Global greenhouse gas emissions continue to rise, with current projections indicating the planet is on a pathway to a temperature increase of approximately 3.2°C by 2100. This has spurred scientific and policy interest in the possible role of solar radiation management and carbon dioxide removal geoengineering activities to help avert passing critical climatic thresholds, or to help societies recover if global temperatures overshoot expectations of safe levels. Marine geoengineering proposals show significant diversity in terms of their purpose, scale of application, likely effectiveness, requisite levels of international cooperation and intensity of environmental risks. This is likely to place significant new demands upon the international law system to govern potential risks and opportunities.