Summary

Anthropogenic climate change is creating impacts that are widespread and severe—and in many cases irreversible—for individuals, communities, economies, and ecosystems around the world. Without decisive action and rapid stabilization of global temperature, risks from a changing climate will increase in the future, with potentially catastrophic consequences. Limiting future warming to substantially less than 2°C above preindustrial levels requires dramatic decreases in the emissions of all greenhouse gases (GHGs) from human activity, with net emissions of carbon dioxide (CO₂) falling to zero in the second half of the 21st century. The potential to rapidly decrease GHG emissions is real, but progress in realizing that potential is insufficient, and global GHG emissions continue at very high levels. In light of these urgent concerns and limited progress with solutions, it is important to have a comprehensive understanding of the feasibility and potential risks and benefits—and consequences for diverse stakeholders—of the wide range of possible policy responses to climate change.

Meeting the challenge of climate change requires a portfolio of options. The centerpiece of this portfolio should be reducing GHG emissions, removing and reliably sequestering carbon from the atmosphere, and pursuing adaptation to climate change impacts that have already occurred or will occur in the future. Concerns that these three options together are not being pursued at the level or pace needed to avoid the worst consequences of climate change—or that even if vigorously pursued will not be sufficient to avoid the worst consequences—have led some to suggest the value of exploring additional response strategies. This includes solar geoengineering (SG),¹ which refers to attempts to moderate warming by increasing the amount of sunlight that the atmosphere reflects back to space or by reducing the trapping of outgoing thermal radiation (see Box S.1 and Figure S.1). To be effective, these SG strategies (like all climate change response efforts) would need to be continuously maintained for very long periods of time.

___________________

¹ See Box 1.1 for a discussion of the committee’s consideration of terminology and rationale for using the term “solar geoengineering” for this report. Many other types of climate intervention strategies have been proposed by investigators around the world—including numerous ways to alter the reflectivity (albedo) of Earth’s land, ocean, and ice surfaces.This particular study focuses specifically on atmospheric-based interventions—both because these strategies are a source of growing research interest and because they pose particularly large governance challenges, given the inherently transboundary, global nature of such interventions.

The available research indicates that SG could reduce surface temperatures and potentially ameliorate some risks posed by climate change (e.g., to avoid crossing critical climate “tipping points”; to reduce harmful impacts of weather extremes). Yet these interventions could also introduce an array of potential new risks, for instance, related to critical atmospheric processes (e.g., loss of stratospheric ozone); important aspects of regional climate (e.g., behavior of the Indian monsoon); or numerous interacting environmental, social, political, and economic factors that can interact in complex, potentially unknowable ways. Predicting and attributing some types of risks, such as the occurrence of and impacts from extreme weather events, will be particularly challenging in the face of significant natural variability. In addition to the concerns about specific harmful impacts, some objections to SG are based on more general ethical concerns, such as the controllability and ownership of the technologies, the lack of agency by those who may be affected by these technologies, and path dependencies that may shape the overall climate change research portfolio.

SG could potentially offer an additional strategy for responding to climate change but is not a substitute for reducing GHG emissions. This is in part because SG

• does not address the underlying driver of climate change (increasing GHG concentrations in the atmosphere) or the key impacts of rising atmospheric CO₂ such as ocean acidification;

• raises concerns about new risks, uncertainties, and unintended impacts on natural ecosystems, agriculture, human health, and other critical areas of concern for society;
• cannot provide a reliable means to restore global or regional climate to some desired prior state; and
• entails unacceptable risk of catastrophically rapid warming if the intervention were ever terminated (if it were used to offset a large amount of warming without simultaneously deploying measures to reduce GHG emissions).

The National Research Council report Climate Intervention: Reflecting Sunlight to Cool Earth (NRC, 2015) reviewed the state of the science and provided high-level findings and recommendations regarding SG methods. This current study was tasked to update the 2015 assessment of the state of understanding and to provide recommendations for how to establish a research program, what to encompass in the research agenda, and what mechanisms to employ for governing this research (see Appendix A for the full Statement of Task). This report draws upon input from numerous experts and stakeholders invited to participate in committee meetings, workshops, and webinars (see Appendix B), as well as a review of the literature and extensive deliberations among the authoring committee.

CURRENT LANDSCAPE FOR SOLAR GEOENGINEERING RESEARCH AND RESEARCH GOVERNANCE

Understanding of some important questions about SG has advanced as a result of research conducted to date, but, at present, this state of understanding remains limited. For instance, with regard to our understanding of the efficacy and impacts of the different SG strategies considered herein, existing physical science research suggests the following:

• SAI. There is substantial modeling and empirical evidence (using volcanic eruptions as a natural analog) that SAI can induce cooling at a global scale, but large uncertainties remain regarding the cooling potential with injection amount, location, and type and regarding the effects of an increased aerosol burden on atmospheric chemistry, transport, and resulting regional and local effects on climate; these contribute to uncertainty in climate response and resulting impacts around the world.
• MCB. Research to date has made clear that adding aerosols to marine clouds can increase cloud reflectivity in some circumstances, and this phenomenon is commonly observed in studies of ship tracks. However, our limited under-

• standing of aerosol/cloud interactions leads to large uncertainty regarding where and by how much cloud albedo can be modified and whether feedback processes will mask or amplify some of the effects. The key processes occur at scales too small to include directly in global climate models, and these process uncertainties will need to be reduced in order to develop reliable large-scale climate impact projections.
• CCT. The efficacy of CCT is currently unknown due to very limited understanding of cirrus cloud properties and the microphysical processes determining how cirrus may be altered. The few existing climate model simulations of CCT have yielded contradictory results because of these uncertainties.

SG research to date is ad hoc and fragmented, with substantial knowledge gaps and uncertainties in many critical areas. There is a need for greater transdisciplinary integration in research, linking physical, social, and ethical dimensions, and inclusion of robust public engagement. There is also a need to expand demographic diversity and inclusiveness among the research community itself.

Research to understand the potential magnitude and distribution of SG impacts—on ecosystems, human health, political and economic systems, and other issues of societal concern—is in a particularly nascent state. Studies published to date do not provide a sufficient basis for supporting informed decisions. The vast majority of research in the natural sciences has focused on climate modeling studies, with large uncertainties in how well climate models can represent some key processes. Social sciences research has been a mix of theoretical and empirical studies, with limited diversity in the participants engaged.

There is currently no coordinated or systematic governance of SG research. Various legal mechanisms developed primarily with other contexts in mind could apply to some aspects of this research, but these mechanisms focus only on concerns about physical impacts. The fragmented state of this research and research governance landscape is a barrier to the effective advancement of knowledge and the associated reduction of uncertainties.

THE CONTEXT AND KEY CONSIDERATION FOR SOLAR GEOENGINEERING RESEARCH

A range of intertwined scientific, societal, and governance issues makes the SG decision space particularly complex, similar in some respects to other emerging technologies (e.g., nanotechnology, synthetic biology, artificial intelligence, robotics, or autonomous vehicles). Factors such as uncertainty in the scope and magnitude of the approaches

under consideration; the lack of social consensus around whether and how to pursue SG research; the relationship with other climate response strategies; and the global, intergenerational dimensions of SG make these emerging technologies challenging to consider.

Knowledge gained from a transdisciplinary SG research program will be critical for informing climate change response strategies, and evidence either in favor or disfavor of SG deployment could have profound value. Such knowledge could be time-critical for policy makers especially if there were intense public or political pressure for a dramatic climate action, or if SG were deployed in the absence of broad international cooperation and safeguards. The pursuit of an SG research program also brings potential risks—for instance, a program could be used as a rationale to undermine efforts to reduce GHG emissions, to legitimize SG as a response to climate change, or to create a community invested in moving toward deployment.

In designing a research program, it is important to take into consideration that research, technology development, and governance are often path dependent. Early decisions about how to structure and govern SG research may create momentum that shapes future research, development, and governance. Commitments to transparency, justice, and broad engagement in the design and implementation of research will facilitate institutionalization of these values and practices going forward.

A principal goal of any research program should be to better characterize and reduce scientific and societal uncertainties concerning the benefits and risks of SG deployment (relative to global warming in the absence of SG). However, there are limits on the level of uncertainty reduction that can be expected, and it is possible that additional research may expand particular uncertainties or reveal new uncertainties, especially for complex interacting factors such as high-resolution spatial patterns of impacts, indirect effects, socioeconomic and political or institutional responses over multidecadal timescales, and attribution for climate- and weather-related extremes. It is also important to recognize that research cannot resolve differences in fundamental values or worldviews among individuals or countries (e.g., regarding what level of certainty is sufficient for making decisions) and that most decision-making processes incorporate many considerations beyond just scientific research results.

Earlier analyses converge on several salient principles for SG research, notably, calling for research and research governance approaches that are

• in the interest of advancing the public good;
• aimed at advancing knowledge while taking into account societal norms and perspectives;
• coordinated and cooperative;

• adaptive and subject to ongoing assessment, check-points, and, if needed, exit ramps;
• inclusive and responsive, including engagement by diverse publics, stakeholders, and governments; and
• fair, equitable, and transparent.

In order to advance these principles, it is important to have a research program that is transdisciplinary, international, and diverse with respect to disciplines and methods, researchers, countries, and perspectives represented and to have research governance strategies that aim to build trust and legitimacy.

PROPOSED FRAMEWORK AND APPROACH FOR SOLAR GEOENGINEERING RESEARCH AND RESEARCH GOVERNANCE

An organized research program can help build the foundation of scientific insights and information that will help decision makers and stakeholders faced with choices about possible future implementation of SG. It is important, however, that such a program ensures that the information developed is as robust as possible, with significant attention to meaningful inclusivity and strong governance strategies. To this end, the committee envisions an integrated framework, illustrated in Figure S.2, that would enable research governance and research activities to evolve hand-in-hand, with ongoing mechanisms for stakeholder engagement and input. This engagement, combined with periodic programmatic assessments, could allow a research program to be responsive to new findings and developments that arise as the program and the knowledge base evolve.

The stepwise, iterative nature of this framework is paramount. Given the many complex features of SG, business-as-usual pathways for establishing a research program will not suffice. Understanding of how to design a robust program that meets all the principles and goals recommended herein is in a nascent state; thus, a research program needs to be sufficiently flexible to allow for improvements and adjustments as understanding grows. The committee offers suggestions for the rough contours of a program but at the same time suggests that expanding engagement with stakeholders around the world will help fill gaps in understanding and perspective.

The committee approached SG research design and coordination from the starting point of efforts within the United States, a choice based largely on practical considerations. Operationally, research agencies of the U.S. federal government already have extensive experience supporting global change research and coordinating that research across agencies; many (although not all) features of SG research can fit into the framework for existing global change research.

SG research and research governance efforts to date have been ad hoc and dispersed. There would be significant value in pursuing more active integration across key research areas—such as modeling, observations, process studies, social and economic studies, and scenario designs—to ensure that research being conducted informs and is informed by other research as efficiently as possible. The United States does not currently have a coordinated federal approach to SG research. Building an effective, transdisciplinary research program will require coordination across multiple agencies, national laboratories, and cooperative institutes. The U.S. Global Change Research Program (USGCRP), charged with coordinating federal global change research across the federal science agencies, is the most logical entity for orchestrating an SG research program.

ROBUST GOVERNANCE FOR SOLAR GEOENGINEERING RESEARCH

The goals of research governance include advancing and coordinating appropriate research, facilitating inclusive and equitable public and stakeholder engagement, and addressing physical risks together with social, ethical, and legal concerns. Table S.1 provides an overview of the governance mechanisms discussed in Chapter 5 of this report, goals and/or principles that they foster, and actors for the chapter’s governance recommendations.

TABLE S.1 Governance Mechanisms Discussed in This Report

Existing U.S. laws and regulations are potentially relevant to SG research but were not crafted with SG research in mind. Laboratory and modeling studies generally would not trigger the application of existing environmental laws. The application of environmental statutes to SG field research would depend on the nature of the research, its location, and the materials used and released. Tort law serves as another potential mechanism for governance of research. For any of these mechanisms, the focus is on physical impacts, not broader social or ethical concerns that frequently surround SG research. Current international law provides a general framework, but it does not explicitly promote, prohibit, or significantly limit SG research; nor does it provide a system of required or recommended research transparency or reporting mechanisms. Some existing international conventions and agreements have explicitly attempted to address geoengineering or could in principle form part of a global system of international SG governance.

This report provides recommendations related to the governance mechanisms identified in Table S.1, many of which could be adopted at both national and international levels. However, it is arguably the case that international governance should not begin with current treaty bodies. With a few important exceptions, global agreements have tended to evolve out of domestic laws and regulations, which currently do not exist for SG. Additionally, attempts at international governance, especially on emerging issues, must confront the reality that achieving multilateral consensus is difficult and that initial multilateral agreements are often weak. At the same time, however, international cooperation among researchers in different countries can still develop in the absence of formal global research governance, providing valuable conduits for information sharing and cooperation.

Simultaneous domestic and international efforts may increase the effectiveness and likelihood of achieving meaningful governance. Governance mechanisms and principles developed domestically can help inform policy makers developing international architectures; in turn, international governance can help reinforce domestic efforts and create expectations of stronger domestic enforcement. Unless and until robust international research governance emerges, it is incumbent on any country where SG research is being conducted to create mechanisms and institutions to govern this work. While international governance practices and institutions ideally should be created as soon as possible, in reality, such mechanisms may emerge only after responsibility has been embraced at the national level and there is commitment by more countries to engage with research, deter unsafe research activities, and regulate activities with potentially significant transboundary impacts.

AN INTEGRATED AGENDA FOR SOLAR GEOENGINEERING RESEARCH

SG research encompasses a diverse array of topics, each involving multiple avenues of investigation. The boundaries between these different research “clusters” are often blurred, with many questions that cut across different types of research. The committee found it useful to organize these research needs under three broad categories:

• Context and Goals for SG Research. This category encompasses studies that help better characterize the context for SG research, development, and possible deployment—with the aim of better understanding the evolving “decision space” for these activities. It includes efforts to clarify the range of possible goals for an SG program, to understand how these goals shape research priorities, to guide development of modeling scenarios, and to identify key considerations for decision making. This area of research will advance exploration of whether and how SG can be developed to generate broadly beneficial outcomes and how to build the capacity needed for countries to engage meaningfully in SG research and research governance.
• Impacts and Technical Dimensions. This category encompasses research to understand the technical feasibility of using different SG strategies to achieve regional-to-global-scale cooling, including chemistry and microphysics research to understand the properties of injected reflective particles and their interactions with clouds and other atmospheric processes; and it

• includes research to understand the possible outcomes for other key climatic variables (e.g., precipitation or wind patterns) and subsequent impacts on human health and numerous ecological and societal systems. It also includes engineering studies of the technical requirements for different SG technologies and research to advance critical monitoring and attribution capabilities.
• Social Dimensions. This category encompasses a wide array of research exploring how to better understand public perceptions of SG research and its possible future deployment; how to fairly govern and effectively engage various publics and stakeholders in SG research, development, and deployment decisions; how to approach domestic and international conflict and cooperation in the SG arena; and how to integrate justice, ethics, and equity considerations.

These diverse research areas need to be investigated in an integrated and interactive manner (see Figure S.3). The proposed research needs vary considerably among the different clusters, reflecting the fact that some research topics are more nascent than others and that there are inherent variations in how different types of research are conducted. The research priorities range from computer modeling and laboratory and field studies to quantitative and qualitative social science investigations—and thus the nature of the steps forward differ accordingly.

CONSIDERATIONS FOR DELIBERATE OUTDOOR EXPERIMENTS

Limited outdoor experimentation could help advance the study of certain core atmospheric processes that are critical for understanding SG. Such activities, however, are controversial, posing significant potential for public concerns and objections. It is the committee’s judgment that, subject to appropriate governance and oversight, outdoor experimentation could feasibly be pursued in a balanced manner that is sufficient in scale to acquire critical observations not available by other means but that is small enough in scale to limit impacts.

The committee considered how to set outdoor experimentation thresholds that address both the impacts of the potential perturbation on the climate and the impacts of the test materials on the environment. The recommended thresholds are intended to err on the conservative side—for instance, limiting the amount of material emitted per experiment to significantly less than other commonly accepted anthropogenic emissions to the atmosphere (e.g., from fireworks or commercial aircraft). Experiments that meet the proposed limitations would be considered on a case-by-case basis, in light of relevance to open questions, expected benefits of the study outcomes, and timing relative to other steps—and subject to approval based on the governance guidelines adopted by the larger SG program.

FUNDING CONSIDERATIONS FOR SOLAR GEOENGINEERING RESEARCH

Implementing the recommended research and research governance will require dedicated resources. To help inform planning for a national SG research program, the committee offers a set of general guidelines for shaping the budget:

• Funding for SG research should not shift the focus from other important global change research, and it should recognize the risk of exacerbating concerns about a slippery slope toward deployment. These guidelines imply that the near-term budget for SG research should be small relative to the overall investment in global change research.
• The research program should support equitably all of the research clusters discussed in this chapter from the outset.
• The budget should be able to accommodate major field campaigns, should proposals for such campaigns meet other requirements outlined in Recommendation 6.2.
• A substantial fraction of the research program should be dynamically allocated in order to allow the research program to flexibly adapt as learning proceeds.
• Research funding should be accompanied by support for implementing research governance and public engagement.

The committee suggests that a reasonable initial investment in SG research is in the range of $100–200 million total over 5 years. A research program of this size would be sufficient to advance all the research topics identified in Recommendation 6.1 but would still represent a small fraction of the national budget for climate change research. The budget for this research program would start small and increase over time, allowing for a thoughtful process of building capacity, adapting plans based on new information, and developing a research community over time.

In addition to funding research itself, support is needed for implementing robust research governance at national and international scales and for public engagement. The committee suggests as a general rule of thumb that these governance and engagement efforts be supported at approximately 20 percent of the level of the total research program support—an investment that would scale with the overall size of the research program.

CONCLUDING THOUGHTS

Based on the evidence available to date, there are indications that SG has the potential to lower Earth’s surface temperature. But there are also indications from research to

date that SG could have unintended negative consequences, for example, providing a rationale to alter GHG mitigation commitments, or consequences for society and ecosystems that arise from unfavorable changes in rainfall or temperature extremes. Much of the discomfort about an SG research program relates to the concern that even early results of the research might serve as a rationale to narrow the range of options going forward and that promising findings could increase pressure for deployment.

For an SG research program to be truly effective, it will need to be structured to minimize the risk that findings from a single discipline, a few studies, or a small slice of the research agenda will drive pressure for or against deployment. In particular, this will require a research program that, from the outset, examines not only the atmospheric effects of SG but also the ecosystem, and economic, political, and ethical implications. It will require a research program and culture without bias or advocacy for any particular outcome, with equal consideration of the factors that make SG either an unattractive or an attractive option. Designing such a program will require a deep commitment to exploring the full range of possible effects, avoiding the temptation to classify indirect effects as secondary or unimportant, and managing adaptively so that the program is shaped by ongoing discoveries.