Submitted as: model evaluation paper 08 Mar 2021

Submitted as: model evaluation paper | 08 Mar 2021

Review status: a revised version of this preprint was accepted for the journal GMD.

Cloud Feedbacks from CanESM2 to CanESM5.0 and their Influence on Climate Sensitivity

John G. Virgin1, Christopher G. Fletcher1, Jason N. S. Cole2, Knut von Salzen2, and Toni Mitovski2,3 John G. Virgin et al.
  • 1Department of Geography & Environmental Management, University of Waterloo, Waterloo, Ontario, Canada
  • 2Canadian Centre for Climate Modelling and Analysis, Environment and Climate Change Canada, Victoria, British Columbia, Canada
  • 3Ministry of Health, Government of British Columbia, Victoria, British Columbia, Canada

Abstract. The newest iteration of the Canadian Earth System Model (CanESM5.0.3) has an Effective Climate Sensitivity (ECS) of 5.65 kelvin, which is a 54 % increase relative to the model's previous version (CanESM2 – 3.67 K), and the highest sensitivity of all current models participating in the sixth phase of the coupled model inter-comparison project (CMIP6). Here, we explore the underlying causes behind CanESM5's increased ECS via comparison of forcing and feedbacks between CanESM2 and CanESM5. We find only modest differences in radiative forcing as a response to CO2 between model versions. Through the use of cloud area fraction output and radiative kernels, we find that more positive shortwave cloud feedbacks – particularly with regards to low clouds across the equatorial pacific, as well as sub/extratropical free troposphere cloud optical depth – are the dominant contributors to CanESM5's increased climate sensitivity. Additional simulations with prescribed sea surface temperatures reveal that the spatial pattern of surface temperature change explains the pattern of change in low cloud fraction, but does not fully explain the increased ECS in CanESM5. The results from CanESM5 are consistent with increased ECS in several other CMIP6 models, which has been primarily attributed to changes in shortwave cloud feedbacks.

John G. Virgin et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gmd-2021-11', Anonymous Referee #1, 31 Mar 2021
  • RC2: 'Comment on gmd-2021-11', Anonymous Referee #2, 13 Apr 2021
  • AC1: 'Comment on gmd-2021-11', John Virgin, 01 Jul 2021

John G. Virgin et al.

John G. Virgin et al.


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Short summary
Equilibrium climate sensitivity, or the amount of warming the Earth would exhibit a result of a doubling of atmospheric CO2, is a common metric used in assessments of climate models. Here, we compare climate sensitivity between two versions of the Canadian Earth System Model. We find the newest iteration of the model (version 5) to have higher climate sensitivity due to reductions in low level clouds, which reflect radiation and cool the planet, as the surface warms.