Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling

Causes of Higher Climate Sensitivity in CMIP6 Models

TitleCauses of Higher Climate Sensitivity in CMIP6 Models
Publication TypeJournal Article
Year of Publication2020
JournalGeophysical Research Letters
Abstract / Summary

Equilibrium climate sensitivity, the global surface temperature response to CO2 doubling, has been persistently uncertain. Recent consensus places it likely within 1.5‐4.5K. 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 1.8‐5.6K across 27 GCMs and exceeding 4.5K 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.

URLhttps://doi.org/10.1029/2019GL085782
DOI10.1029/2019GL085782
Journal: Geophysical Research Letters
Year of Publication: 2020
Publication Date: 01/2020

Equilibrium climate sensitivity, the global surface temperature response to CO2 doubling, has been persistently uncertain. Recent consensus places it likely within 1.5‐4.5K. 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 1.8‐5.6K across 27 GCMs and exceeding 4.5K 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.

DOI: 10.1029/2019GL085782
Citation:
Zelinka, MD, TA Myers, DT McCoy, S Po-Chedley, PM Caldwell, P Ceppi, SA Klein, and KE Taylor.  2020.  "Causes of Higher Climate Sensitivity in CMIP6 Models."  Geophysical Research Letters.  https://doi.org/10.1029/2019GL085782.