A drying out of the stratosphere may help explain recent temperature trends at the Earth’s surface

THE stratosphere—specifically, the lower stratosphere—has, it seems, been drying out. Water vapour is a greenhouse gas, and the cooling effect on the Earth’s climate due to this desiccation may account for a fair bit of the slowdown in the rise of global temperatures seen over the past ten years. These are the somewhat surprising conclusions of a paper by Susan Solomon of America’s National Oceanic and Atmospheric Administration and her colleagues, which was published online by Science on January 28th. Whether the trend will continue, stop or reverse itself, though, is at present unknown.

The stratosphere sits on top of the troposphere, the lowest, densest layer of the atmosphere. The boundary between the two, the tropopause, is about 18km above your head, if you are in the tropics, and a few kilometres lower if you are at higher latitudes (or up a mountain). The tropopause separates a rowdy below from a sedate above. In the troposphere, the air at higher altitudes is in general cooler than the air below it, an unstable situation in which warm and often moist air below is endlessly buoying up into cooler air above. The resultant commotion creates clouds, storms and much of the rest of the world’s weather. In the stratosphere, the air gets warmer at higher altitudes, which provides stability. …