Perhaps the most serious remaining danger to the ozone shield is the black market in cfcs. Their production has been halted in most nations, but their use is still legal--for example, to recharge air conditioners in old cars. According to the Montreal Protocols, replacement cfcs should come from inventories or recycled stocks. But as the supply has dwindled, the cost has soared. In the U.S., the price of cfcs is now five times what it is in the few countries--notably Russia and China--still producing the chemicals. The difference creates a thriving illicit trade, and U.S. authorities have intercepted thousands of tons of cfcs brought into the country illegally. Even so, the situation is far better than in the late 1970s, when 1 million tons of ozone-eating compounds were released each year. Russia, which may be the largest source for black-market cfcs, has reduced its manufacturing capacity from 40,000 tons a year in 1996 to 17,000 tons in 1997.

That gives scientists strong hope that earth's shield will soon be on the mend. Charles Jackman, an atmospheric modeler at the National Aeronautics and Space Administration in Washington, expects the ozone to reach a low ebb at decade's end and then start recovering, perhaps regaining its 1979 level by 2030. "That assumes the Montreal Protocols hold," says Jackman. They should, although the global environment--and our treatment of it--is always full of surprises.