A new study of the Eastern Pacific region, published in the latest issue of Science, finds that climate warming in recent decades has decreased cloud cover—which leads to more warming.
Hear an interview with study author Amy Clement by Stewart Wills, host of this week's Science Podcast.
The effect of low-level clouds on climate is one of the largest obstacles standing in the way of more confident predictions of global climate change. Low-level clouds are typically composed of water droplets and include the darker, flatter clouds often associated with a cloudy day.
Some models project that warming may increase this type of cloudiness. But the clouds themselves may cause cooling—a "negative feedback"—by blocking incoming solar radiation. On the other hand, if warming cuts down on this cloudiness, thus allowing in more solar radiation, the diminishing clouds may then create a "positive feedback" effect, in which warming begets more warming.
In the 24 July issue of Science, Amy Clement of the University of Miami and colleagues analyzed several datasets recording cloud cover over five decades, as measured by both human eyes and satellite instruments, over a large patch of the subtropical Eastern Pacific, roughly between Hawaii and Mexico. They found that low-level clouds do in fact have a positive feedback effect, on a decadal timescale.
Changes in cloud cover in that region appeared to be linked to changes in both local sea surface temperatures and large-scale atmospheric circulation. During a warming episode in the 1970s, for example, cloudiness decreased while sea surface temperatures rose; the opposite occurred during a cooling event in the late 1990s.
The authors then tested leading climate models and found that only one simulated this positive feedback effect realistically. This model also predicts the climate system will be relatively sensitive to a doubling of atmospheric carbon dioxide.
"We have to acknowledge the fact that there is a wide range of predicted warmings for the 21st century, and this study doesn't tell us which trajectory we're on," Clement said in an interview with Stewart Wills, who hosts this week's Science Podcast. "It does indicate that perhaps we should be giving serious consideration to the high end, the high range of future warming."