Causes of Higher Climate Sensitivity in CMIP6 Models

TLDR

Equilibrium climate sensitivity, the temperature response to CO₂ doubling, remains uncertain but is now broadly constrained to 1.5–4.5 K, and global climate models estimate it through CO quadrupling experiments. The study aims to evaluate the plausibility of the higher sensitivity values produced by CMIP6 models, given their societal implications. CMIP6 models represent clouds such that extratropical low cloud cover and water content respond weakly to unforced surface temperature changes, weakening their feedbacks. Effective climate sensitivity in CMIP6 has risen to 1.8–5.6 K, exceeding 4.5 K in ten models, largely due to stronger positive cloud feedbacks from reduced extratropical low cloud coverage and albedo.

Abstract

Abstract Equilibrium climate sensitivity, the global surface temperature response to CO 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 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.