Because of its transboundary effects and because states will be the primary actors, large-scale solar geoengineering and its governance are matters of international law. This is the second of four chapters that consider international legal rules, here regarding the climate and the atmosphere. Climate change is, and solar geoengineering would be, foremost atmospheric phenomena. The UN Framework Convention on Climate Change and its protocols – the central legal regime for international cooperation to limit climate change and its impacts – offer surprisingly limited guidance for solar geoengineering. However, the regime could provide an institutional site for future multilateral governance. Some provisions of the Vienna Convention for the Protection of the Ozone Layer and the Convention on Long-Range Transboundary Air Pollution (and their protocols), which regulate substances that contribute to stratospheric ozone depletion and to acid rain, respectively, would be applicable, depending on circumstances. The International Law Commission has approved Draft Guidelines on the Protection of the Atmosphere, one of which addresses activities aimed at its intentional large-scale modification.

Governance includes nonstate actors and nonlegal instruments. Nonstate governance – that which is developed, implemented, and/or enforced by nonstate actors – can fill roles that state law cannot or does so poorly. This chapter considers the extent to which nonstate actors do, could, and should contribute to solar geoengineering governance. It introduces key concepts of nonstate governance. The extant governance that is specific to solar geoengineering is largely nonstate. This is particularly evident in the development, influence, and apparent compliance with multiple sets of principles, such as the Oxford principles. Notably, for the most part, these sets substantively agree. The chapter closes with an analysis of nonstate governance’s potential. It concludes that nonstate governance should contribute because solar geoengineering’s characteristics – such as technically complexity, dynamism, reliance upon experts’ knowledge, transboundary impacts, and researchers' shared yet undifferentiated reputational sensitivity – are favorable to nonstate governance, while states are taking no significant steps toward governance.

Solar geoengineering holds the potential for both benefit and harm. Actors such as states could ask ex ante for assurances of compensation, possibly as a precondition for not opposing the activity, or demand ex post compensation for actual or claimed harm. Legal rules could indicate that those who conducted or approved an activity would be liable to pay damages. There could be a basis – at least in principle – in customary international law for state liability for transboundary harm caused by solar geoengineering that was contrary to international law. Although space-based solar geoengineering is presently prohibitively expensive, states would be strictly liable for harm arising from it. Compensation for other potential harm would face substantial political, institutional, and theoretical challenges, including what damages to compensate, the injurers’ and victims’ identities, and mechanisms and reasons for securing compensation. While recognizing states’ strong resistance to compensation, the chapter suggests an international compensation fund for harm from large-scale outdoor solar geoengineering research and offers initial thoughts regarding that from deployment.

Greenhouse gas emissions abatement, negative emissions technologies, and adaptation are not, and most likely will not, be enough to prevent dangerous climate change and its deleterious impacts on humans, other species, and ecosystems. Some scientists and others are increasingly considering and researching solar geoengineering, which would reflect or block some of the sun's incoming solar radiation, as a potential complementary response. This introductory chapter offers an initial explanation of climate change and solar geoengineering, including its leading proposed techniques of stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning. Solar geoengineering should be taken seriously, as its governance is both important and challenging. Among the major challenges is that solar geoengineering presents a high-stakes risk-risk tradeoff under conditions of great uncertainty. Another is that although earlier governance can be more effective, little is then known of such an emerging technology’s salient characteristics. The chapter outlines the topics covered by the remainder of the book and makes the author’s prior assumptions explicit.

Solar geoengineering is being considered and researched as a potential response to anthropogenic climate change. After exploring the causes and risks of climate change and other responses to it, this chapter describes solar geoengineering’s history and proposed methods, including stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning. The current evidence regarding their potential capabilities, costs, and technical feasibility is presented. Evidence from models and natural analogs indicates that a moderate deployment of solar geoengineering would globally reduce climate change. It also appears to be technically feasible, rapid in its effects, inexpensive in its direct deployment costs, and reversible. Among solar geoengineering’s physical risks are imperfect compensation of climatic changes and consequent residual climatic anomalies, delayed recovery of stratospheric ozone, and irresolvable uncertainty. Social challenges include decision-making regarding deployment, problematic uni- or minilateral implementation, strained international relations, displacement of emissions abatement, biased future decision-making, and disagreement regarding ethics.