The effective climate sensitivity estimates the equilibrium response of near-surface temperature to doubling atmospheric carbon dioxide concentration and is a widely used metric to characterize potential global warming. Earth system models participating in phase 6 of the Coupled Model Intercomparison Project (CMIP6) exhibit considerable spread in effective climate sensitivity estimates. Cloud feedbacks are thought to be the cause of this, with marine boundary layer clouds over the Southern Ocean playing an important role. Here, we show that Southern Ocean deep convection is a major contributor to the CMIP6 intermodel spread in effective climate sensitivity. By comparing two Earth system models with very different sensitivities, we find that greater storage of heat at depth can delay the Southern Ocean surface warming and associated cloud response, thereby delaying global surface warming by centuries. The link between Southern Ocean convection and effective climate sensitivity is seen across 41 CMIP6 models, with low-sensitivity models exhibiting substantial deep ocean warming. Our results reveal the influence of Southern Ocean convection on potential long-term climate warming.
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All CMIP data used are available from the Earth System Grid Federation (ESGF) server: CMIP5 model output (https://esgf-node.llnl.gov/search/cmip5/) and CMIP6 model output (https://esgf-node.llnl.gov/search/cmip5/). The data from the hosing experiment, NorESM2-hosing, used to produce panels a and c in Fig. 5 are available for download from Uninett Sigma2 (https://archive.sigma2.no/pages/public/datasetDetail.jsf?id=10.11582/2021.00053)59. The NorESM2-LM and NorESM2-MM experiments piClim-p4K used for the feedback comparison with CESM2 are available for download from Uninett Sigma2 (https://archive.sigma2.no/pages/public/datasetDetail.jsf?id=10.11582/2021.0005460 and https://archive.sigma2.no/pages/public/datasetDetail.jsf?id=10.11582/2021.0005661). Source data are provided with this paper.
All code used for the analysis and plotting can be obtained from https://github.com/adagj/ECS_SOconvection. The kernels used in the feedback analysis in Figs. 1–3 can be obtained from https://github.com/apendergrass/cam5-kernels. The CESM2 source code can be accessed at https://github.com/ESCOMP/CESM. The NorESM2 source code can be accessed from https://github.com/NorESMhub/NorESM.
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A.G., D.O., Ø.S and M.S. received support from the European Framework Programme Horizon 2020 project CRESCENDO (Coordinated Research in Earth Systems and Climate: Experiments, Knowledge, Dissemination and Outreach, grant agreement no. 641816). All authors received support from the Norwegian Research Council funded projects INES (270061) and KeyClim (295046). High-performance computing and storage resources were provided by UNINETT Sigma2, the Norwegian infrastructure for computational science.
The authors declare no competing interests.
Peer review information Nature Geoscience thanks Levi Silvers and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Tom Richardson.
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Gjermundsen, A., Nummelin, A., Olivié, D. et al. Shutdown of Southern Ocean convection controls long-term greenhouse gas-induced warming. Nat. Geosci. 14, 724–731 (2021). https://doi.org/10.1038/s41561-021-00825-x