Climate Feedback

Cloud shields breached by warming

clouds.jpgBlankets of low clouds shield and cool the Earth’s surface – but in a warming climate, will this safety blanket thicken, or will it deteriorate? That question has bugged climatologists for decades. A paper published in Science today (subscription) now offers convincing evidence that warming leads to fewer clouds, and thus exposure to more warming.

The positive feedback effect was observed in the northeast Pacific Ocean, where the relationship between short-term local meteorological conditions and cloud cover is fairly well understood. In contrast, scientists hadn’t previously said much about how this relationship plays out on a decadal scale – in part because any long-term data set would be viewed askance. Satellite cloud records go back only 25 years, and their accuracy can be wrecked by instrument drift and data gaps. Another type of cloud data, eyeball observations made from ships, is considered suspiciously subjective.

But in the new study, Amy Clement of the University of Miami and colleagues looked at both types of measurements from the northeast Pacific and found they were remarkably consistent. Clement says in a press release,

“The agreement we found between the surface-based observations and the satellite data was almost shocking. These are subtle changes that take place over decades. It is extremely encouraging that a satellite passing miles above the earth would document the same thing as sailors looking up at a cloudy sky from the deck of a ship.”

The data showed that over the climatic ups and downs since the 1950s, periods of warming have dispersed the clouds. The authors then compared 18 global climate models to see if they could match the observations. Of these, just two showed the correct relationship between temperature, circulation and clouds – and only one of the pair, from the UK’s Hadley Centre, also responds to greenhouse gas forcing in a manner that matches the average response of the 18 models.

The Hadley model also happens to be one with highly sophisticated methods for simulating clouds, so it may point the way to incorporating the group’s discovery into future models.

Anna Barnett

Image: Jason Pratt, Creative Commons License

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