Climate researchers have consistently emphasized that the response of clouds to climate change will play a critical role in the future of the globe’s climate, but assessing the magnitude, and direction, of this feedback mechanism has proven extremely problematic in climate change modeling. A new study in the journal Science, Clement, Burgman & Norris, Observational and Model Evidence for Positive Low-Level Cloud Feedback, is a valuable contribution in this context, assessing the two best, long-term records of cloud behavior over a rectangle of ocean that nearly spans the subtropics between Hawaii and Mexico. The study’s conclusions are sobering. During a warming episodes, that started around 1976, ship-based data showed that cloud cover—especially low-altitude cloud layers—decreased in the study area as ocean temperatures rose and atmospheric pressure fell. This, in turn, appears to result in more sunlight being let in, further warming the ocean, an apparent positive, and potent, feedback mechanism.
Another implication of the study is in the context of the accuracy of global climate models. The study tested 18 GCMs to ascertain if they were accurately modeling the cloud positive feedback mechanism, and concluded that only two of them were, with the stand out in this context being the HadGEM1 model from the U.K. Met Office’s Hadley Center in Exeter. This is not particularly good news since this model is among the most sensitive of the 18 models to added greenhouse gases. When carbon dioxide is doubled, the model warms the world by 4.4°C; the median of the models for a doubling is 3.1°C.
This study is thus a good way to demonstrate to students the importance of seeking to operationalize the precautionary principle in the UNFCCC. As the lead author of the study concluded, “We tend to focus on the middle of the range of model projections and ignore the extremes . . . I think it does suggest serious consideration should be given to the upper end of the range.”