Introduction
From 2024 to 2026, many U.S. state legislatures introduced legislation aimed at prohibiting or strictly regulating outdoor research and deployment of solar geoengineering or solar radiation modification (SRM; see Appendix). Also known as solar geoengineering, SRM encompasses a set of proposed technologies to cool Earth.1 Some of the recent legislation proposes to ban specific technologies, like stratospheric aerosol injection (SAI) and marine cloud brightening (MCB), also known as cloud seeding (see the next section, What Is SRM?). Other legislation bans “weather modification,” which is different from but often conflated with SRM.2 Two U.S. states—Tennessee and Florida—have banned the deployment of SRM technologies.3 Although SRM is not yet available for large-scale deployment, some scientists anticipate its widespread use within the twenty-first century.4
Attitudes toward outdoor SRM research and deployment are mixed, with high levels of skepticism from some quarters alongside an increasing willingness to consider SRM among scientists and entrepreneurs. However, research into the attitudes held about SRM in the Global South and among youth and Indigenous populations is notably lacking.5
Emerging SRM technologies are underresearched and poorly understood, so significant uncertainty remains about their proposed uses and potential effects—both desirable and undesirable. Proposed outdoor SRM experiments have generated immense distrust (see the section on Experiments, Entrepreneurship, and Community Pushback). In the United States, some anti-SRM sentiment is rooted in the technology’s conflation with conspiracy theories about chemtrails—a conflation that has led to much of the aforementioned U.S. legislation.6 These factors have spurred a protracted debate about SRM’s desirability, how it should be governed, and whether research on the topic should move forward.
This primer briefly describes what SRM is, arguments for and against it, international efforts to govern it, notable simulations and outdoor experiments, and remaining questions for policymakers and scholars to consider.