Existing norms, rules, and institutions seem insufficient to govern solar geoengineering in the long term. This chapter recommends additional and feasible measures to help ensure that it is researched, developed, and – if appropriate – used in ways that improve human well-being, are sustainable, and are consistent with widely shared norms. This will be challenging for reasons such as political contestation, conflicting desires for early governance and the reduction of uncertainty, and the endeavor's speculative nature. The suggestions are divided into rough stages of indoor and small-scale outdoor research, large-scale outdoor research, and deployment. The suggested forms of governance are norms, standards, and best practices; cooperation among nonstate actors; institutionally affiliated committees; state law; international institutions, including one dedicated to solar geoengineering; and – most speculatively – a multilateral agreement. Among governance's functions are reducing uncertainty and environmental risks, synthesis and assessment of results, public engagement, and preventing premature implementation, international tensions, and harmful abatement displacement.

Governance includes nonstate actors and nonlegal instruments. Nonstate governance – that which is developed, implemented, and/or enforced by nonstate actors – can fill roles that state law cannot or does so poorly. This chapter considers the extent to which nonstate actors do, could, and should contribute to solar geoengineering governance. It introduces key concepts of nonstate governance. The extant governance that is specific to solar geoengineering is largely nonstate. This is particularly evident in the development, influence, and apparent compliance with multiple sets of principles, such as the Oxford principles. Notably, for the most part, these sets substantively agree. The chapter closes with an analysis of nonstate governance’s potential. It concludes that nonstate governance should contribute because solar geoengineering’s characteristics – such as technically complexity, dynamism, reliance upon experts’ knowledge, transboundary impacts, and researchers' shared yet undifferentiated reputational sensitivity – are favorable to nonstate governance, while states are taking no significant steps toward governance.