrality. On the other hand, the mitigation cost of relying exclusively on land-based biological CDR processes for China's carbon neutrality goals could be approximately 30 % higher than that of an integrated approach (on an annual average basis). In conclusion, China's policymakers and stakeholders should carefully assess different combinations of diverse CDR approaches, considering the associated impacts on the country's energy-land-water system and policy costs, to make informed decisions that help maximize benefits and minimize negative consequences in pursuit of carbon neutrality goals.

Ocean carbon dioxide removal (OCDR) is rapidly attracting interest, as climate change is putting ecosystems at risk and endangering human communities globally. Due to the centrality of the ocean in the global carbon cycle, augmenting the carbon sequestration capacity of the ocean could be a powerful mechanism for the removal of legacy excess emissions. However, OCDR requires careful assessment due to the unique biophysical characteristics of the ocean and its centrality in the Earth system and many social systems. Using a sociotechnical system lens, this review identifies the sets of considerations that need to be included within robust assessments for OCDR decision-making. Specifically, it lays out the state of technical assessments of OCDR approaches along with key financial concerns, social issues (including public perceptions), and the underlying ethical debates and concerns that would need to be addressed if OCDR were to be deployed as a carbon dioxide removal strategy.

This paper introduces GCAM-CDR 1.0, an integratedassessment model for climate policy based on the open-source Global ChangeAnalysis Model (GCAM). GCAM-CDR extends GCAM v5.4 by enabling users to modeladditional carbon dioxide removal (CDR) technologies and additional policiesand controls related to CDR. New CDR technologies include terrestrialenhanced weathering with basalt, ocean liming, and additional versions ofdirect air capture. New CDR policies and controls include integration ofbioenergy with carbon capture and storage (BECCS) into the CDR market,interregional trade in CDR, exogenous control over the rate of growth ofCDR, the ability to set independent targets for emissions abatement and CDR,and a variety of mechanisms for setting demand for CDR at the regionaland/or global level. These extensions enhance users' ability to study thepotential roles of CDR in climate policy.

Making informed future decisions about solar radiation modification (SRM; also known as solar geoengineering)-approaches such as stratospheric aerosol injection (SAI) that would cool the climate by reflecting sunlight-requires projections of the climate response and associated human and ecosystem impacts. These projections, in turn, will rely on simulations with global climate models. As with climate-change projections, these simulations need to adequately span a range of possible futures, describing different choices, such as start date and temperature target, as well as risks, such as termination or interruptions. SRM modeling simulations to date typically consider only a single scenario, often with some unrealistic or arbitrarily chosen elements (such as starting deployment in 2020), and have often been chosen based on scientific rather than policy-relevant considerations (e.g., choosing quite substantial cooling specifically to achieve a bigger response). This limits the ability to compare risks both between SRM and non-SRM scenarios and between different SRM scenarios. To address this gap, we begin by outlining some general considerations on scenario design for SRM. We then describe a specific set of scenarios to capture a range of possible policy choices and uncertainties and present corresponding SAI simulations intended for broad community use.

Responding to climate change will involve some combination of mitigation (emissions abatement), carbon dioxide removal, adaptation, rectification, and perhaps solar geoengineering. This chapter surveys the moral or ethical reasons for climate action, as they relate to climate policy, including both consequentialist and justice- or rights-based approaches, and the implications of those moral justifications for different ways of addressing climate change.

The Intergovernmental Panel on Climate Change assessments (IPCC) Special Report on 1.5 °C of global warming is clear. Nearly all pathways that hold global warming well below 2 °C involve carbon removal (IPCC, 2015). In addition, solar geoengineering is being considered as a potential tool to offset warming, especially to limit temperature until negative emissions technologies are sufficiently matured (MacMartin et al., 2018). Despite this, there has been a reluctance to embrace carbon removal and solar geoengineering, partly due to the perception that these technologies represent what is widely termed a “moral hazard”: that geoengineering will prevent people from developing the will to change their personal consumption and push for changes in infrastructure (Robock et al., 2010), erode political will for emissions cuts (Keith, 2007), or otherwise stimulate increased carbon emissions at the social-system level of analysis (Bunzl, 2008). These debates over carbon removal and geoengineering echo earlier ones over climate adaptation. We argue that debates over “moral hazard” in many areas of climate policy are unhelpful and misleading. We also propose an alternative framework for dealing with the tradeoffs that motivate the appeal to “moral hazard,” which we call “risk-response feedback.”

Understanding possible climate futures that include carbon dioxide removal (CDR) and solar radiation modification (SRM) requires thinking not just about staying within the remaining carbon budget, but also about politics and people. However, despite growing interest in CDR and SRM, scenarios focused on these potential responses to climate change tend to exclude feedbacks between social and climate systems (a criticism applicable to climate change scenarios more generally). We adapted the Manoa MashUp method to generate scenarios for CDR and SRM that were more integrative, creative, and dynamic. The method was modified to identify important branching points in which different choices in how to respond to climate change (feedbacks between climate and social dynamics) lead to a plurality of climate futures. An interdisciplinary group of participants imagined distant futures in which SRM or CDR develop into a major social-environmental force. Groups received other "seeds" of change, such as Universal Basic Income or China's Belt and Road Initiative, and surprises, such as permafrost collapse that grew to influence the course of events to 2100. Groups developed narratives describing pathways to the future and identified bifurcation points to generate families of branching scenarios. Four climate-social dynamics were identified: motivation to mitigate, moral hazard, social unrest, and trust in institutions. These dynamics could orient toward better or worse outcomes with SRM and CDR deployment (and mitigation and adaptation responses more generally) but are typically excluded from existing climate change scenarios. The importance of these dynamics could be tested through the inclusion of social-environmental feedbacks into integrated assessment models (IAM) exploring climate futures. We offer a step-by-step guide to the modified Manoa Mash-up method to generate more integrative, creative, and dynamic scenarios; reflect on broader implications of using this method for generating more dynamic scenarios for climate change research and policy; and provide examples of using the scenarios in climate policy communication, including a choose-your-own adventure game called Survive the Century (https://survivethecentury.net/), which was played by over 15,000 people in the first 2 weeks of launching.

What if solar geoengineering were enacted not through careful intergovernmental deliberations or the actions of a rogue state, but by millions of private citizens taking matters into their own hands? This thought experiment—the subject of a scenario exercise at the Sixth International Geoengineering Governance Summer School—produced a range of critical reflections on international responses to such grassroots deployment as participant teams developed and critiqued governance proposals. Consideration of decentralized solar geoengineering, while unlikely, provides compelling insights for the governance of both irregular and more conventional geoengineering scenarios. Participants stressed that concerns with the collecting and reporting of data, hemispheric imbalances, and multidirectional questions of legitimacy are likely to arise in such contexts. Grassroots activity could further serve to galvanize wider state-led deliberations around geoengineering governance, forcing—but not necessarily deciding—the question of whether and how solar geoengineering should be implemented.

This article adds conceptual discipline to a well-rehearsed but largely intuitive argument within the climate engineering community that carbon dioxide removal (CDR) and solar radiation management (SRM) should be treated separately - 'split' rather than 'lumped' - in policy discussions. Specifically, we build the first, theoretically derived argument for 'splitting'. We do this by engaging a set of theoretical insights from the international relations literature, having to do with the relationship between problem structure and institutional design. Centrally, we apply some key elements of problem structure - which allows us to compare policy issues along variables such as geographic scope, costs, and actor number and asymmetries - to the cases of SRM and CDR. By analyzing their problem structures, we demonstrate that SRM and CDR are different in ways that are likely to yield different state preferences for institutional design, and thus policy proposals that split SRM and CDR are more likely to be adopted by states. In short, we construct a theoretical argument for 'splitting' SRM and CDR governance in global policy discussions.

Carbon capture and storage (CCS) remains a polarizing issue in climate policy. In a recent One Earth review, Martin-Roberts et al. examine why CCS failed to deliver on early promises and how it might nevertheless help mitigate climate change. Here I synthesize their findings and discuss implications for a just transition.

Carbon dioxide removal (CDR) is often characterized as separate from climate change mitigation. Discussion of CDR governance – despite enjoying growing interest – tends to overlook how key provisions on mitigation apply. Similarly, many climate policy processes have ignored CDR. CDR may have been discursively held separate from ‘mitigation’ due to a partial conceptual overlap with ‘geoengineering’. We unpack how the ‘mitigation of climate change’ – as defined in the United Nations Framework Convention on Climate Change and its Paris Agreement – includes CDR as defined by the Intergovernmental Panel on Climate Change. We point to important implications and opportunities for strengthening governance by enhanced clarity regarding parties’ obligations, principled equitable distribution of removal efforts, prioritization of rapid emissions reductions and careful paths to long-term removals

Over the past decade or so, several commentators have called for mission-driven research programs on solar geoengineering, also known as solar radiation management (SRM) or climate engineering. Building on the largely epistemic reasons offered by earlier commentators, this paper argues that a well-designed mission-driven research program that aims to evaluate solar geoengineering could promote justice and legitimacy, among other valuable ends. Specifically, an international, mission-driven research program that aims to produce knowledge to enable well-informed decision-making about solar geoengineering could (1) provide a more effective way to identify and answer the questions that policymakers would need to answer; and (2) provide a venue for more efficient, effective, just, and legitimate governance of solar geoengineering research; while (3) reducing the tendency for solar geoengineering research to exacerbate international domination. Thus, despite some risks and limitations, a well-designed mission-driven research program offers one way to improve the governance of solar geoengineering research relative to the 'investigator-driven' status quo.

Carbon dioxide removal (CDR) is rising up the climate-policy agenda. Four principles for thinking about its role in climate policy can help ensure that CDR supports the kind of robust, abatement-focused long-term climate strategy that is essential to fair and effective implementation.

This article identifies diverse rationales to call for anticipatory governance of solar geoengineering, in light of a climate crisis. In focusing on governance rationales, we step back from proliferating debates in the literature on 'how, when, whom, and where' to govern, to address the important prior question of why govern solar geoengineering in the first place: to restrict or enable its further consideration? We link these opposing rationales to contrasting underlying visions of a future impacted by climate change. These visions see the future as either more or less threatening, depending upon whether it includes the possible future use of solar geoengineering. Our analysis links these contrasting visions and governance rationales to existing governance proposals in the literature. In doing so, we illustrate why some proposals differ so significantly, while also showing that similar-sounding proposals may emanate from quite distinct rationales and thus advance different ends, depending upon how they are designed in practice.

Carbon dioxide removal and climate change adaptation are rarely analyzed together, yet it is critical to consider the interactions between these forms of climate response. We identify ways to foreground adaptation in carbon removal policies and project designs and to incorporate carbon removal into adaptation efforts. Attempts at aligning adaptation and carbon removalmay genuinely increase adaptive capacity or introduce new vulnerabilities, depending on policy and project design. Based upon four case studies of addressing adaptation needs with adaptive carbon removal, we find that effective implementation is likely to hinge upon predictable climate policy, innovative technical policies such as rigorous life-cycle assessment, and project design with local ecological conditions in mind. We propose three simple principles for integrating carbon removal and adaptation: identify opportunities for adaptive carbon removal in planning, prioritize adaptive value of projects, and give credit for carbon removed.

Solar geoengineering research in the social sciences and humanities has largely evolved in parallel with research in the natural sciences. In this article, we review the current state of the literature on the ethical, legal, economic, and social science aspects of this emerging area. We discuss issues regarding the framing and futures of solar geoengineering, empirical social science on public views and public engagement, the evolution of ethical concerns regarding research and deployment, and the current legal and economic frameworks and emerging proposals for the regulation and governance of solar geoengineering.

Solar radiation management (SRM) technologies would reflect a small amount of incoming solar radiation back into space before the radiation can warm the planet. Although SRM may emerge as a useful component of a global response to climate change, there is also good reason for caution. In June 2017, the Academic Working Group on Climate Engineering Governance released a policy report, “Governing Solar Radiation Management”, which developed a set of objectives to govern SRM in the near-term future: (1) keep mitigation and adaptation first; (2) thoroughly and transparently evaluate risks, burdens, and benefits; (3) enable responsible knowledge creation; and (4) ensure robust governance before any consideration of deployment. To advance the governance objectives identified above, the working group developed twelve recommendations, grouped into three clusters: (1) create politically legitimate deliberative bodies; (2) leverage existing institutions; and (3) make research transparent and accountable. This communication discusses the rationale behind each cluster and elaborates on a subset of the recommendations from each cluster.

Talk of removing carbon dioxide from the atmosphere is spilling out of scientific circles and into the realm of climate politics and policy. Civil society has a vital role to play in ensuring that this conversation is responsive to social needs and develops in constructive and appropriate ways. This report provides a starting point to help environmental and social justice non-governmental organizations (NGOs) engage in the growing conversation about carbon removal. Section 1 of the report argues that the world needs to grapple seriously with large-scale carbon removal and long-term storage, exploring both the rationale for considering carbon removal and the associated risks and downsides. Section 2 provides a brief introduction to the range of options for large-scale carbon removal. Section 3 surveys the existing conversation in different domains, from academia to philanthropy to civil society. Section 4 considers the relationship between carbon removal and mitigation. Section 5 sketches a research agenda for assessing various approaches to carbon removal.