Each spring for the past ten years, an ozone hole has appeared over the Antarctic. Researchers have figured out that in the cold stratosphere above the South Pole, man-made chemicals known as chlorofluorocarbons (CFCs) can be transformed into ozone-destroying chlorine atoms. Yet over the North Pole, the ozone escaped relatively unharmed--until January 1995. Although the loss was not as severe as the Antarctic hole, some regions above Siberia saw ozone levels dip by 25 percent--and for a few days by as much as 40 percent. Atmospheric scientists believe the decrease was triggered by an unusually cold winter.
A key step in ozone destruction--the production of chlorine molecules (Cl2) from CFC derivatives--takes place on the surface of ice particles in stratospheric clouds. The clouds form in the cold darkness of polar winters, and when sunlight strikes them in spring, it splits the chlorine molecules into two charged chlorine atoms, which then destroy ozone. In the Arctic, warm air from the tropics has always dispersed the clouds before this can happen. But last year’s record-breaking cold in the Arctic stratosphere kept the clouds intact until spring, thus triggering ozone destruction. Although Arctic ozone levels rebounded by the summer, the long-term global outlook remains bleak, says atmospheric physicist Rumen Bojkov of the World Meteorological Organization. We are destroying more ozone than natural mechanisms are regenerating, he says. During the last 15 years, we have lost more than 6 percent of global ozone. Global recovery, he predicts, will not begin for at least 20 years, at which point the international ban on CFCs will bring down levels of stratospheric chlorine.