Dry cold stratosphere

Drying out

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

which extends up to about 55km, where the mesosphere begins—is made even less weather-prone by the absence of water vapour, and thus of the clouds and precipitation to which it leads. This is because the top of the troposphere is normally very cold, causing ascending water vapour to freeze into ice crystals that drift and fall, rather than continuing up into the stratosphere.