Lawrence Livermore National Laboratory scientists evaluated how well models perform at simulating climate feedbacks using atmosphere-only simulations that agree with those derived from computationally expensive fully coupled simulations (~ 150 years). They found that the cloud feedback correspondence between the two simulations remains in the response of global mean, individual cloud properties, and spatial distribution. Finally, they found that 1-year atmosphere-only simulation is enough to capture the inter-model spread of the global mean cloud feedback from coupled simulations.
This study suggests that short-term atmosphere-only simulations are both scientifically and technically effective to understand the cloud feedback that is generally estimated from fully coupled simulations. This finding enhances the confidence to estimate the cloud feedback using short-term atmosphere-only simulations in climate model development.
Atmosphere-only experiments are widely used to investigate climate feedbacks simulated in more computationally expensive fully-coupled global climate model simulations. Lawrence Livermore National Laboratory scientists 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 global-mean cloud feedbacks, they 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) and holds even when considering atmosphere-only simulations of only 1-year duration. For regional cloud feedbacks, the correspondence between the two experiments is generally present at every geographic location except for the tropical Pacific but takes longer experiments to reveal. For the lapse rate and surface albedo feedbacks, the correspondence between the two experiments is weaker due to the non-uniform warming pattern and loss of sea ice in the coupled experiment. For the across-model relationship between 4xCO2 effective radiative forcing and feedback, they 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.