On the rise and fall of Earth's strong clear-sky hemispheric albedo asymmetry | CU Experts

A striking feature of the Earth system is that the Northern and Southern; Hemispheres reflect identical amounts of sunlight. This hemispheric; albedo symmetry comprises two asymmetries: The Northern Hemisphere is; more reflective in clear skies, whereas the Southern Hemisphere is; cloudier. The traditional explanation is that the clear-sky asymmetry is; primarily due to the relatively-bright continents being; disproportionately located in the Northern Hemisphere. Here we show that; this explanation is inadequate because the clear-sky asymmetry is; dominated by atmospheric aerosol, not surface reflection, and the; greater reflection from Northern Hemisphere land is largely offset by; greater reflection from the Antarctic surface than the Arctic surface.; Climate model simulations suggest that aerosol emissions since the; pre-industrial era have driven a large increase in the clear-sky; asymmetry that would reverse in future low-emission scenarios featuring; rapid decarbonization and decreases in co-emitted aerosol. High-emission; scenarios also show a decrease in asymmetry, but instead driven by; declines in Northern Hemisphere ice and snow cover. Strong clear-sky; hemispheric albedo asymmetry is therefore a transient, rather than; fixed, feature of Earth’s climate. If all-sky symmetry is maintained,; compensating cloud changes would have uncertain but important; implications for Earth’s energy balance, the hydrological cycle, and; atmosphere-ocean circulations.

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