Climate change is a defining challenge of the 21st century. Since the beginning of the industrial revolution, Earth’s average surface temperature has warmed by more than 1°C. This warming has already resulted in impacts on every continent and in the oceans. Observed impacts range from more frequent heat waves to increased coastal flooding associated with rising sea level (Herring et al., 2019; IPCC, 2014b). Warming temperatures are changing the distribution and composition of ecosystems, shifting cropping seasons and cultivars, and causing intensified conflicts over water resources. In the oceans, the warming and increased acidification caused by rising carbon dioxide (CO2) levels is damaging coral reefs, with Australia’s Great Barrier Reef experiencing its third major bleaching event in the past 5 years. Powerful new analytical techniques are revealing impacts of warming that has already occurred on crop yields, wildfires, and economic inequality (Abatzoglou and Williams, 2016; Diffenbaugh and Burke, 2019; Duffy et al., 2019).
The impacts depend on the amount of warming that occurs, with risks that are widespread, severe, and irreversible (IPCC, 2014b). If the global mean surface temperature rise were limited to 1.5°C, many risks would be substantially moderated (IPCC, 2018). Risks rise rapidly based upon further warming, and some risks may reach high levels even if warming is limited to 2°C. Stabilizing global temperature requires decreasing net emissions of CO2 to zero. Because the warming effects of CO2 persist for thousands of years, every ton emitted pushes the temperature higher, and the resulting temperature is a nearly linear function of cumulative CO2 emissions since the beginning of the industrial revolution (IPCC, 2013).
The main lever one has for limiting warming is to constrain net emissions of CO2 and other greenhouse gases (GHGs). Dedicated efforts to remove CO2 from the atmosphere through natural or industrial processes can offset GHG emissions. These “negative emissions” strategies have the potential to be important parts of the climate solutions portfolio, and their development is advancing rapidly (Davis et al., 2018; IPCC, 2014a), but there remain many unanswered questions about capacity, cost, and unintended consequences (NASEM, 2019a). The challenge of decarbonization is also complicated by the long lifetimes and the high retirement costs of fossil infrastructure (Davis et al., 2010).
Meanwhile global anthropogenic (human-caused) GHG emissions are continuing to increase. In 2019, global CO2 emissions were projected to reach an all-time high of ~37
gigatons (GT) of CO2 from fossil sources and 6 GT from land-use change (GCP, 2020). Emissions of other GHGs add to the forcing of climate change, amplifying the warming effect by about one-third over that of CO2 alone. The Intergovernmental Panel on Climate Change (IPCC) analyses suggest that total allowable emissions cannot exceed 420 GT CO2 (post-2017) in order to have a substantial probability (66 percent chance) of stabilizing warming at 1.5°C or less. Based on 2019 emission rates, this emission total will be exceeded in less than a decade (IPCC, 2018). The emissions limit for stabilizing at 2°C is somewhat larger (allowing an additional 800 GT CO2); reaching either target requires rapid and sustained emissions reductions, on the order of halving emissions every decade (Rockström et al., 2017).
Meeting the challenge of climate change requires a portfolio of options. This portfolio must involve reducing GHG emissions to the atmosphere (mitigation), and removing carbon from the atmosphere and reliably sequestering it. In addition, it must involve adaptation to climate change impacts that have already occurred or will occur in the future. But given the possibility that these three options will not be pursued swiftly or broadly enough to provide sufficient protection against unacceptable climate change impacts, some suggest there may be value in exploring additional response strategies—including possible strategies to moderate warming by altering the abundance or properties of small reflective particles (aerosols) or droplets in the atmosphere or by modifying cloud properties. In 2015, the National Academies released Climate Intervention: Reflecting Sunlight to Cool Earth (NRC, 2015), which reviewed the state of the science and provided high-level findings and recommendations on this set of possible strategies.
Two of the main conceptual approaches for reflecting sunlight involve increasing the reflection of solar radiation away from Earth. Stratospheric aerosol injection (SAI) proposes to accomplish this through increasing the number of small reflective particles in the stratosphere. Marine cloud brightening (MCB) focuses on increasing the abundance or reflectivity of clouds over particular parts of the oceans. Cirrus cloud thinning (CCT), the third approach, aims to modify the properties of high-altitude clouds, increasing the atmosphere’s transparency to outgoing thermal radiation.1
The available research indicates that such approaches have the potential to reduce temperature and ameliorate some risks of climate change, but they also might introduce an array of potential risks. Such risks could be related to processes in the atmosphere (e.g., ozone loss from SAI); important aspects of regional climate (e.g., behavior of the Indian monsoon); or numerous environmental, ethical, social, political, and
1 While CCT is not truly a “reflecting sunlight” strategy like SAI and MCB, it is sufficiently related to these other methods to be included in the study scope.
economic factors that can interact in complex, potentially unknowable ways. The NRC (2015) study committee highlighted two potential risks in particular. First is the concern that with a heavy concentration of physical climate modeling research (relative to a focus on broader solar geoengineering [SG] impacts), enthusiasm for SG deployment
might get ahead of the research. Second is the concern that SG deployment might be inexpensive enough that it could potentially be undertaken by a single nation or other actor, thus pointing to needs for rapid detection and attribution methods.
These different types of risks are highly diverse and likely to be perceived very differently across nations, communities, and individuals. Moreover, one does not (and, indeed, cannot) know the future climatic and sociopolitical conditions under which expanded SG research or potential deployment might be considered, and how the differing types of risks will be perceived by future decision makers and society at large. Very little research to date has attempted to address the full cascade of potentially interacting processes.
1.1 ORIGINS OF THIS STUDY
NRC (2015) made six recommendations. Recommendation 1 discusses strategies that should be the core of the climate solutions portfolio—emissions reduction and adaptation. Recommendation 2 speaks to the importance of additional research and development on technologies for CO2 removal. Recommendation 3 states that albedo modification at scales sufficient to alter climate should not be deployed at this time. Recommendation 4 argues for a research program on albedo modification, pointing to the potential for research targeted at advancing fundamental knowledge as well as evaluating potential applications. Recommendation 5 emphasizes the importance of improving monitoring of the atmospheric radiation budget as a strategy for detecting secret deployments. Recommendation 6 points to the need for a serious deliberative process to explore and develop appropriate mechanisms for governing SG research.
As a follow on to NRC (2015), the National Academies of Sciences, Engineering, and Medicine launched the present study to develop a research agenda and recommend research governance approaches for SG intervention strategies, focusing on SAI, MCB, and CCT. The study was deliberately designed to address research needs and research governance in tandem, such that the understanding and thinking on each informs the other. This study considers transdisciplinary research2 that integrates understanding across factors such as the baseline chemistry, radiative balance, and other characteristics of the atmosphere; potential impacts (both positive and nega-
2 As described in Toomey et al. (2015), whereas multidisciplinary research draws on knowledge from different disciplines, and interdisciplinary research synthesizes and harmonizes links between disciplines, transdisciplinary work moves beyond this bridging of divides within academia to also engage directly with the production and use of knowledge outside of the academy. Societal impact is a central aim of the research.
tive) of SG interventions on the atmosphere, climate system, natural and managed ecosystems, and human systems; the technological feasibility of these interventions; detection and monitoring of such impacts; ethical implications and public perceptions of SG research and possible deployment; and optimal strategies for governing such activities. The study explores and recommends appropriate research governance mechanisms at international, national, and sub-national scales, as well as self-governance by the research community. It considers the research governance that already exists and lessons from research governance mechanisms currently being used or considered for other areas of scientific inquiry (see full Statement of Task in Appendix A).
This report is intended for the broadest range of audiences interested in SG. The committee’s focus was on research to support the information needs of those who may be involved in decisions about the scale, scope, direction, and organization of the SG research enterprise—including the appropriateness of certain kinds of studies, especially field experiments. Ultimately, SG research should help support decisions about whether or not to include these strategies in the portfolio of climate responses and even to understand who should be involved in these decision-making processes. As decision-making priorities evolve over time, this points to the need for a research portfolio that is iterative and adaptive in nature. Some of the information most relevant for policy decisions in this space can contribute to increasing our understanding of basic functions of Earth and its atmosphere, ecosystems, oceans, and societies; however, advancing “basic knowledge” was not the primary driver for the current study.
Funding for this study came from three very different types of entities. Reflecting its assessment of the importance of the topic, some funding came from the Arthur L. Day fund of the National Academy of Sciences. Four private foundations—the BAND Foundation, the Christopher Reynolds Foundation, the John D. and Catherine T. MacArthur Foundation, and the V. Kann Rasmussen Foundation—provided support. Three federal agencies also provided support for the study: the U.S. Department of Energy, the National Aeronautics and Space Administration, and the National Oceanic and Atmospheric Administration.
1.2 SCOPE AND MOTIVATION OF THIS REPORT
Available information is inadequate to provide the needed input to decisions about whether, when, and how SG should be included in the portfolio of climate response strategies, and a detailed agenda to define the relevant scientific research has thus far not been developed or implemented. A well-designed and well-governed research
program could provide a great deal of critical information, but such a research program entails risks if its focus is too narrow, if stakeholders are not appropriately engaged, or if research decision making is not sufficiently transparent or inclusive. There are inherent limits to the questions that a research program can resolve for decision makers—including values-based questions about whether or not society should use SG as an option in the future, how to balance trade-offs in the potential impacts of SG, and how much uncertainty in outcomes is tolerable for decision makers or broader society—but research can provide useful insights that help inform these difficult questions.
The components of an SG research program and the interactions among them depend upon the contexts (i.e., social, economic, cultural, technological, and ethical) in which the research unfolds. While some questions that must be addressed are purely technical (e.g., Could a particular technology, under ideal circumstances, change radiative forcing by some desired amount?), other questions involve complex interactions between physical and social dimensions (e.g., Is it possible to manage the risk of an unintended damaging change in regional rainfall?), or they involve ethical considerations (e.g., How should trade-offs be evaluated when SG might improve the welfare of many but erode the welfare of others?).
Defining a research framework broadly perceived as fair, especially for stakeholders who lack political power or financial resources, is a major challenge. An important element to consider is the approach used for evaluating benefits and risks. For instance, one possible approach, a risk-risk framework, sets the objective of evaluating the benefits and risks of a given action in comparison to the benefits and risks of alternative actions, or compared to no action. An underlying challenge of such evaluations is the landscape of deep uncertainties surrounding climate change and SG.
An SG research program can encompass elements as diverse as scenario development; modeling; laboratory studies; field studies; and socioeconomic, political, governance, ethical, and public perception studies. Data sources will range widely—from stakeholder interviews, to laboratory experiments, to observations collected from satellites, aircraft, and ships. Of all the possible lines of research, field experiments with controlled dispersal of particles raise especially challenging issues. Some researchers have proposed that small-scale field studies are already the logical next step to advance understanding, and a few research teams in the United States and elsewhere are moving forward with planning for field experiments. But there is scientific debate about whether small-scale field experiments can provide useful insights about large-scale deployment; the need for caution in pursuing such proposals has been raised by many. For instance, NRC (2015) recommended that field experiments designed to
inject material into the atmosphere should not proceed until key governance issues are addressed and appropriate structures are in place. Several nongovernmental organizations (NGOs) are on record as being strongly opposed to field experiments, while others accept them under highly specified conditions. Other perspectives point to the importance of suitable public engagement to explore whether there is “social license” to proceed with field experiments.
The committee’s recommendations are grounded in the conviction that in order to maximize scientific value and prospects for social acceptance, an SG research program needs to be highly interdisciplinary, open to broad participation, as transparent as possible, and structured to actively foster coordination and knowledge sharing across nations.
Several existing reports and organizations address aspects of SG governance. For example, groups of scholars have proposed principles and best practice guidelines for operating norms. The Carnegie Climate Governance Initiative is focused on catalyzing policy discussions with governments and in international bodies to expand understanding of SG risks and benefits, and to prevent deployment of these technologies without having effective governance in place. The Solar Radiation Management Governance Initiative is a partnership among several NGOs convening conversations about SG in countries around the world, with an emphasis on engaging developing country researchers. Yet despite these many efforts, and progress being made in expanding the community of scholars, policy makers, and NGOs engaged in this topic, discussions are still mostly in the early stages, and no consensus has yet been reached about protocols for research governance.
Governance of SG research will also need to deal with the opportunities and challenges associated with engagement of the private sector. Private-sector involvement in research and development can spur innovation, attract capital investment, and accelerate the development of effective and lower cost technologies. At the same time, however, there are concerns that for-profit efforts may neglect social, economic, and environmental risks, that research transparency will be compromised by data that are not open and accessible, and that some companies may develop financial interests in moving from research to deployment and seeking private ownership of globally relevant technologies.
1.3 SOLAR GEOENGINEERING IS NOT A SUBSTITUTE FOR MITIGATION
The starting position of the committee is that SG is not a substitute for mitigation, nor does it lessen the urgency for pursuing mitigation actions. Four main lines of evidence
underscore this position. The first is that SG does not address some of the key impacts of elevated CO2 concentrations, including impacts on ocean acidification (with ramifications for the structure and function of ocean ecosystems) and impacts on terrestrial plants (altering growth rates, competitive interactions, and crop nutritional values). Second, there is abundant evidence that SG cannot restore the climate with high fidelity to any specific prior state but rather leads to outcomes that differ from prior states in terms of spatial and temporal temperature and precipitation patterns, as well as extreme events, which introduce a new set of challenges all their own. Third, SG may lead to a variety of unintended consequences and impacts. Fourth, offsetting a large amount of warming through SG (something that might be advocated in the absence
of stronger future mitigation) requires that the intervention be sustained for very long periods of time and entails unacceptable risks of catastrophically rapid warming if the intervention were ever terminated.
This is a critical framing point for all discussion of SG research and research governance, stemming from not only technical calculations but also considerations about social acceptability, ethics and justice, and the other social dimensions discussed in this report. No matter what the research concludes, climate change mitigation must be a central element of society’s future. The goal for research is to determine whether SG can be a complement to mitigation, not a substitute, and whether and under what conditions it could be part of the portfolio of climate response strategies.
1.4 THE STUDY PROCESS
The study was developed and overseen jointly by the National Academies’ Board on Atmospheric Sciences and Climate and the Committee on Science, Technology, and Law. The members of the study committee had expertise in diverse areas such as atmospheric physics, chemistry, ecology, economics, policy studies, law, ethics, and international governance and negotiations. Several committee members have a long record of contributions to SG scholarship, while some were chosen to bring perspectives from other research domains.
The committee held five in-person meetings, during which (as per National Academies’ rules and procedures) all of the information-gathering sessions were open to the public, while internal deliberations and report writing were held in closed session. These included the following:
- Meeting #1 (April/May 2019; Washington, DC) included presentations from leading researchers, overviews of existing efforts to explore SG governance, input from project sponsors regarding their motivation for requesting this study, and presentations from stakeholders representing civil society, governments, and NGOs.
- Meeting #2 (August 2019; Boulder, CO) included a workshop to gather insights about the current state of SG research. Invited experts addressed the current status of modeling studies, observational studies, research on impacts across many sectors, and work on engineering development for relevant technologies.
- Meeting #3 (September 2019; Stanford, CA) included a workshop on research governance issues. Invited experts discussed questions about ethics and scientific responsibility, engagement and representation, governing research for collective benefit, perspectives on existing frameworks for SG governance, and lessons learned from governance of research in other complex, ethically fraught fields (e.g., related to biotechnology).
- Meeting #4 (October 2019; Washington, DC) and Meeting #5 (January 2020; Vancouver, BC, Canada) were closed to the public as the committee debated
- key report messages and supporting arguments and collaborated to develop text for this report.
The committee also held three virtual information-gathering sessions. One session focused on learning more about SG research activities being advanced in China and in Australia. Two other sessions were organized to seek insights from individuals who could offer “decision-maker” perspectives (based upon their experience as leaders in various national and international organizations) about the types of information they would need from the scientific community to help inform decisions related to SG research, research governance, and possible deployment. To aid these discussions, the committee developed a set of hypothetical scenarios about potential SG research and/or deployment for speaker consideration (see Appendix C).
The committee also received a wide variety of written input from interested organizations and individuals, which was reviewed and discussed among the group. These information-gathering steps were followed by several months of work (carried out by calls, emails, and other virtual means among subgroups and the full committee) to finish deliberations and to facilitate the process of completing its report. Following standard National Academies’ procedures, the draft report then underwent a rigorous process of external peer review prior to publication.
1.5 THE REPORT ROADMAP
The rest of the report is organized as follows:
Chapter 2 reviews the “landscape” of SG-related research (i.e., the current state of understanding and key knowledge gaps that need to be addressed—across both natural and social science realms), as well as the landscape of existing governance and legal structures that could be relevant to this research.
Chapter 3 explores the complex “decision space” surrounding this issue, including the types of information needed by decision makers; the many societal considerations that shape research and research governance planning; and the principles for SG research that have been highlighted in past work.
Chapter 4 presents the committee’s core recommendations for a national program of SG research and research governance, considering how such a program could be organized, managed, and funded.
Chapter 5 recommends key mechanisms to pursue, at national and international levels, for governance of SG research that help assure robust research oversight and
regulation and adherence to critical goals such as legitimacy, transparency, and stakeholder engagement.
Chapter 6 defines a broad transdisciplinary agenda for research to fill the key knowledge and information gaps identified in the earlier chapters and explores the special considerations related to outdoor experimentation.