WCD-07. Maintenance of Earth's hemispheric albedo symmetry despite changes in aerosol and ice cover during the twenty-first century

Abstract
One striking feature of the observed Earth system is that the amount of sunlight reflected in any given year from the Northern Hemisphere is essentially equal to that reflected from the Southern Hemisphere. This is surprising for at least two reasons: 1) Because land surfaces are brighter than ocean surfaces in general, the Northern Hemisphere has a much larger surface albedo than the Southern Hemisphere; and 2) state-of-the-art global climate models generally do not simulate this hemispheric symmetry. Differences in cloudiness between the hemispheres compensate for the clear-sky asymmetry. We use the new CERES "FluxByCldTyp" Level 3 product to break down reflected shortwave contributions by cloud type (defined using cloud top pressure and optical thickness bins) and analyze trends over the 2003-2019 data record. Over the course of the twenty-first century, the Northern Hemisphere has darkened, due in part to changes in aerosol concentrations and warming-related reductions in snow and ice cover. Interestingly, the Southern Hemisphere has darkened by approximately the same amount, despite having very different sea ice and aerosol dynamics. We test the hypothesis that shifts in deep convective clouds in the Intertropical Convergence Zone can explain how the hemispheres maintain symmetry and assess the roles played by other cloud types (e.g., stratiform marine clouds) and regions (i.e., midlatitudes and poles).