Polar alert

时间:2019-03-08 04:01:04166网络整理admin

By Jeff Hecht in Boston GLOBAL warming threatens to destroy a natural safety mechanism that has been preserving Arctic ozone. As a result, a hole the size of the one above Antarctica could develop even as chlorine levels decline, says a scientist from California. While climate change is warming the atmosphere closer to the ground, it is cooling the lower stratosphere. Standard models predict that the temperature will drop about 2 °C per decade, but over the past five years scientists have observed an even sharper fall in temperature over the poles, and this has caused alarm (“Chill in the air,” New Scientist, 1 May, p 28). Although they are not entirely clear about the mechanism, most scientists accept that colder temperatures worsen the depletion of ozone. One reason for this may be that the cold causes polar stratospheric clouds to last longer. These clouds remove nitrogen oxides that normally deactivate the chlorine monoxide (ClO) molecules that catalyse ozone destruction. Azadeh Tabazadeh of the NASA Ames Research Center in Moffett Field warned the American Geophysical Union meeting in Boston last week that just a few degrees of cooling of the stratosphere over the Arctic could have devastating results. The modest amount of cooling that is predicted by the standard models of climate change could remove the nitrogen oxides that protect ozone—and do so much more quickly than scientists realise. Tabazadeh argues that the presence of nitrogen oxides over the Arctic has been largely responsible for preventing the formation of an ozone hole as severe as the one that has developed over the Antarctic. There, the stratosphere is about 10 °C colder during the winter. If stratospheric temperatures drop by just 3 °C, clouds over the Arctic would be very similar to those in the Antarctic in June, Tabazadeh says, and they will persist as long. At that time of year, both denitrification and ozone depletion are severe. “Extensive denitrification of the Arctic will occur sometime during the 2010 decade,” she predicts. This could, she warns,