Because of its transboundary effects and because states will be the primary actors, large-scale solar geoengineering and its governance are matters of international relations. The divergent problem structures among the responses to climate change help explain extant and likely action. Abatement and negative emissions technologies are aggregate effort global public goods and consequently undersupplied. Solar geoengineering would be a single best effort and a mutual restraint global public good, implying that it might be oversupplied. Uni- or minilateral deployment would be a problem if it were premature or contrary to the international community’s consensus. Solar geoengineering could pose other challenges to international relations such as legitimate decision-making, potential disagreements, cost-sharing, and security risks. Its sudden and sustained termination would have severe negative environmental impacts, but the probability of this is uncertain and may be low. Some economists have explicitly modeled states as rational actors to understand, explain, and predict their behavior in this domain. Given the approach’s limitations, these results should be interpreted with caution.

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 first of four chapters that consider rules from custom, treaty, and principles as well as international organizations. Although there are no international instruments that are legally binding, in force, and specific to solar geoengineering, international law provides both a substantial extant governance framework and a foundation upon which future norms, rules, procedures, and institutions specific to solar geoengineering could be built. This chapter introduces how international law operates and discusses several general international legal principles that would guide its interpretation and development with respect to solar geoengineering. It describes one source of binding international law – that of states’ customary behavior – and what it might mean for solar geoengineering, emphasizing procedural obligations. The chapter reviews some relevant nonbinding multilateral environmental agreements and activities of intergovernmental organizations, such as the Intergovernmental Panel on Climate Change.

This brief final chapter offers some closing observations concerning solar geoengineering. Environmentalism is a diverse domain of thought, and its central norms – such as whether humans’ ideal role is treading as lightly as possible or being stewards of managed nature – are sometimes in tension. Solar geoengineering lies, in many ways, astride this division and its controversy should not be surprising. Yet my central concern is that the solar geoengineering discourse is unduly driven by intuition and ideology instead of empiricism and rationality. The seeming neglect and possible dismissal of solar geoengineering is a gamble that emissions abatement, adaptation, and negative emissions technologies will be sufficient to prevent dangerous climate change. Given the stakes, solar geoengineering should be taken seriously, including through governance that facilitates its responsible research, development, and – if warranted – use.

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 final of four chapters that consider international legal rules, here those that fall outside the previous chapters’ scopes. Multilateral agreements govern activities and impacts in areas beyond national jurisdiction, including Antarctica, outer space, and the oceans. The comprehensive UN Convention on the Law of the Sea governs solar geoengineering that would take place in the marine environment or would likely result in deleterious effects there. The parties to the Convention on Biological Diversity, a far-reaching agreement, have issued three nonbinding decisions concerning geoengineering, the only such negotiated consensuses from representatives of most countries. The Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques proscribes solar geoengineering's hostile use. Among the four major international legal forums that help resolve disputes in broad ranges of issues, the UN Security Council could address problematic or contentious solar geoengineering.