Practical paths to risk-risk analysis of solar radiation modification

Abstract Solar radiation modification (SRM) is increasingly discussed as a potential strategy—in addition to ongoing greenhouse gas emission reduction, carbon dioxide removal, and adaptation—for reducing climate change risks. SRM, in particular stratospheric aerosol injection (SAI), could cool the earth, reducing many of the adverse impacts of rising global temperature; but it could also have unintended consequences both positive and negative, and both biophysical and societal. Because the potential benefits and harms of each SRM option are multiple and uncertain, they need to be analyzed using a comprehensive framework that compares the risks of courses of action that include SRM against those that do not, where the definition of risk captures both the severity and likelihood of impacts. Here we outline such a risk-risk framework for SRM with a specific application to SAI. Four practical steps are needed to perform a risk-risk analysis: (i) specify the candidate risk reduction action(s) to be analyzed, (ii) catalog all important potential benefits and harms of each candidate action, (iii) define the events that constitute the risks of harms and less-than-expected benefits, and estimate their likelihood, magnitude, timing, distribution, and other relevant dimensions, including uncertainty about these estimates, and (iv) compare the risks across different candidate risk reduction actions with the aim of informing decisions that reduce overall risk. We perform an initial cataloging, estimation, and comparison of important risks of a specified SAI deployment in comparison to a non-SAI scenario. We also suggest ways to overcome some key challenges to applying the risk-risk framework across a broad array of possible actions, impacts, and scenarios. We recommend an international assessment of SRM options and their risk-risk profiles.