The Earth's protective ozone layer is thinning twice as fast over the Northern Hemisphere as scientists had previously believed, according to preliminary NASA satellite observations.
The new measurements of depletion -- made from 1978 to 1990 -- strengthen the case that man-made chemicals are responsible for destroying the ozone layer, and they greatly increase estimates of how many skin cancer deaths can be expected during the next 50 years. The Environmental Protection Agency, in making the data public, said yesterday that it would anticipate 200,000 additional deaths in the United States.
"Clearly things have gotten worse in the last five years, and when youare talking about the atmosphere, that is rapid change," said F. Sherwood Rowland, a chemist at the University of California at Irvine and a leading authority on ozone depletion.
As a result of the new measurements -- taken over Europe, Canada and the United States -- EPA officials said yesterday that they would urge a speedier conversion to the use of chemicals that do not harm the ozone layer, a shield that protects the Earth from the sun's harmful rays. Chlorofluorocarbons -- found in air conditioners, foam insulation and refrigerators -- are believed to be responsible for destroying ozone in the upper atmosphere.
Seventy nations, including the United States, have signed a treatythat calls for the phase-out of CFCs by 2000 and of other ozone-eating chemicals by 2005.
The most serious thinning occurs during the winter and spring, said Richard S. Stolarski, a research scientists at the Goddard Space Flight Center, who found a 4 percent to 5 percent average decrease over the 11-year period he studied. During the winter, the depletion was measured at 8 percent to 9 percent of the layer.
Until the recent release of the National Aeronautics and Space Administration satellite data, scientists had used ground-based measurements to calculate a smaller depletion over the Northern Hemisphere. But analysis of the ground measurements has not been made since 1986.
"I think after this analysis I have more confidence in the ozone data than at any time in the past," said Mack MacFarland, a Du Pont Co. scientist who has reviewed Dr. Stolarski's work. "I think it emphasizes our position to phase out as soon as possible." Du Pont, a major producer of CFCs, announced recently that it would close the world's largest CFC plant, in New Jersey, in May.
If the depletion continues at its current rate, Dr. MacFarland said, it could be expected to double in the next decade.
The average person probably will not be able to detect a difference because of the depletion, Dr. Stolarski said, although some fair-skinned people may get a sunburn slightly more quickly. "I certainly haven't done much to change my lifestyle because of it."
Dr. Stolarski, while cautious about drawing conclusions from the new data, did note that the findings demonstrated that the effects of man's use of the atmosphere as a dumping ground for chemicals can be seen on a global scale.
David Doniger, an attorney at the Natural Resources Defense Council, an environmental group, said, "The information goes from worse to worst and is extremely alarming. I would applaud [EPA chief William K.] Reilly for drawing attention to this."
He urged Mr. Reilly to use his authority to step up the phase-out of CFCs in the United States and to issue strong requirements for recycling those chemicals now in use.