Unexpected failure of regional marine cloud brightening in a warmer world

Abstract Marine cloud brightening is a solar geoengineering1–3 proposal to cool atmospheric temperatures and reduce some impacts of climate change. To-date, modeling studies of solar geoengineering have primarily focused on large-scale schemes with objectives of stabilizing or mediating changes in global mean temperature4–7. However, these global proposals pose substantial governance challenges8–10, making regional interventions tailored toward targeted climate outcomes potentially more attractive in the near-term. In this study, we investigate the efficacy of regional marine cloud brightening in the North Pacific designed to mitigate extreme heat in the Western United States. We find cloud brightening in a remote mid-latitude region cools our target region more than brightening in a proximate subtropical region, but both schemes reduce the relative risk of dangerous summer heat exposure under present-day conditions, by 39% and 25% respectively. However, the same cloud brightening interventions under mid-century warming produce significantly hotter rather than cooler summers, both in the Western U.S. and other areas of the world. We trace this loss of efficacy to a nonlinear response of the Atlantic Meridional Overturning Circulation to the combination of greenhouse gas driven warming and regional cloud brightening. Our result demonstrates a risk in assuming that regional interventions that are effective under certain conditions will remain effective as the climate continues to change.