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Science 24 July 2009:
Vol. 325. no. 5939, pp. 460 - 464
DOI: 10.1126/science.1171255
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Reports

Observational and Model Evidence for Positive Low-Level Cloud Feedback

Amy C. Clement,1,* Robert Burgman,1 Joel R. Norris2

Feedbacks involving low-level clouds remain a primary cause of uncertainty in global climate model projections. This issue was addressed by examining changes in low-level clouds over the Northeast Pacific in observations and climate models. Decadal fluctuations were identified in multiple, independent cloud data sets, and changes in cloud cover appeared to be linked to changes in both local temperature structure and large-scale circulation. This observational analysis further indicated that clouds act as a positive feedback in this region on decadal time scales. The observed relationships between cloud cover and regional meteorological conditions provide a more complete way of testing the realism of the cloud simulation in current-generation climate models. The only model that passed this test simulated a reduction in cloud cover over much of the Pacific when greenhouse gases were increased, providing modeling evidence for a positive low-level cloud feedback.

1 Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Division of Meteorology and Physical Oceanography, MSC 362, 4600 Rickenbacker Causeway, Miami, FL 33149, USA.
2 Scripps Institution of Oceanography, University of California-San Diego, La Jolla, CA 92093–0224, USA.

* To whom correspondence should be addressed. E-mail: aclement{at}rsmas.miami.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)