Scientists at NASA's Goddard Space Flight Center say they have the first conclusive evidence that the Earth's protective ozone layer is being eroded by man-made chemical products, and not by natural events like volcanic eruptions.
Data gathered by NASA's Upper Atmosphere Research Satellite UARS), they said, have found ozone-destroying chlorine in the stratosphere over Antarctica along with fluorine. It's a coincidence, the scientists say, that can be explained only by the breakdown of chlorofluorocarbons -- the chief suspect in ozone destruction that contains both elements.
"We don't see any alternative," said Dr. Mark Schoeberl, project scientist at Goddard's Laboratory for Atmospheres. "The data confirm that chlorofluorocarbons are the major source of chlorine in the stratosphere and are the cause of the ozone loss we are seeing in the stratosphere."
"This is nailed," said his deputy, Dr. Anne Douglass. "There is no other possibility."
John W. Passacantando, executive director for Ozone Action, a Washington-based environmental group, welcomed the Goddard findings and urged accelerated international action to stop the manufacture and use of CFCs and to replace them with products less damaging to the ozone layer.
But Dr. Schoeberl said such measures were too costly to warrant moving faster. "I think if we stick to what we planned, that's pretty good, and the CFCs are going to come down," he said.
Chlorofluorocarbons (CFCs) are compounds invented in the 1930s as a safer alternative to the ammonia then used in refrigerators. They have been used since as refrigerants, propellants in spray cans and as cleaning agents.
Ozone is a three-atom form of oxygen. Formed naturally in the upper atmosphere, it acts as a barrier to harmful ultraviolet radiation from the sun.
When scientists noticed in the 1970s that the ozone layer seemed to be thinning each spring over the Antarctic, they began to suspect it was being broken down in a seasonal chemical reaction with chlorine more rapidly than it could be replaced by nature.
CFCs soon became suspect as the source of the chlorine, and some scientists warned that a continuing loss of ozone could lead to an increased incidence of human skin cancer, crop losses and environmental damage.
As scientific support for the theory grew, governments responded. Since a 1987 treaty called the Montreal Protocol, 87 nations have agreed to halt production of CFCs by Jan. 1, 1996.
Thanks to that agreement and other unilateral actions, Dr. Schoeberl said, CFC levels in the upper atmosphere should level off by 1998, and start to decline by 2005. By 2020, they should have returned to levels that existed in 1979 when the ozone hole was first measured.
Some consumers, businesses and developing nations have complained that the alternatives to CFCs are too costly, and that there was too little proof that CFCs were at fault to justify the expense of replacing them.
A few critics argued that the ozone layer was being eroded by chlorine that found its way to the stratosphere from volcanic eruptions or ocean biology.
Yesterday's data appear to have closed the argument. "We believe this data eliminates the possibility that there are major natural sources for the chlorine in the stratosphere," Dr. Schoeberl said.
UARS instruments scanned the globe for a variety of elements and compounds produced during the breakdown of ozone in the upper atmosphere. They found not only the chlorine compounds that actually destroy the ozone, but also fluorine, which could only have come from chlorofluorocarbons.
"The real key to it is that we are only seeing high chlorine amounts in areas where we are seeing high fluorine amounts, and they match," said Dr. Douglass.
"The detection of stratospheric fluorine gases, which are not natural, eliminates the possibility that chlorine from volcanic eruptions or some other natural source is responsible for the ozone hole," Dr. Schoeberl said.
The annual ozone losses are worst over Antarctica because the chemical reactions involved are more efficient in the extreme cold of the South Polar winter. But less dramatic ozone depletion does occur over the warmer North Pole, and in the Earth's middle latitudes, where most people live and grow crops.
The Goddard findings have not fully resolved the details of middle latitude ozone losses, because the chemistry in the warmer regions is more complex and its effects more subtle.
Although chlorine is still the prime suspect there, Dr. Schoeberl said, more research using UARS and other instruments is needed to sort it out: "That's the challenge now."
Dr. Schoeberl said ozone depletion in the middle latitudes runs between 8 percent and 10 percent each spring. That's far less than the 60 percent depletion at the South Pole, but still enough to cause an extra 30,000 cases of skin cancer per decade.