Abstract
Shortwave radiative feedbacks from Southern Ocean clouds are a major
source of uncertainty in climate projections. Much of this uncertainty
arises from changes in cloud scattering properties and lifetimes that
are caused by changes in cloud thermodynamic phase. Here we use
satellite observations to infer the scattering component of the
cloud-phase feedback mechanism and determine its relative importance by
comparing it with an estimate of the overall temperature-driven cloud
feedback. The overall feedback is dominated by an optical thinning of
low-level clouds. In contrast, the scattering component of cloud-phase
feedback is an order of magnitude smaller and is primarily confined to
free-tropospheric clouds. The small magnitude of this feedback component
is a consequence of counteracting changes in albedo from cloud optical
thickening and shifts in the scattering direction of cloud particles.
These results indicate that shortwave cloud feedback is likely positive
over the Southern Ocean and that changes in cloud scattering properties
arising from phase changes make a small contribution to the overall
feedback. The feedback constraints shift the projected 66% confidence
range for the global equilibrium temperature response to doubling
atmospheric CO2 by about +0.1 K relative to a recent
consensus estimate of cloud feedback.