Global climate models predict warming in response to increasing GHG concentrations, partly due to decreased tropical low-level cloud cover and reflectance. We use satellite observations that discriminate stratocumulus from shallow cumulus clouds to separately evaluate their sensitivity to warming and constrain the tropical contribution to low-cloud feedback. We find an observationally inferred low-level cloud feedback two times smaller than a previous estimate. Shallow cumulus clouds are insensitive to warming, whereas global climate models exhibit a large positive cloud feedback in shallow cumulus regions. In contrast, stratocumulus clouds show sensitivity to warming and the tropical inversion layer strength, controlled by the tropical Pacific sea surface temperature gradient. Models fail to reproduce the historical sea surface temperature gradient trends and therefore changes in inversion strength, generating an overestimate of the positive stratocumulus cloud feedback. Continued weak east Pacific warming would therefore produce a weaker low-cloud feedback and imply a more moderate climate sensitivity (3.47 ± 0.33 K) than many models predict.
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The CALIPSO-GOCCP CASCCAD statistical datasets (Cesana et al.2) can be downloaded from the GISS website (https://data.giss.nasa.gov/clouds/casccad/). CERES-EBAF 4.0 SW TOA fluxes were downloaded from the CERES website (https://ceres.larc.nasa.gov/data/#energy-balanced-and-filled-ebaf). The CMIP6 GCM outputs were downloaded from the ESGF (https://esgf-node.llnl.gov/search/cmip6/). ERA5 files were downloaded from climserv (https://climserv.ipsl.polytechnique.fr/fr/les-donnees/era-5.html). HadISST1.1 files were downloaded from https://www.metoffice.gov.uk/hadobs/hadisst/. ERSSTv5 files were downloaded from the NOAA National Centers for Environmental Information website (https://www1.ncdc.noaa.gov/pub/data/cmb/ersst/v5/netcdf/). NCEP/DOE reanalysis2, NCEP-NCAR reanalysis1 and NOAA/CIRES/DOE 20th Century Reanalysis V3 were downloaded from the NOAA ESRL Physical Sciences Division website (http://www.esrl.noaa.gov/psd/data/).
The codes used to produce the figures and to compute the different derivatives and feedbacks are available from the corresponding author on request.
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G.V.C. and A.D.D. were supported by a CloudSat-CALIPSO RTOP at the NASA Goddard Institute for Space Studies. We thank NASA and CNES for giving access to CALIPSO and CloudSat observations, and Climserv for giving access to CALIPSO-GOCCP observations and CMIP6 model outputs and for providing computing resources. We acknowledge the World Climate Research Programme’s Working Group on Coupled Modeling, which is responsible for CMIP, and we thank the climate modelling groups (listed in Supplementary Tables 1 and 3) for producing and making available their model output. G.V.C. thanks M. Richardson for proofreading the first draft of the manuscript and providing useful comments and M. Zelinka for providing an updated version of supplementary table 1 of his study27.
The authors declare no competing interests.
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Cesana, G.V., Del Genio, A.D. Observational constraint on cloud feedbacks suggests moderate climate sensitivity. Nat. Clim. Chang. 11, 213–218 (2021). https://doi.org/10.1038/s41558-020-00970-y
Nature Climate Change (2021)