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Distinguishing Between Low and High Climate Sensitivity in Climate Models


Clouds can both cool climate by reflecting sunlight and warm it by absorbing and re-emitting thermal radiation. As a result, depending on the specific cloud properties, clouds can either strengthen or weaken greenhouse gas-induced warming. This effect is known as cloud feedback. A recent paper, published in Geophysical Research Letters, investigates how cloud feedback is represented in climate models. The work by Princeton University and University of Miami researchers distinguishes between climate models with low and high climate sensitivity due to aerosol-cloud interactions, helping scientists to better understand and reduce uncertainty in future climate projections.

The newest generation of climate models (CMIP6) features a number of models more sensitive to changes in climate, largely due to a strong greenhouse gas (GHG)-temperature amplification from cloud feedback. However, this study shows that, in response to increasing GHGs, models with a more positive cloud feedback also tend to have a stronger cooling effect from aerosol-cloud interactions. These two effects offset each other during much of the 20th century as both anthropogenic aerosols and GHGs emissions increased. Thus, despite their significantly increased sensitivity, it is hard to distinguish CMIP6 models from prior generations of climate models solely on the basis of how they reproduce historical global mean temperatures. 

Interhemispheric temperature differences, that is the temperature differences between separate hemispheres, however, does distinguish the sensitive CMIP6 models from the prior, less-sensitive models. That’s because, over the twentieth century, changes in anthropogenic aerosols were mostly concentrated in the Northern Hemisphere. Consequently, models with strong aerosol-cloud interactions produce different warming trends over the hemispheres than models with weak aerosol-cloud interactions and less positive cloud feedback. The models with lower climate sensitivity and weaker aerosol-cloud interactions are more consistent with interhemispheric asymmetric warming patterns during the 20th century. This hemispheric asymmetry of surface warming over the historical period can provide an important constraint, or check, on future climate projections in the context of important processes such as tropical cyclones and rainfall. 

This project was funded in part by the CPO Modeling, Analysis, Predictions, and Projections (MAPP) program as part of the Climate Sensitivity program award, as well as the CPO Climate Observations and Modeling (COM) program. 

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