Who Could Deploy Stratospheric Aerosol Injection? The <scp>United States</scp> , China, and Large‐Scale, Rapid Planetary Cooling

ABSTRACT Stratospheric aerosol injection, which would reflect a small fraction of sunlight away from the Earth to lower temperatures, involves many unanswered questions. One of these is, who could deploy it? We consider this with reference to a scenario in which global temperatures are reduced by 1°C by midcentury; we term this a ‘PLUS’ deployment—Planetary, Large‐scale, Uninterrupted, and Speedy. The technical requirements of a PLUS deployment—a fleet of a hundred or more specialized aircraft—limit the number of capable actors to ten states. The geopolitical requirements—broad‐spectrum capabilities sufficient to overcome external constraints—mean that only the US and China are capable of implementing unilaterally against strong opposition. As such, the US and China will be decisive in determining whether and how a PLUS‐type deployment takes place. In particular, the degree of Sino‐American alignment on this issue will strongly influence the likelihood of a PLUS deployment and its disruptive potential. We examine three cases in which activities with the potential to harm global commons were debated during the Cold War: scientific research in Antarctica, atmospheric testing of nuclear weapons, and experiments in outer space. Backed by evidence from these cases, we then consider several implications of our findings.