According to a Japanese study, the ozone hole over the Antarctic may slam shut within four decades. Tatsuya Nagashima of the National Institute for Environmental Studies in Ibaraki simulated the gases seeping into the ozone layer, 10 to 15 miles high in the stratosphere. He found that declining levels of chlorine from recently banned chlorofluorocarbons should enable the ozone to return to early-1980s levels by 2040. But another study, conducted by physicist Drew Shindell of NASA Goddard Institute for Space Studies in New York City, suggests that recovery of the ozone layer will be delayed 10 to 20 years by rising levels of two gasesmethane and nitrous oxidethat also contribute to greenhouse warming. At low latitudes, methane in the stratosphere breaks down into hydrogen oxides, which attack ozone. Nitrous oxide can decompose to form ozone-eating nitrogen oxides. "In 60 or 70 years, there should be no more depletion from chlorine, but since there will be all these greenhouse gases, it will not look like it used to in the 1970s," Shindell says. Fortunately, the ozone layer should still be robust enough to provide protection from solar ultraviolet rays.