Strong Dependence of Atmospheric Feedbacks on Mixed‐Phase Microphysics and Aerosol‐Cloud Interactions in HadGEM3

Bodas‐Salcedo, Alejandro; Mulcahy, Jane; Andrews, Timothy; Williams, Keith D.; Ringer, Mark A.; Field, Paul R.; Elsaesser, Gregory S.

doi:10.1029/2019ms001688

Cited by 121 publications

(156 citation statements)

References 93 publications

(156 reference statements)

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“…Hence throughout the extratropics, the multimodel mean feedback is substantially more positive in CMIP6 (Figure a black line). These changes are similar to those seen in CESM1‐CAM5 experiments in which the treatment of mean‐state supercooled liquid fraction is modified to better match observations (Frey & Kay, ; Tan et al, ), and in the evolution of HadGEM3‐GC2 to HadGEM3‐GC3.1 (Bodas‐Salcedo et al, ) and CESM1 to CESM2 (Gettelman et al, ).…”

Section: Resultssupporting

confidence: 77%

“…A plausible reason for these responses is an increase in mean‐state supercooled liquid water in mixed‐phase clouds—manifest as an increased liquid condensate fraction (LCF)—in CMIP6 (Text S6 and Figure S10). Models with larger mean‐state LCF have been shown to experience weaker LWP increases with warming (Bodas‐Salcedo et al, ; Gettelman et al, ; McCoy et al, ; Tan et al, ), qualitatively consistent with CMIP6 versus CMIP5 differences. To the extent that condensate sinks related to ice processes weaken with warming (Ceppi et al, ), condensate lifetime increases, thereby increasing cloud cover or opposing cloud cover reductions caused by other processes.…”

Section: Conclusion and Discussionmentioning

confidence: 74%

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Causes of Higher Climate Sensitivity in CMIP6 Models

Zelinka

Myers

McCoy

et al. 2020

Geophysical Research Letters

1,041

917

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Equilibrium climate sensitivity, the global surface temperature response to CO 2 doubling, has been persistently uncertain. Recent consensus places it likely within 1.5–4.5 K. Global climate models (GCMs), which attempt to represent all relevant physical processes, provide the most direct means of estimating climate sensitivity via CO 2 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 1.8–5.6 K across 27 GCMs and exceeding 4.5 K 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.

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Section: Resultssupporting

confidence: 77%

Section: Conclusion and Discussionmentioning

confidence: 74%

Causes of Higher Climate Sensitivity in CMIP6 Models

Zelinka

Myers

McCoy

et al. 2020

Geophysical Research Letters

1,041

917

View full text Add to dashboard Cite

show abstract

“…The high sensitivity of the model to CO 2 concentrations and emissions, as evidenced by the high values of ECS, TCR, and TCRE will also be a focus of further study. Bodas-Salcedo et al (2019) show that some of the changes responsible for the high sensitivity represent significant improvements in process evaluation, but…”

Section: Discussionmentioning

confidence: 99%

“…The UKESM1 values of ECS, TCR, and TCRE are all higher than those of CMIP5 models (respectively 2.1 K to 4.7 K, 1.0 K to 2.6 K, and 0.8 K TtC−1 to 2.4 K TtC−1; Andrews et al, 2012;Gillett et al, 2013). Elsewhere in this special issue, Bodas-Salcedo et al (2019) analyze the increase in atmospheric climate feedbacks in HadGEM3-GC3.1 relative to the previous version of HadGEM3, whose ECS (3.2 K; Senior et al, 2016) is within the range of CMIP5 models. They find that the feedbacks have become more positive as a result of improvements to cloud microphysics and cloud-aerosol interactions.…”

Section: Emergent Behavior Of the Coupled Modelmentioning

confidence: 93%

UKESM1: Description and Evaluation of the U.K. Earth System Model

Sellar

Jones

Mulcahy

et al. 2019

J Adv Model Earth Syst

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601

461

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We document the development of the first version of the U.K. Earth System Model UKESM1.The model represents a major advance on its predecessor HadGEM2-ES, with enhancements to all component models and new feedback mechanisms. These include a new core physical model with a well-resolved stratosphere; terrestrial biogeochemistry with coupled carbon and nitrogen cycles and enhanced land management; tropospheric-stratospheric chemistry allowing the holistic simulation of radiative forcing from ozone, methane, and nitrous oxide; two-moment, five-species, modal aerosol; and ocean biogeochemistry with two-way coupling to the carbon cycle and atmospheric aerosols. The complexity of coupling between the ocean, land, and atmosphere physical climate and biogeochemical cycles in UKESM1 is unprecedented for an Earth system model. We describe in detail the process by which the coupled model was developed and tuned to achieve acceptable performance in key physical and Earth system quantities and discuss the challenges involved in mitigating biases in a model with complex connections between its components. Overall, the model performs well, with a stable pre-industrial state and good agreement with observations in the latter period of its historical simulations. However, global mean surface temperature exhibits stronger-than-observed cooling from 1950 to 1970, followed by rapid warming from 1980 to 2014. Metrics from idealized simulations show a high climate sensitivity relative to previous generations of models: Equilibrium climate sensitivity is 5.4 K, transient climate response ranges from 2.68 to 2.85 K, and transient climate response to cumulative emissions is 2.49 to 2.66 K TtC −1 . Plain Language SummaryWe describe the development and behavior of UKESM1, a novel climate model that includes improved representations of processes in the atmosphere, ocean, and on land. These processes are inter-related: For example, dust is produced on the land and blown up into the atmosphere where it affects the amount of sunlight falling on Earth. Dust can also be dissolved in the ocean, where it affects marine life. This in turn changes both the amount of carbon dioxide absorbed by the ocean and the material emitted from the surface into the atmosphere, which has an affect on the formation of clouds. UKESM1 includes many processes and interactions such as these, giving it a high level of complexity. Ensuring realistic process behavior is a major challenge in the development of our model, and we have carefully tested this. UKESM1 performs well, correctly exhibiting stable results from a continuous pre-industrial simulation (used to provide a reference for future experiments) and showing good agreement

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“…What model developments have led to the models relatively high EffCS? Bodas‐Salcedo et al () tested the impact of all atmospheric model developments on λ NET between HadGEM3‐GC3.1‐LL and the previous HadGEM configuration HadGEM3‐GC2.0 (Williams et al, ), which had a “typical” EffCS of ~3 K (Senior et al, ). (Note that the CMIP5 configuration was HadGEM2‐ES, but the vast amount of model developments since then means a traceable link between model development and changes in climate sensitivity is impractical).…”

Section: Global Sensitivity and Feedbacks Metrics To Idealized Co2 Chmentioning

confidence: 99%

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

Andrews

Bodas‐Salcedo

et al. 2019

J Adv Model Earth Syst

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100

120

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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.

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Strong Dependence of Atmospheric Feedbacks on Mixed‐Phase Microphysics and Aerosol‐Cloud Interactions in HadGEM3

Cited by 121 publications

References 93 publications

Causes of Higher Climate Sensitivity in CMIP6 Models

Causes of Higher Climate Sensitivity in CMIP6 Models

UKESM1: Description and Evaluation of the U.K. Earth System Model

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

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