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Title: Causes of Higher Climate Sensitivity in CMIP6 Models

Journal Article · · Geophysical Research Letters
 [1];  [1];  [2];  [1];  [1];  [3];  [1];  [1]
  1. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
  2. Univ. of Leeds (United Kingdom). Institute of Climate and Atmospheric Sciences
  3. Imperial College, London (United Kingdom). Grantham Inst.

Equilibrium climate sensitivity, the global surface temperature response to CO2 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 CO2 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.

Research Organization:
Lawrence Livermore National Laboratory (LLNL), Livermore, CA (United States)
Sponsoring Organization:
USDOE National Nuclear Security Administration (NNSA); LDRD; European Union Horizon 2020 Project
Grant/Contract Number:
AC52-07NA27344
OSTI ID:
1642367
Report Number(s):
LLNL-JRNL--791582; 991009
Journal Information:
Geophysical Research Letters, Journal Name: Geophysical Research Letters Journal Issue: 1 Vol. 47; ISSN 0094-8276
Publisher:
American Geophysical UnionCopyright Statement
Country of Publication:
United States
Language:
English

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