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.