This paper explores the governance options surrounding geoengineering—the deliberate, large-scale manipulation of the Earth's climate system to counteract climate change. The authors focus solely on methods that affect the incoming solar radiation to the atmosphere, referred to as solar radiation management (SRM). They examine whether an international governance framework for SRM is needed, how it should be designed, and whether it is feasible. The authors propose a governance regime that initially has small membership and weak legalization, and is flexible in that future institutional reforms allow for broader membership and deeper commitments. The article provides supporting evidence for key aspects of the regime through past international treaties in arms control and environmental protection, including the Antarctica, Outer Space, and Montreal Protocol treaty regimes. For these cases, acting early and treating the respective problems as part of the “regulation of unexplored territory” produced more effective outcomes than the “national appropriation” approach that characterizes arms control.