THERE is "limited good news" about Earth's protective ozone layer. The rate at which man-made ozone-destroying chemicals are accumulating in the atmosphere has begun to decline.
Reporting this during the annual meeting of the American Association for the Advancement of Science (AAAS), atmospheric chemist F. Sherwood Rowland of the University of California at Irvine explained that this news is "good" because it shows that the worldwide effort to cut down on release of these chemicals is working. But, he added, he takes "limited" encouragement from this finding because the atmospheric concentration of the chemicals is still growing, although at a lesser rate.
Dr. Rowland, one of the first scientists to warn of the threat to the ozone layer, said the new finding underscores the recent United States Senate resolution calling for rapid phaseout of the chemicals. He said that the fact that the present international effort to phase out the chemicals by the year 2000 is already showing results demonstrates that nations can cooperate effectively to curb this type of pollution. Thus, he added, they probably can work together to phase out the chemicals at an even fast er rate.
The chemicals at issue are the chlorofluorocarbon (CFC) compounds now used as refrigerator/air conditioner coolants, industrial solvents, and cleaning compounds. When released to the atmosphere, they make their way into the stratosphere. There they take part in complex chemical reactions that destroy the ozone, which screens out biologically damaging ultraviolet radiation from the sun.
Rowland and his colleagues regularly sample the atmosphere's CFC concentration. He said it looks as though the emission rate for one of the chemicals, CFC-11, is about half of what it was in 1988. For all types of CFCs taken together, the data show an overall emission decline of somewhat less than 50 percent.
In 1988, industrial nations and many developing countries signed the so-called Montreal Protocol, which set a goal of cutting CFC production in half by the year 2000. In 1990, they changed that goal to a total phaseout of CFC production in industrial countries by 2000 and in developing nations by 2010. This was in response to growing scientific evidence that the threat to the ozone layer might be more serious than originally estimated. Ongoing international studies have reinforced that suspicion. During the 1980s, scientists linked CFCs to ozone loss over Antarctica. These CFCs cause the famous "ozone hole" that develops during the Antarctic spring. More recently, scientists have found evidence of similar - although less severe - ozone destruction over the Arctic regions as well.
The recent National Aeronautics and Space Administration aircraft surveys, which found ozone-destroying chemicals in the stratosphere as far south as Maine, emphasized that the threat to the ozone layer is global in scope. This spurred on the Senate resolution to speed up CFC phaseout. The Bush administration, which had opposed such a policy, now supports it.
It is hard to know how rapid the CFC phaseout can be. Leo Manzer of the Wilmington, Del., experimental station of the Du Pont Company - a major CFC manufacturer - told a AAAS symposium that the CFC industry has identified several probable CFC substitutes. They belong to chemical families called hydrochlorofluorocarbons and hydrofluorocarbons. But he warned development of these CFC substitutes is running on a fast track that bypasses some of the normal time-consuming testing.
"Commercial plants are being designed based on a minimal amount of data, with the potential risk that problems could occur during the pilot plant work requiring very significant and costly changes," he said. This could cause unexpected delays.
While there may be uncertainty as to the timing of the CFC phaseout, there no longer seems to be any question that it must be done. Mr. Manzer noted that there is "compelling evidence that production and use of CFCs should be phased out to reduce risk of ozone depletion."