Causes of Higher Climate Sensitivity in CMIP6 Models

By M. Zelinka, T. Myers, D. McCoy, S. Po‐Chedley, Peter M. Caldwell, P. Ceppi, Stephen A. Klein, K. Taylor. Geophysical Research Letters, 2020

1,274 citations recorded by the source corpus.

Equilibrium climate sensitivity, the global surface temperature response to CO 2 doubling, has been persistently uncertain. Recent consensus places it likely within 1.5–4.5 K. Global climate models (GCMs), which attempt to represent all relevant physical processes, provide the most direct means of estimating climate sensitivity via CO 2 quadrupling experiments. Here we show that the closely related effective climate sensitivity has increased substantially in Coupled Model Intercomparison Project phase 6 (CMIP6), with values spanning 1.8–5.6 K across 27 GCMs and exceeding 4.5 K in 10 of them. This (statistically insignificant) increase is primarily due to stronger positive cloud feedbacks from decreasing extratropical low cloud coverage and albedo. Both of these are tied to the physical representation of clouds which in CMIP6 models lead to weaker responses of extratropical low cloud cover and water content to unforced variations in surface temperature. Establishing the plausibility of these higher sensitivity models is imperative given their implied societal ramifications.

Subfields: Atmospheric Science, Global and Planetary Change

View Paper: https://www.semanticscholar.org/paper/b0a76aa576df8a81baecd893ad2e0e5c5e674f4c

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Citations in the corpus

High-resolution (1 km) Köppen-Geiger maps for 1901–2099 based on constrained CMIP6 projections (2023, background): “Several climate models included in CMIP6 have unrealistically hot projections 27,29,30 , which is largely due to the representation of clouds and their response to increased CO 2 concentrations 52,53 .”
Reduced global warming from CMIP6 projections when weighting models by performance and independence (2020, methodology): “Such different responses to the same forcing can emerge due to different processes and feedbacks as well as due to the parametrization used in the different models, among other things (e.g., Zelinka et al., 2020).”
Quantifying Progress Across Different CMIP Phases With the ESMValTool (2020, background): “Figure 10d supports the results of Zelinka et al. (2020) that there is an increase in the shortwave cloud feed-back parameter over the Southern Ocean in CMIP6 compared with CMIP5 (in many regions understood as a decrease, or even sign change, in the size of a negative shortwave cloud feedback).”
Global surface air temperatures in CMIP6: historical performance and future changes (2020, result): “5 in CMIP6, the warming in most regions increases by 4 ◦C–12 ◦C, which may be due to the higher climate sensitivity compared with previous versions of CMIP5 (Flynn andMauritsen 2020, Zelinka et al 2020).”
Opinion: Cloud-phase climate feedback and the importance of ice-nucleating particles (2020, methodology): “Whether these high ECS values (Gettelman et al., 2019; Zelinka et al., 2020; Bodas-Salcedo et al., 2019) are possible or probable is a matter of debate (Palmer, 2020; Forster et al., 2020; Sherwood et al., 2020).”