Abstract
AbstractClimate forcing, sensitivity, and feedback metrics are evaluated in both the United Kingdom's physical climate model HadGEM3‐GC3.1 at low (‐LL) and medium (‐MM) resolution and the United Kingdom's Earth System Model UKESM1. The effective climate sensitivity (EffCS) to a doubling of CO2 is 5.5 K for HadGEM3.1‐GC3.1‐LL and 5.4 K for UKESM1. The transient climate response is 2.5 and 2.8 K, respectively. While the EffCS is larger than that seen in the previous generation of models, none of the model's forcing or feedback processes are found to be atypical of models, though the cloud feedback is at the high end. The relatively large EffCS results from an unusual combination of a typical CO2 forcing with a relatively small feedback parameter. Compared to the previous U.K. climate model, HadGEM3‐GC2.0, the EffCS has increased from 3.2 to 5.5 K due to an increase in CO2 forcing, surface albedo feedback, and midlatitude cloud feedback. All changes are well understood and due to physical improvements in the model. At higher atmospheric and ocean resolution (HadGEM3‐GC3.1‐MM), there is a compensation between increased marine stratocumulus cloud feedback and reduced Antarctic sea‐ice feedback. In UKESM1, a CO2 fertilization effect induces a land surface vegetation change and albedo radiative effect. Historical aerosol forcing in HadGEM3‐GC3.1‐LL is −1.1 W m−2. In HadGEM3‐GC3.1‐LL historical simulations, cloud feedback is found to be less positive than in abrupt‐4xCO2, in agreement with atmosphere‐only experiments forced with observed historical sea surface temperature and sea‐ice variations. However, variability in the coupled model's historical sea‐ice trends hampers accurate diagnosis of the model's total historical feedback.
Highlights
Comprehensive models of the global climate system, known as coupled atmosphere‐ocean general circulation models (AOGCMs), are essential tools for understanding climate processes and projecting future climate changes (e.g., Collins et al, 2013; Flato et al, 2013)
While the effective climate sensitivity (EffCS) is larger than that seen in the previous generation of models, none of the model's forcing or feedback processes are found to be atypical of models, though the cloud feedback is at the high end
The EffCS to a doubling of CO2 is found to be 5.5 K for HadGEM3.1‐GC3.1‐LL and 5.4 K for UKESM1 using the benchmark method of Andrews, Gregory, et al (2012) that was adopted in IPCC AR5 (Flato et al, 2013)
Summary
Comprehensive models of the global climate system, known as coupled atmosphere‐ocean general circulation models (AOGCMs), are essential tools for understanding climate processes and projecting future climate changes (e.g., Collins et al, 2013; Flato et al, 2013). Diagnosing a model's radiative forcings, feedbacks, and climate sensitivities is a useful first step to understanding its characteristic behavior in response to forcing (e.g., Andrews, Gregory, et al, 2012) Such metrics can be used to understand a climate model's simulation of historical global temperature change as well as its projection of 21st‐century climate change for a given emission scenario (e.g., Forster et al, 2013). Of Kuhlbrodt et al, 2018)—are explicitly required and a retuning of the model (e.g., to the top‐of‐atmosphere [TOA] radiative balance) is not needed since resolution‐dependent parameterizations are avoided (Williams et al, 2017) This mean differences in forcing, feedback, and climate sensitivity between the ‐LL and ‐MM configurations can confidently be attributed to differences in horizontal resolution between the configurations rather than any change in physical parameters that might have arisen if the models had needed to be retuned.
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