Hemispheric Planetary Albedo Symmetry and Climate Sensitivity

Abstract:

Earth radiation budget observations show that the Northern and Southern Hemisphere reflect the same amount of short-wave radiation when averaged over several years. There is no fundamental physical reason why this “albedo symmetry” occurs. Planetary albedo -- influenced by physical processes related to sea ice, snow and clouds -- has the potential to change substantially under global warming and act as a radiative feedback. Climate models vary tremendously in their ability to replicate the observed hemispheric albedo symmetry. The fraction of models with positive or negative biases is the same in the CMIP5 and CMIP6 ensemble. Even though many models are tuned to the global mean top of the atmosphere radiative fluxes, the albedo symmetry is usually not a tuning target. We show that model biases in albedo symmetry correlate with the models’ short-wave cloud radiative effect and effective climate sensitivity. Models with a too bright Southern or too dark Northern Hemisphere tend to have positive short-wave cloud radiative effect and high effective climate sensitivity, while models with a too dark Southern or too bright Northern Hemisphere tend to have small or negative short-wave cloud radiative effects and a low effective climate sensitivity. We discuss how increased process understanding of albedo symmetry might help constraining climate sensitivity of the Earth.