Clouds cool the planet by reflecting sunlight back into space. Clouds warm the planet by trapping heat. Both statements can be true, depending on what kind of clouds you’re talking about.
Add to that the fact that some types of clouds might increase in a warming world and some might decrease, and it becomes clear why clouds are such a headache for people trying to project where temperatures are likely to go over the next century. If the net effect is to trap heat, clouds will act as a positive feedback, amplifying the underlying warming from greenhouse gases. If the main effect is to reflect sunlight, their feedback will be negative, tamping the warming down.
So far, computer climate models generally agree that cloud feedbacks will be positive, but they don’t agree on how much—and even that general agreement isn’t a guarantee. Models are constructed differently, but, says Andrew Dessler, an atmospheric scientist at Texas A&M University, “they share DNA. One’s a baboon and one’s an organgutan. They could all agree and the answer could still be wrong.”
It’s also hard to make actual measurements of what clouds do in the real world, because their structures are so complex and varied, and they change so rapidly (that’s why they’re hard to model in the first place). So Dessler came at the problem in an entirely different way: he analyzed cloud feedbacks by looking at everything but clouds. The result, described in a paper just published in Science: overall, clouds are likely to amplify warming—or at the very least, to hold it back by the tiniest amount.
What Dessler did was to take look at the data from NASA’s Terra satellite—specifically from an instrument called CERES, which measures how much energy Earth radiates into space in the form of both reflected light and heat coming from the surface and lower atmosphere (NASA has a nice animation showing how CERES works). He also looked at the changes in global average temparature over the past decade, the period when Terra has been in orbit. The temperature hasn’t risen a huge amount overall—global warming is inexorable, but not especially rapid—but in 2008, the planet went through a cooling La Niña event, and then rebounded in a warming El Niño the following year.
Dessler looked at how the escaping radiation changed during those relatively dramatic temperature swings. Part of that change had to do with changes in cloud cover—but part also came from other feedbacks, including changes in water vapor (water vapor itself is a greenhouse gas) and changes in ice cover (more ice means more reflected sunlight).
When he subtracted out these other well known feedback effects, what was left over was the effect of clouds. He never had to measure that elusive quantity directly. “It really simplified the analysis,” he says.
The result was a boost for climate models, which generally come up with the same results in their virtual world that Dessler did for the real one. But it’s possible that the long-term warming caused by greenhouse gases would trigger different sorts of cloud responses than the short-term warming from an El Niño. “During an El Niño,” says Dessler, “most of the warming happens in the Pacific. With global warming it happens more at the poles.”
To get a truly accurate picture of cloud feedbacks with his method, he says, “you’d need observations for 50 0r 100 years. We obviously can’t wait that long.” On the other hand, says Dessler, “if the models were doing a terrible job with cloud feedbacks on the shorter timescale, it would make you worry. In my mind, the fact that they do a good job should us confidence.”