Abstract
Greenhouse gases (GHGs) are gases that absorb and emit thermal energy.
In a warming climate, GHGs modulate the thermal cooling to space from
the surface and atmosphere, which is a fundamental feedback process that
affects climate sensitivity. Previous studies have stated that the
thermal cooling to space with global warming is primarily emitted from
the surface, rather than the atmosphere. Using a millennium-length
coupled general circulation model (Geophysical Fluid Dynamics
Laboratory’s CM3) and accurate line-by-line radiative transfer
calculations, here we show that the atmospheric cooling to space
accounts for 12 % to 50 % of Earth’s clear-sky longwave feedback
parameter from the poles to the tropics. The atmospheric cooling to
space is an efficient stabilizing feedback process because water vapor
and non-condensable GHGs tend to emit at higher temperatures with
surface warming as the thermodynamic structure of the atmosphere
evolves. A simple yet comprehensive model is proposed in this study for
predicting the clear-sky longwave feedback over a wide range of surface
temperatures. It achieves good spectral agreement when compared to
line-by-line calculations. Our study provides a theoretical way for
assessing Earth’s climate sensitivity, with important implications for
Earth-like planets.