The buildup of chlorofluorocarbons in the atmosphere appears to be slowing for the first time since the compounds came under attack as destroyers of the planet's protective ozone layer, says one of the scientists who first identified the problem.
"It's a tough call," says F. Sherwood Rowland, a chemistry professor at the University of California at Irvine. The slowdown noted in the past year is tiny and "very difficult to measure. . . . But there's definitely a slowdown." He says it appears that the annual growth in atmospheric chlorofluorocarbons (CFCs) is down by several tenths of a percent.
Rowland and Mario J. Molina of the Massachusetts Institute of Technology discovered the CFC-ozone connection in 1974. Both will address the issue again this week in Baltimore at the Chemrawn VII conference on atmospheric chemistry, at the Stouffer Harborplace Hotel.
Sponsored by the International Union of Pure and Applied Chemistry and the American Chemical Society, the meeting will bring together nearly 200 scientists from around the world. They will report on ozone depletion, global warming, smog and other topics related to global atmospheric change.
The slowdown in the accumulation of CFCs in the atmosphere is likely to continue as manufacturers who had used them as coolants, fire-control gases, blowing gases for polyurethane foams and aerosol propellants continue to switch to more environmentally friendly chemicals, Rowland says.
International agreements and national regulations have banned CFCs in some countries and have begun a gradual phase-out of their manufacture.
But Rowland says no real reduction in global atmospheric CFC levels is likely until production stops, and all the CFCs contained in refrigerator and air conditioner coils have leaked out and worked their way up to the stratosphere.
That will take "at least 10 years, maybe 20," he says. And some types of CFC molecules already released will continue to destroy ozone molecules for 65 to 100 years before they decay.
In a recent interview, Rowland said that "knowledgeable scientists" no longer debate the role of CFCs in destroying the stratospheric ozone that shields Earth from most damaging ultraviolet radiation.
Ongoing measurements have documented seasonal "holes" in the ozone layer over Antarctica, where the loss of stratospheric ozone is "nearly total" during the southern spring. The annual depletion there tripled between 1988 and 1990.
The losses are less in the northern hemisphere -- perhaps 10 percent at some latitudes compared with measurements 20 years ago.
"But this is in places where people live," Rowland said. "On average for the globe, there is probably 4 to 5 percent less ozone than there was 20 years ago."
And for each 1 percent loss of ozone, scientists expect a 2 percent increase in UV-B, the ultraviolet radiation most likely to produce skin cancer in humans.
Some 500,000 people get skin cancer in the United States each year, up from 300,000 15 years ago.
Most of the rising cancer rate can be blamed on the cumulative damage inflicted by changing lifestyles that have increased exposure to the sun, Rowland said. Ozone depletion has added only a small number.
"But those numbers will climb if there is no change in lifestyle," he said.
The U.S. Environmental Protection Agency has estimated 200,000 additional skin cancer deaths over the next 50 years in the United States.
Some people have gotten the message. New Zealanders are now warned to keep their children off the beaches when high altitude ozone is low.
"And I think we notice on the beaches here in Southern California that . . . the first thing people do . . . is they pull out a beach umbrella and put lotion [with sun block] all over the kids," Rowland said.
It's not clear yet just how bad the ozone losses will get in populated regions before things get better.
Researchers are searching for more substitutes for CFCs. Although scientists identified safe substitutes in the late 1970s, research on them stopped in 1981 when the Reagan administration made it clear that the federal government would not force a switch.
Without government regulations, there would be no market for the alternative chemicals, which would be three or four times more costly, Rowland said.
Consumer pressure in 1975 and 1976 forced aerosol manufacturers in the United States and a few other countries to stop using CFCs as a propellant. But it wasn't so easy for consumers to put pressure on manufacturers of air conditioners and refrigerators.
An international agreement in 1990 calls for a complete phase-out of CFCs by the end of the decade, with exceptions for developing countries.
Some countries imposed more rapid phase-outs. In the United States, Rowland said, "industries led the way." The biggest CFC maker, DuPont, announced in 1988 that it would soon get out of the business entirely.