Friday, August 22, 2014

Why the Long Pause...?

As the bartender asked of the polar bear...

The apparent slowdown in global surface warming since 2000 is both a treasured talking point of the climate skeptocracy and a persistent puzzle for climate scientists. Suspicion has focused on the ocean heat content, mostly the Pacific, but according to this article in The Economist, the Atlantic now looks more like the culprit.

The most likely explanation is that it is hiding in the oceans, which store nine times as much of the sun’s heat as do the atmosphere and land combined. But until this week, descriptions of how the sea might do this have largely come from computer models. Now, thanks to a study published in Science by Chen Xianyao of the Ocean University of China, Qingdao, and Ka-Kit Tung of the University of Washington, Seattle, there are data.

Dr Chen and Dr Tung have shown where exactly in the sea the missing heat is lurking. As the left-hand chart below shows, over the past decade and a bit the ocean depths have been warming faster than the surface. This period corresponds perfectly with the pause, and contrasts with the last two decades of the 20th century, when the surface was warming faster than the deep. The authors calculate that, between 1999 and 2012, 69 zettajoules of heat (that is, 69 x 1021 joules—a huge amount of energy) have been sequestered in the oceans between 300 metres and 1,500 metres down. If it had not been so sequestered, they think, there would have been no pause in warming at the surface.

Hidden depths

The two researchers draw this conclusion from observations collected by 3,000 floats launched by Argo, an international scientific collaboration.

This sort of heat storage is probably part of a multi-decadal cycle, suggesting that when warming returns, it will return with a vengance.

UPDATE: More details in this Climatewire story.