Testing Cloud Adjustment Hypotheses for the Maintenance of Earth's
Hemispheric Albedo Symmetry with Observed Trends and Natural Experiments
Abstract
Earth’s Northern and Southern Hemispheres reflect essentially equal
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 the surface,
clear-sky atmosphere, and different cloud types as defined by cloud
effective pressure and optical thickness. Climatologically, greater
reflection by the surface, aerosols, and high clouds in the Northern
Hemisphere is balanced by greater low and mid-level cloudiness in the
Southern Hemisphere. Both hemispheres have darkened at similar rates
over the past two decades; whether the decrease in clear-sky asymmetry
from declining aerosol is breaking all-sky symmetry due to more rapid
Northern Hemisphere darkening remains uncertain. Regional trends and
natural experiments including sea ice loss and volcanic eruptions
provide strong evidence against the hypothesis that extratropical low
clouds compensate changing clear-sky asymmetries on decadal timescales
but some evidence that tropical high cloud shifts may do so.