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 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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