Patrick W Keys , Elizabeth A Barnes, Noah S Diffenbaugh, James W Hurrell, Curtis M Bell

As anthropogenic activities warm the Earth, the fundamental solution of reducing greenhouse gas emissions remains elusive. Given this mitigation gap, global warming may lead to intolerable climate changes as adaptive capacity is exceeded. Thus, there is emerging interest in solar radiation modification, which is the process of deliberately increasing Earth's albedo to cool the planet. Stratospheric aerosol injection (SAI) — the theoretical deployment of particles in the stratosphere to enhance reflection of incoming solar radiation — is one strategy to slow, pause or reverse global warming. If SAI is ever pursued it will likely be for a specific aim, such as affording time to implement mitigation strategies, lessening extremes, or reducing the odds of reaching a biogeophysical tipping point. Using an ensemble climate model experiment that simulates the deployment of SAI in the context of an intermediate greenhouse gas trajectory, we quantify the probability that internal climate variability masks the effectiveness of SAI deployment on regional temperatures. We find that, while global temperature is stabilized, substantial land areas continue to experience warming. For example, in the SAI scenario we explore, up to 55% of the global population experiences rising temperatures over the decade following SAI deployment, and large areas exhibit high probability of extremely hot years. These conditions could cause SAI to be perceived as a failure. Countries with the largest economies experience some of the largest probabilities of this perceived failure. The potential for perceived failure could therefore have major implications for policy decisions in the years immediately following SAI deployment.