Double Trouble: Cloud Feedbacks and Rapid Adjustments to CO2 are Positive in Current GCMs
Clouds have a tremendous amount of leverage on the Earth's energy budget, and even small changes in their properties can strongly amplify or diminish the amount of warming that occurs in response to increased greenhouse gases. Climate models forced by increased CO2 concentrations predict a wide range of cloud changes, which lead to a wide range of possible future climates. The best predictor of how much the planet warms due to increased greenhouse gases is the response of clouds. Thus, it is important to understand the details of how cloud properties change in models, the types of cloud changes for which the models disagree, and the features that all models robustly predict.
In this study we diagnose in detail the changes that clouds undergo in experiments in which CO2 is instantaneously quadrupled and held fixed. We use a "cloud radiative kernel" technique that was described in previous work (here and here), which allows us to separately quantify the radiative impact of changes in individual cloud types at a variety of altitudes and optical depths (albedos). The cloud changes in these experiments are characterized by a rapid response to the abrupt CO2 quadrupling, as well as a slower, temperature-mediated response as the planet warms. In all of the five models analyzed, clouds exhibit a rapid decrease in fractional coverage and optical depth (albedo), as well as an increase in altitude upon quadrupling of CO2. These changes lead to roughly 1 W/m2 of planetary heating, which enhances the heating due directly to CO2. Rapid reductions in mid-level clouds and optically thick clouds are especially important in causing a reduction in the planetary albedo immediately upon quadrupling CO2. As the planet subsequently warms, cloud fraction decreases fairly uniformly at low- and mid-latitudes, cloud optical depth decreases slightly at low latitudes and increases substantially at high latitudes, and cloud top altitude increases nearly everywhere. Thus, the global mean net cloud feedback is positive in all but one model, and arises from a positive cloud amount feedback, negative cloud optical depth feedback, and positive cloud altitude feedback.
Current fully coupled climate models robustly predict that clouds will become fewer, thinner, and higher immediately upon quadrupling of CO2. These changes lead to a large increase in planetary heating on top of what is caused directly by CO2. In addition, as the planet subsequently warms due to CO2, models robustly predict clouds become fewer, thicker, and higher. This leads to positive cloud amount and altitude feedbacks that are slightly opposed by a negative optical depth feedback, and the global mean net cloud feedback is positive in all but one model.
We acknowledge the World Climate Research Programme's Working Group on Coupled Modelling, which is responsible for CMIP, and we thank the climate modeling groups for producing and making available their model output. For CMIP, the U.S. Department of Energy's (DOE's) Program for Climate Model Diagnosis and Intercomparison provides coordinating support and led development of software infrastructure in partnership with the Global Organization for Earth System Science Portals. The work of MDZ, SAK, and KET was supported by the DOE Office of Science Regional and Global Climate Modeling Program and was performed under the auspices of the DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. TA, MJW and JMG were supported by the Joint DECC/Defra Met Office Hadley Centre Climate Programme (GA01101). MJW is also supported by funding from the European Union, Seventh Framework Programme (FP7/2007-2013) under grant agreement No. 244067 via the EU Cloud Intercomparison and Process Study Evaluation project (EUCLIPSE).