The thinning of the ozone layer over the Arctic could be much worse than we thought, because of a side-effect of global warming. If the upper reaches of the Arctic atmosphere get colder – a predicted consequence of climate change – then the rate of ozone depletion could be three times greater than currently forecast, according to Markus Rex of the Alfred Wegener Institute for Polar and Marine Research in Potsdam, Germany, and his co-workers.
“I was surprised to see these results,” says Drew Shindell, an atmospheric scientist at NASA’s Goddard Institute for Space Studies, New York. “We never suspected the models were this far out of whack,” he says. Rex and his colleagues studied climate conditions in the Arctic over the past ten winters to calculate how ozone destruction depends on the weather. They found a surprisingly strong relationship between ozone loss and the amount of polar stratospheric clouds, they report in Geophysical Research Letters.
These clouds form 20 kilometres above the ground in winter-time, and are sometimes called ‘mother-of-pearl clouds’ because of their shimmering appearance. But they are not harmless things of beauty: the clouds provide reaction surfaces for chemicals eating away the Earth’s protective ozone. Chemical reactions in the clouds convert chlorine from industrially produced compounds, such as the chlorofluorocarbons (CFCs) found in old refrigerators, into a reactive form that breaks apart ozone molecules. The destruction of ozone allows more ultraviolet rays from the sun through to the surface of the planet, harming humans and the ecosystem close to the poles.
Colder air in the stratosphere is thought to promote the formation of these clouds and the destruction of ozone. But it has proven difficult to appreciate the scale of the problem. On average the Arctic stratosphere has cooled barely perceptibly over the past few years, but Rex and colleagues say the winter-time conditions are getting more conducive to ozone destruction. The amount of stratospheric cloud has been climbing steadily since at least the late 1960s, they say.
If Rex’s findings and models prove correct, then all our predictions about future ozone depletion are under-estimates, says Shindell. It is not all bad news: Rex points out that even if polar stratospheric clouds continue to increase in size and number, the amount of CFCs should decline as the chemicals have largely been phased out. However, they do hang about in the atmosphere for a long time, so they will continue to cause ozone depletion for several decades yet.