On the Correspondence between Atmosphere-Only and Coupled Simulations
for Radiative Feedbacks and Forcing from CO2
Abstract
Atmosphere-only experiments are widely used to investigate climate
feedbacks simulated in more computationally expensive fully-coupled
global climate model simulations. We confirm that this remains a valid
approach by comparing the radiative feedbacks and forcing between
coupled and atmosphere-only simulations for the latest models taking
part in the 6th phase of the Coupled Model Intercomparison Project
(CMIP6). For cloud feedbacks, we find a better than previously known
correspondence between these experiments, which applies even to the
response of individual cloud properties (amount, altitude and optical
depth), is present at nearly every geographic location, and holds even
when considering atmosphere-only simulations of only 1 year duration. In
the tropics, the correspondence between the two experiments is better
revealed when considering feedbacks stratified by vertical motion rather
than by geography, owing to the non-uniform warming pattern in the
coupled experiment. For the lapse rate and surface albedo feedbacks, the
correspondence between the two experiments is weaker due to the lack of
sea-ice changes in the atmosphere-only experiment. For the across-model
relationship between 4xCO2 radiative forcing and
feedback, we find a different behavior across experiments in CMIP6 than
in CMIP5, casting doubt on the physical significance of previous results
that highlighted an anti-correlation between the two quantities.
Overall, these results confirm the utility of atmosphere-only
experiments particularly to study cloud feedbacks, which are the
dominant source of inter-model spread in climate sensitivity.