What if solar geoengineering were enacted not through careful intergovernmental deliberations or the actions of a rogue state, but by millions of private citizens taking matters into their own hands? This thought experiment—the subject of a scenario exercise at the Sixth International Geoengineering Governance Summer School—produced a range of critical reflections on international responses to such grassroots deployment as participant teams developed and critiqued governance proposals. Consideration of decentralized solar geoengineering, while unlikely, provides compelling insights for the governance of both irregular and more conventional geoengineering scenarios. Participants stressed that concerns with the collecting and reporting of data, hemispheric imbalances, and multidirectional questions of legitimacy are likely to arise in such contexts. Grassroots activity could further serve to galvanize wider state-led deliberations around geoengineering governance, forcing—but not necessarily deciding—the question of whether and how solar geoengineering should be implemented.