Potential Climate Sensitivity Above 5°C from Cloud Feedback Mechanisms

Nature Geoscience, October 26

Updated CMIP6 climate models (produced for the next IPCC assessment AR6) consider the radiative effects of clouds and may show higher values of warming for a doubling of CO2 in the atmosphere.  If these new estimates are borne out, the upper limit of warming for a doubling of CO2 would rise from 4.1°C, to values exceeding 5°C.  In clouds with both ice and water particles, a rise in the portion of water due to higher global temperatures makes the clouds reflect sunlight more effectively.  This slows warming, especially over the Southern Ocean.  However, as temperatures continue to rise due to increases in CO2, and clouds become predominantly liquid, this protective cloud feedback vanishes, resulting in greater warming. Cloud dynamics are extremely complex; and the authors underscore the urgency of improving our understanding of ocean heat transport and atmospheric processes to better understand and quantify the upper limit of future warming should CO2 levels continue to rise.

https://www.nature.com/articles/s41561-020-00649-1

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