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High climate sensitivity in CMIP6 model not supported by paleoclimate

Nature Climate Change volume 10, pages 378–379 (2020)Cite this article

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To the Editor — Equilibrium climate sensitivity (ECS) is the long-term response of global mean surface temperature (GMST) to a doubling of atmospheric CO2 concentrations. It is poorly constrained with a ‘likely’ range of 1.5–4.5 °C, which has remained nearly unchanged since the Charney report 40 years ago1. Ten in twenty-seven of the available climate models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6) have an ECS higher than the upper end of this range, in contrast to two in twenty-eight CMIP5 models2,3. For example, the ECS in the Community Earth System Model version 2 (CESM2)4 — a CMIP6 model — is 5.3 °C (ref. 5). Determining whether this high ECS is realistic is paramount for estimating future climate and crafting effective policies and adaptation plans. Without a historical benchmark to test the ECS of CMIP6 models against, they can be evaluated against past warm periods, such as the Early Eocene Climate Optimum (EECO), a period of sustained high GMST ~53–50 mya6,7.

CESM2 GMSTs are substantially higher than results using its predecessors, CESM1 (ref. 12) and the Community Climate System Model version 4 (CCSM4)13. In a CESM1 EECO simulation with 6× piCO2, GMST is 29.8 °C and the MTG is 76%, agreeing well with proxy evidence9. In CCSM4, CO2 levels of 16× piCO2 are necessary to attain an EECO GMST, values that are much higher than proxy estimates. Sensitivity to the non-CO2 EECO climate forcings — paleogeography, vegetation, and the removal of anthropogenic aerosols and land ice sheets — are estimated to be 9.4 °C, 5.1 °C and 2.9 °C in CESM2, CESM1 and CCSM4, respectively, showing a monotonic but nonlinear dependence between model sensitivity and its pre-industrial ECSs, which are 5.3 °C, 4.2 °C and 3.2 °C (refs. 5,14), respectively. The nonlinear relationship results from the increase of ECS with GMST9 and potential increases in the effectiveness of non-CO2 climate forcings between model versions, for which the underlying mechanisms merit further research. The dependence of ECS on model versions and GMST has been attributed to the cloud feedback — that is, the amplification of surface warming through changes in clouds5,9.

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Fig. 1: Model-data comparison of GMST and MTG.

Data availability

CESM1 simulation data are available in the Zenodo repository at https://zenodo.org/record/2642536#.Xo7xa5NKjyI. CESM2 data can be requested from J.Z.

Code availability

CESM2 model code is available at https://github.com/ESCOMP/CESM/releases/tag/release-cesm2.1.1. CESM1 and CCSM4 model code is available at https://svn-ccsm-models.cgd.ucar.edu/cesm1/release_tags/cesm1_2_2_1/.

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Acknowledgements

This work was supported by Heising-Simons Foundation (grant nos. 2016-05 and 2016-12) and National Science Foundation (NSF; grant no. 2002397) to C.J.P. We acknowledge the computational resources provided by the CESM Paleoclimate Working Group for high-performance computing support from Cheyenne (https://www2.cisl.ucar.edu/user-support/acknowledging-ncarcisl) provided by the National Center for Atmospheric Research’s CISL, sponsored by NSF. The CESM project is supported primarily by the NSF. This material is based upon work supported by the National Center for Atmospheric Research, which is a major facility sponsored by the NSF under Cooperative Agreement no. 1852977.

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Authors and Affiliations

  1. Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI, USA

    Jiang Zhu & Christopher J. Poulsen

  2. Climate and Global Dynamics Laboratory, National Center for Atmospheric Research, Boulder, CO, USA

    Bette L. Otto-Bliesner

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  1. Jiang Zhu
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Correspondence to Jiang Zhu.

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Zhu, J., Poulsen, C.J. & Otto-Bliesner, B.L. High climate sensitivity in CMIP6 model not supported by paleoclimate. Nat. Clim. Chang. 10, 378–379 (2020). https://doi.org/10.1038/s41558-020-0764-6

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