Don’t Count on It: Reasons to doubt a strong negative cloud feedback

Friday, December 18, 2015 - 08:00
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In CMIP3 and CMIP5 climate models, positive global mean cloud amount and altitude feedbacks are opposed by negative cloud optical depth feedbacks, resulting in a net positive cloud feedback in nearly every model. Is it possible that the relative strengths of these feedbacks are incorrect, such that the net cloud feedback is overestimated in models? In this study we quantify the individual components that govern the magnitude of the LW high cloud altitude feedback and the SW low cloud optical depth feedback. The feedbacks are shown to depend on both mean-state cloud properties and the response of cloud properties to warming. Comparing with observations and exploiting relationships between forced and unforced cloud changes, we demonstrate that models likely underestimate the positive LW high cloud altitude feedback and likely overestimate the magnitude of the negative SW low cloud optical depth feedback. We conclude that it is unlikely that the net cloud feedback is strongly negative.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. It is supported by the Regional and Global Climate Modeling Program of the Office of Science at the DOE. IM release #: LLNL-ABS-669552