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The complex international regime for climate change has evolved over the past three decades, from the Framework Convention on Climate Change and the Kyoto Protocol through the Paris Agreement and beyond. We assess this evolution from the 1990s to the 2020s, and its potential future evolution from the 2020s to the 2050s, across three main policy strategies: mitigation, adaptation, and reflection. In its first three decades, the regime has focused predominantly on the mitigation of net emissions and on engaging all major emitting countries in that effort. More recently, as progress on mitigation has been slow and as the impacts of climate change have risen around the world, the regime has begun to address adaptation. The next three decades may see the rise of a third strategy, reflection, if actors (collectively or unilaterally) perceive an urgent need to alleviate peak climate damages through fast-acting but controversial and risky climate interventions known as sunlight reflection methods or solar radiation modification (SRM). Several major international groups have recently issued reports on SRM, yet the international climate change regime has not yet constructed a governance regime for assessment or management of SRM. We recommend and outline comprehensive risk-risk tradeoff analyses of SRM to help avoid harmful countervailing risks. We suggest the development of an adaptive governance regime, starting early and embracing iterative and inclusive learning and updating over time. We urge that among the first key steps should be the development of a transparent international monitoring system for SRM. Such a monitoring system could provide early warning and help deter any unilateral SRM, assess the intended and unintended global and regional impacts of any research or eventual deployment of SRM, foster collective deliberation and reduce the risk of international conflict over SRM, help attribute adverse side effects of SRM to assist those adversely affected, and aid learning to improve the system adaptively over time. Thus, any reflection (of sunlight) should involve ongoing reflection (analysis and revision). Such an SRM monitoring regime is needed before SRM might be deployed, and can be developed at the same time that the focus of current efforts remains on mitigation and adaptation. 

As the prospect of average global warming exceeding 1.5°C becomes increasingly likely, interest in supplementing mitigation and adaptation with solar geoengineering (SG) responses will almost certainly rise. For example stratospheric aerosol injection to cool the planet could offset some of the warming for a given accumulation of atmospheric greenhouse gases ( 1 ). However, the physical and social science literature on SG remains modest compared with mitigation and adaptation. We outline three research themes for advancing policy-relevant social science related to SG: (i) SG costs, benefits, risks, and uncertainty; (ii) the political economy of SG deployment; and (iii) SG’s role in a climate strategy portfolio.