Studies of Climate Sensitivity

The CMIP3 Archive of climate model results created by PCMDI has made intercomparisons of the world’s climate model global warming projections far easier than in the past. We have utilized this archive heavily in the work described below.

Radiative feedbacks in climate models

In standard feedback analyses of the response to global warming, the response of global mean temperature is decomposed into components related to the changes in water vapor, clouds, and changes in surface reflectivity due to changes in snow and ice, as well as the base response in the absence of these feedbacks.. There are a number of challenges in actually performing such analysis in comprehensive climate models, owing to time dependence and correlations between clouds and water vapor. Recent papers on the methodology of feedback analyses in climate models in which we have participated include Soden, Held, Coleman, Shell, Kiehl, and Shields (2008)  and Soden and Held (2006). This work has highlighted the previously under-appreciated fact that cloud feedbacks are positive, or near zero, in all of the climate models used in the IPCC’s AR4 assessment (it had earlier been thought that some models had positive and some negative cloud feedbacks.)

The response of the hydrological cycle to warming

It is important to differentiate between those hydrological responses to warming that are robust across the world’s climate models and those that are model-dependent. Held and Soden (2006) have reviewed the robust aspects of the modeled hydrological responses, and discussed the underlying physical basis for these responses. The robust responses include:

• an increase in the poleward moisture transport that results in an increase in the convergence of water into regions where there is a time-averaged convergence (“wet” regions) and an increased divergence of water out of regions where there is a time-averaged divergence (“dry” regions) — resulting in wet regions getting wetter and dry regions drier — for further discussion of the resulting pattern of rainfall changes — click here.
• an increase in the temporal variance of this moisture convergence-divergence, resulting in greater likelihood of extreme droughts and wet periods
• a small increase in global mean precipitation or evaporation — smaller than the increase in atmospheric water content
• a reduction in the rate of circulation of mass from the surface layers to the free troposphere
• a reduction in poleward eddy sensible heat flux on long time scales