Forcings, Feedbacks, and Climate Sensitivity in HadGEM3‐GC3.1 and UKESM1

Abstract

Climate 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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-4xCO 2 , 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. Plain Language SummaryA new generation of climate models-called HadGEM3-GC3.1 and UKESM1-have been developed in the United Kingdom and will be used widely in the Coupled Model Intercomparison Project Phase 6 (CMIP6). Evaluating the models' benchmark climate sensitivity and feedback metrics is a useful first step to understanding their characteristic response to forcing. The effective climate sensitivities are found to be higher than that seen in the previous generation of models, in common with other recently developed climate models. Reasons for this are discussed.

U.K. Community Earth System Modeling for CMIP6

“…As discussed in Andrews et al. (2019), the net feedback in the new models is toward the top end of (but not outside of) the CMIP5 range. The EffCS lies outside of the CMIP5 range due to this feedback combining with a relatively “normal” sized forcing.…”

The HadGEM3-GA7.1 radiative kernel: the importance of a well-resolved stratosphere

“…Aerosols were neglected and greenhouse gases, including the prescribed CMIP6 monthly climatology in ozone concentrations, were set to their pre-industrial (1850) values. Following the protocol for RFMIP (Pincus et al, 2016), sea-surface temperatures and sea-ice distributions from 50 years of the HadGEM3-GC3.1 coupled model were used to build the climatology (Andrews et al, 2019).…”

Tuning the MPI‐ESM1.2 Global Climate Model to Improve the Match With Instrumental Record Warming by Lowering Its Climate Sensitivity

“…In this case the temperature is nevertheless colder than observed in the 1960s to early 2000s, which is a consequence of the temporal evolution of the aerosol forcing that increased up until the 1970s and then changed only little afterwards as greenhouse gas forcing rose more steadily in time (Figure ). It is therefore difficult to compensate a high climate sensitivity only with strong aerosol cooling and obtain a realistic temporal evolution (Zhao et al, ), and the behavior seen in the two‐layer model simulation here can be seen in several of the recent CMIP6 models with high ECS (Andrews et al, ; Flynn & Mauritsen, ; Golaz et al, ; Held et al, ). The planetary imbalance in year 2011 in both cases, 0.76 W m −2 for the standard setup, and 0.80 W m −2 with high climate sensitivity and strong aerosol cooling, are close to but slightly higher than the observed estimate of 0.71 W m −2 ± 0.10 for the period 2005–2015 (Johnson et al, ).…”