Earth's Observed Hemispheric Albedo Symmetry by Cloud Type: Climatology, Trends, and Tests of Cloud Adjustment Hypotheses

Abstract

Earth's Northern and Southern Hemispheres reflect identical amounts of sunlight. How -and whether -this hemispheric albedo symmetry is maintained remains a mystery. We decompose Earth's hemispheric albedo symmetry into components associated with different cloud types as defined by cloud effective pressure and optical thickness. Greater reflection by the surface, clear-sky atmosphere, and high clouds in the Northern Hemisphere is balanced by low and mid clouds (dominated by stratocumulus) in the Southern Hemisphere. Both hemispheres have darkened by ˜0.5-0.8 W/m2/decade due to decreasing low and mid cloud and surface reflection, partially offset by increasing high cloud reflection. Cloud reflection trends largely follow cloud fraction, with the exception of decreasing stratocumulus albedo in both hemispheres. Hypotheses that all-sky symmetry is maintained despite clear-sky changes via adjustments in high clouds within the Intertropical Convergence Zone or in low and mid clouds in the Southern Ocean are not supported at interannual or decadal timescales.