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

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.