Anthropogenic aerosol and cryosphere changes drive Earth's strong but transient clear-sky hemispheric albedo asymmetry

Climate model simulations indicate that the observed strong clear-sky hemispheric albedo asymmetry is likely a transient feature of Earth's climate that may diminish with future declines in anthropogenic aerosol emissions and Arctic sea ice. 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. Here we show that the hemispheric reflection contrast from differences in continental coverage is offset by greater reflection from the Antarctic than the Arctic, allowing the net clear-sky asymmetry to be dominated by aerosol. Climate model simulations suggest that historical anthropogenic aerosol emissions drove a large increase in the clear-sky asymmetry that would reverse in future low-emission scenarios. High-emission scenarios also show decreasing asymmetry, instead driven by declines in Northern Hemisphere ice and snow cover. Strong clear-sky hemispheric albedo asymmetry is therefore a transient 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 and hydrological cycle.

Automated summary

Climate model simulations suggest that the strong clear-sky hemispheric albedo asymmetry observed on Earth is a temporary feature that may weaken with the decline in anthropogenic aerosol emissions and Arctic sea ice. The Northern Hemisphere appears more reflective in clear skies, while the Southern Hemisphere is cloudier. The difference in continental coverage is balanced by greater reflection from Antarctica than the Arctic, resulting in aerosols dominating the net clear-sky asymmetry. Historical anthropogenic aerosol emissions have significantly increased the clear-sky asymmetry, but this trend is expected to reverse in low-emission scenarios. High-emission scenarios also show decreasing asymmetry due to declining Northern Hemisphere ice and snow cover. If all-sky symmetry is maintained, changes in cloud cover could have uncertain but important implications for Earth's energy balance and hydrological cycle.