Review of Cloud Feedbacks in Climate Models
RGCM-funded scientists contributed to an article reviewing the current state of knowledge of cloud feedbacks in global climate models. The authors review the primary cloud feedbacks in global climate models, explain how they are diagnosed, describe their underlying physical mechanisms, characterize how well these mechanisms are represented in models, and discuss the various sources of inter-model spread. The authors describe the extent to which each feedback is supported by theory, high-resolution modeling, and/or observations.
Cloud feedback – the change in planetary heating resulting from the cloud response to global warming – constitutes by far the largest source of uncertainty in the climate response to carbon dioxide forcing simulated by global climate models (GCMs). This article provides a much needed review of the state of scientific understanding of cloud feedback mechanisms that operate in climate models. The article describes recent advances in diagnostics, in the theoretical and observational basis for feedbacks, and in observational constraints on feedback sign and strength, while also providing an outlook of areas in need of continued research at the frontiers of the science.
The authors review the main mechanisms for cloud feedbacks, and discuss their representation in climate models and the sources of inter-model spread. Global-mean cloud feedback in GCMs results from three main effects: (1) rising free-tropospheric clouds (a positive longwave effect); (2) decreasing tropical low cloud amount (a positive shortwave effect); (3) increasing high-latitude low cloud optical depth (a negative shortwave effect). These cloud responses simulated by GCMs are qualitatively supported by theory, high-resolution modeling, and observations. Rising high clouds are consistent with the Fixed Anvil Temperature (FAT) hypothesis, whereby enhanced upper-tropospheric radiative cooling causes anvil cloud tops to remain at a nearly fixed temperature as the atmosphere warms. Tropical low cloud amount decreases are driven by a delicate balance between the effects of vertical turbulent fluxes, radiative cooling, large-scale subsidence, and lower-tropospheric stability on the boundary-layer moisture budget. High-latitude low cloud optical depth increases are dominated by phase changes in mixed-phase clouds. The causes of inter-model spread in cloud feedback are discussed, focusing particularly on the role of unresolved parameterized processes such as cloud microphysics, turbulence, and convection.