Clouds warm things up

Over the past decade, during short-term climate changes, clouds trapped heat in the Earth’s atmosphere and warmed the planet, a new study suggests.

The work is the most detailed look at how clouds affect climate — one of the biggest scientific unknowns in how much global warming to expect (SN: 12/4/10, p. 24). Some researchers have suggested that clouds might cool the planet overall, but the study supports the opposite idea, that clouds make things toastier.

“This is really the first quantitative test of the total cloud feedback in climate models,” says Andrew Dessler, an atmospheric scientist at Texas A&M University in College Station. “The results suggest that our understanding of the cloud feedback, and the simulation of the cloud feedback by models, is actually quite good.”

Dessler’s paper appears in the December 10 issue of Science.

Only about a third of the roughly 3 degree Celsius warming expected over the next century will come directly from heat-trapping greenhouse gases like carbon dioxide, says Dessler. The rest comes from a variety of feedbacks, or processes that act to amplify the solar heating. Climate scientists understand almost all of these feedbacks very well, except for clouds.

That’s because clouds can both cool the planet, by reflecting sunlight back into space, and warm it, by absorbing heat reradiating from the Earth’s surface and preventing the heat from escaping. Researchers haven’t been able to figure out which of these effects is more important overall.

To tackle the question, Dessler used an instrument on NASA’s Terra satellite to track global radiation coming into and leaving Earth’s atmosphere between March 2000 and February 2010.

By subtracting complicating factors like the amount of atmospheric water vapor and how reflective the Earth’s surface was, Dessler was left with the effects of changing cloud cover. And although the data could potentially be interpreted as clouds showing a mild cooling effect, the best explanation, he says, is that clouds amplify whatever warming might be going on.

Running several computer models over the same period also produced a positive feedback, an indication that the climate simulations deal with cloud effects relatively well.

“It’s very encouraging to see analysis of observations placing some greater constraints on estimates of the magnitude of cloud feedback,” says Dennis Hartmann, an atmospheric scientist at the University of Washington in Seattle.

The biggest source of planetary climate change during the decade studied was the El Ni±o Southern Oscillation, a climate pattern that can temporarily raise temperatures by several tenths of a degree Celsius. More work is needed, says Hartmann, to understand how clouds will behave during long-term changes like the global warming expected from greenhouse gases.

But having some numbers on cloud feedback is better than having no numbers at all, says Dessler. He next plans to look more closely at how clouds are distributed around the globe, and how the feedback might vary locally.