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.