Abstract
Abstract. The newest iteration of the Canadian Earth System Model (CanESM5.0.3) has an effective climate sensitivity (EffCS) of 5.65 K, 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 EffCS 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. We find small increases in the surface albedo and longwave cloud feedback, as well as a substantial increase in the SW cloud feedback in CanESM5. Through the use of cloud area fraction output and cloud radiative kernels, we find that more positive low and non-low shortwave cloud feedbacks – particularly with regards to low clouds across the equatorial Pacific, as well as subtropical and 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 exerts controls on the magnitude and spatial distribution of low-cloud fraction response but does not fully explain the increased EffCS in CanESM5. The results from CanESM5 are consistent with increased EffCS in several other CMIP6 models, which has been primarily attributed to changes in shortwave cloud feedbacks.
Highlights
Equilibrium climate sensitivity (ECS), defined as the global annual mean surface warming the Earth would exhibit as a response to a doubling of CO2, is a frequently cited emergent property from simplified climate models (Charney et al, 1979), as well as modern Earth system models (ESMs) (Andrews et al, 2012; Vial et al, 2013)
We examine the high ECS of the Canadian Earth System Model 5 (CanESM5) in relation to the previous model version that was contributed to CMIP5 (CanESM2)
Relative to CanESM2, CanESM5 has a weaker net feedback parameter (−0.64 W m−2 K−1) and higher effective climate sensitivity (EffCS) (5.65 K), meaning that EffCS has increased by 54 % between CanESM versions 2 and 5
Summary
Equilibrium climate sensitivity (ECS), defined as the global annual mean surface warming the Earth would exhibit as a response to a doubling of CO2, is a frequently cited emergent property from simplified climate models (Charney et al, 1979), as well as modern Earth system models (ESMs) (Andrews et al, 2012; Vial et al, 2013). In the latest phase of the Coupled Model Inter-comparison Project (CMIP6), the range of ECS from participating models has widened (1.8–5.5 K), with the mean shifting towards higher values than the previous phase of CMIP (3.2 to 3.7 K from CMIP5 to CMIP6) (Flynn and Mauritsen, 2020; Zelinka et al, 2020). Understanding cloud feedback uncertainty and its influence on the ECS of ESMs has been an imperative of researchers in recent decades – with regards to properties such as cloud optical depth, which determine the amount of reflected shortwave (SW) radiation and help cool the planet (Vial et al, 2013; Tan et al, 2016; Zelinka et al, 2020; Bjordal et al, 2020). SW cloud feedbacks can be separated based on latitude; middle-latitude SW cloud feedbacks are mostly negative from the optical thickening of clouds due to phase transition towards liquid in ice- and mixed-phase clouds
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