On the rise and fall of Earth's strong clear-sky hemispheric albedo asymmetry
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 most-cited explanation is that the clear-sky asymmetry is primarily due to the relatively-bright continents being disproportionately located in the Northern Hemisphere. However, it is the atmosphere, not the surface, that contributes most to the clear-sky asymmetry. Here we show that the continent-based component of the clear-sky surface asymmetry is largely offset by greater reflection from the Southern Hemisphere poles, allowing the clear-sky asymmetry to be dominated by aerosol. 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. 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 despite changes in the clear-sky asymmetry, compensating cloud changes would have uncertain but important implications for Earth’s energy balance and hydrological cycle.
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