Southern Ocean Clear‐Sky Brightening From Sea Spray Aerosol Increase Drives Departure From Hemispheric Albedo Symmetry

Abstract Observations reveal significant negative trends in reflected shortwave radiation over the 21st century. The global‐mean darkening is primarily driven by clouds, while the global‐mean atmospheric clear‐sky signal is near zero due to offsetting trends in the two hemispheres. Northern Hemisphere clear‐sky darkening is dominated by reduced anthropogenic aerosol emissions over population centers. The Southern Hemisphere (SH), in contrast, exhibits an unexpected widespread atmospheric clear‐sky brightening, particularly over the remote Southern Ocean (SO). We ascribe this brightening to an increase in aerosol optical depth (AOD). Based on observed correlations between monthly‐mean wind speed and AOD over the SO, we conclude the trends in AOD come from enhanced wind‐driven sea spray aerosol emissions driven by increasing near‐surface winds. We discuss the implications of SO brightening for deviations away from the observed hemispheric albedo symmetry and more generally as a potential negative Earth system feedback.