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Discussion papers
https://doi.org/10.5194/acp-2019-1175
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-1175
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 24 Jan 2020

Submitted as: research article | 24 Jan 2020

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This preprint is currently under review for the journal ACP.

On the Climate Sensitivity and Historical Warming Evolution in Recent Coupled Model Ensembles

Clare Marie Flynn and Thorsten Mauritsen Clare Marie Flynn and Thorsten Mauritsen
  • Department of Meteorology, Stockholm University, Stockholm, Sweden

Abstract. The Earth's equilibrium climate sensitivity (ECS) to a doubling of atmospheric CO2, along with the transient 35 climate response (TCR) and greenhouse gas emissions pathways, determines the amount of future warming. Coupled climate models have in the past been important tools to estimate and understand ECS. ECS estimated from Coupled Model Intercomparison Project Phase 5 (CMIP5) models lies between 2.0 and 4.7 K (mean of 3.2 K), whereas in the latest CMIP6 the spread has increased: 1.8–5.5 K (mean of 3.7 K), with 5 out of 25 models exceeding 5 K. It is thus pertinent to understand the causes underlying this shift. Here we compare the CMIP5 and CMIP6 model ensembles, and find a systematic shift between CMIP eras to be unexplained as a process of random sampling from modeled forcing and feedback distributions. Instead, shortwave feedbacks shift towards more positive values, in particular over the Southern Ocean, driving the shift towards larger ECS values in many of the models. These results suggest that changes in model treatment of mixed-phase cloud processes and changes to Antarctic sea ice representation are likely causes of the shift towards larger ECS. Somewhat surprisingly, CMIP6 models exhibit less historical warming than CMIP5 models; the evolution of the warming suggests, however, that several of the models apply too strong aerosol cooling resulting in too weak mid 20th Century warming compared to the instrumental record.

Clare Marie Flynn and Thorsten Mauritsen

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Clare Marie Flynn and Thorsten Mauritsen

Clare Marie Flynn and Thorsten Mauritsen

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Latest update: 01 Apr 2020
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Short summary
The range of climate sensitivity of models participating in CMIP6 has increased relative to models participating in CMIP5, due to decreases in the total feedback parameter. This is caused by increases in the shortwave all-sky and clear-sky feedbacks, particularly over the Southern Ocean. These shifts between CMIP6 and CMIP5 did not arise by chance. Both CMIP5 and CMIP6 models are found to exhibit aerosol forcing that is too strong, causing too much cooling relative to observations.
The range of climate sensitivity of models participating in CMIP6 has increased relative to...
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