Equilibrium climate sensitivity above 5 °C plausible due to state-dependent cloud feedback

Abstract

The equilibrium climate sensitivity of Earth is defined as the global mean surface air temperature increase that follows a doubling of atmospheric carbon dioxide. For decades, global climate models have predicted it as between approximately 2 and 4.5 °C. However, a large subset of models participating in the 6th Coupled Model Intercomparison Project predict values exceeding 5 °C. The difference has been attributed to the radiative effects of clouds, which are better captured in these models, but the underlying physical mechanism and thus how realistic such high climate sensitivities are remain unclear. Here we analyse Community Earth System Model simulations and find that, as the climate warms, the progressive reduction of ice content in clouds relative to liquid leads to increased reflectivity and a negative feedback that restrains climate warming, in particular over the Southern Ocean. However, once the clouds are predominantly liquid, this negative feedback vanishes. Thereafter, other positive cloud feedback mechanisms dominate, leading to a transition to a high-sensitivity climate state. Although the exact timing and magnitude of the transition may be model dependent, our findings suggest that the state dependence of the cloud-phase feedbacks is a crucial factor in the evolution of Earth’s climate sensitivity with warming.

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Fig. 1: Near-surface temperature and radiative responses to a quadrupling of atmospheric CO2.
Fig. 2: Total and decomposed cloud feedbacks with time.
Fig. 3: Maps of the net optical depth feedback at the beginning and end of the simulation.
Fig. 4: Cloud water and temperature profiles for summer over the Southern Ocean.

Data availability

All CESM2 simulation output is available for download from Uninett Sigma: https://archive.sigma2.no/pages/public/datasetDetail.jsf?id=10.11582/2020.00028. The standard CloudSat and CALIPSO data products (version R05) used in this study (2B-CWC-RO, 2C-ICE, ECMWF-AUX) were downloaded from the CloudSat Data Processing Center’s (at Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins) website (http://www.cloudsat.cira.colostate.edu/). The historical surface air temperature from CESM2 used in Fig. 1a is available through the Earth System Grid Federation (ESGF) server (https://esgf-data.dkrz.de/search/cmip6-dkrz/) under CMIP6, ‘source ID’ CESM2-FV2 and ‘experiment ID’ historical. The data used in this article were downloaded on 20 April 2020. The dataset containing observed surface temperature, GISTEMP v4, was downloaded from https://data.giss.nasa.gov/gistemp/ on 6 May 2020. Source data are provided with this paper.

Code availability

The code used to analyse satellite data is available here: https://github.com/tim-carlsen/satellite-ecs-so. The code used to calculate cloud feedbacks using radiative kernels is available here: https://github.com/mzelinka/cloud-radiative-kernels.

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Acknowledgements

This work was supported by the European Research Council (ERC) through Grant StG 758005 and by the Norwegian Research Council through grant 281071. The computations and simulations were performed on resources provided by UNINETT Sigma2, the National Infrastructure for High Performance Computing and Data Storage in Norway. We acknowledge the World Climate Research Programme, which, through its Working Group on Coupled Modelling, coordinated and promoted CMIP6. We thank the climate modelling groups for producing and making available their model output, the Earth System Grid Federation (ESGF) for archiving the data and providing access, and the multiple funding agencies who support CMIP6 and ESGF.

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Contributions

J.B. ran all model simulations, performed most of the analyses, produced Figs. 24 and helped write the paper. T.S. designed the study and wrote the paper with help from all authors. K.A. performed analysis, produced Fig. 1 and helped write the paper. T.C. performed the observational analysis that went into Extended Data Fig. 6 and helped write the paper.

Corresponding author

Correspondence to Trude Storelvmo.

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The authors declare no competing interests.

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Peer review information Primary Handling Editors: Tamara Goldin; Heike Langenberg.

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Extended data

Extended Data Fig. 1 Cloud feedbacks as a function of surface air temperature change.

15-year average net a, LW b, and SW c, cloud feedbacks for the 150 years of the CESM2 simulation after quadrupling of CO2. Dots are placed at the middle of each 15-year interval, for the total cloud feedback (black) and separated into the cloud amount feedback (yellow), the cloud altitude feedback (red) and the cloud optical depth feedback (blue). Source data

Extended Data Fig. 2 Regional cloud feedbacks as a function of time since CO2 quadrupling.

15-year average regional cloud feedbacks for the 150 years of the CESM2 simultion with quadrupling of CO2, with dots placed at the middle of each 15-year intercal. Showing total cloud feedback (black) as well as the amount (yellow), altitude (pink), and optical depth (blue) feedbacs, for the northern extratropics (30–90°N) a, the tropics (30°N-30°S) b. the southern extratropics (30–90°S) c, and the Southern Ocean (45–60°S) d. Source data

Extended Data Fig. 3 Cloud water and temperature profiles for autumn over the Southern Ocean.

The autumn (MAM) grid-box average of cloud water amount and temperature profiles (red lines) for the pre-industrial control simulation a, the first 15 years b, and the last 15 years c, of the 150-year simulation with quadrupling of CO2. The ice fraction is light blue and the liquid fraction in dark blue. The mixed-phased cloud region (temperatures between 0 and −38 °C) is shown in grey shading. Source data

Extended Data Fig. 4 Cloud water and temperature profiles for winter over the Southern Ocean.

Same as Extended Data Fig. 3, but for winter (JJA). Source data

Extended Data Fig. 5 Cloud water and temperature profiles for spring over the Southern Ocean.

Same as Extended Data Fig. 3, but for spring (SON). Source data

Extended Data Fig. 6 Observed and simulated cloud water and temperature profiles over the Southern Ocean.

Grid-box average cloud water amounts including snow and rain, and temperature profiles for the pre-industrial control simulation and from satellite observations (see Methods) for Summer a, b, Fall c, d, Winter e, f, and Spring g, h.The ice (cloud ice + snow) is light blue and the liquid (cloud liquid + rain) in dark blue. The mixed-phase cloud region (temperatures between 0 and −38 °C) is shown in grey shading. Note different x-axes. Source data

Source data

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Source Data Fig. 3

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Source Data Fig. 4

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Source Data Extended Data Fig. 1

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Source Data Extended Data Fig. 2

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Source Data Extended Data Fig. 3

Numerical data to generate graphs.

Source Data Extended Data Fig. 4

Numerical data to generate graphs.

Source Data Extended Data Fig. 5

Numerical data to generate graphs.

Source Data Extended Data Fig. 6

Numerical data to generate graphs.

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Bjordal, J., Storelvmo, T., Alterskjær, K. et al. Equilibrium climate sensitivity above 5 °C plausible due to state-dependent cloud feedback. Nat. Geosci. 13, 718–721 (2020). https://doi.org/10.1038/s41561-020-00649-1

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