NASA satellite observations have confirmed that atmospheric ozone over Antarctica plunges as much as 60 percent for a month every year, confounding elaborate computer models used to forecast the impact of air pollutants and setting off a frenzied search for a scientific explanation.
The latest NASA data, collected by the agency's Nimbus 7 satellite last year and circulated among scientists earlier this month, add a note of urgency to the debate over a seasonal phenomenon known as the "hole in the ozone."
Identified by British researchers in the mid-1970s, the "hole" now covers an area as large as the continental United States, stretching from Antarctica to the southern tip of South America. Ozone levels in this area drop sharply every October, the beginning of spring in the Southern Hemisphere, and the phenomenon has become more pronounced in recent years.
The most recent NASA data show that ozone decreased as much as 60 percent in the hole last year, compared to a 45 percent depletion the previous year. The hole begins to disappear in November, as the months-long polar night ends and wind currents bring in air from other parts of the globe.
The observations have stirred a furor among atmospheric scientists, some of whom believe that the Earth's protective layer of ozone is being destroyed more quickly than expected, with potentially catastrophic consequences.
Ozone in the upper atmosphere screens out more than 99 percent of the sun's ultraviolet radiation. Researchers estimate that losing as little as 2.5 percent of the ozone could harm plants and animals and cause a half-million additional cases of human skin cancer each year.
The ozone reduction in Antarctica is "unprecedented anywhere else in the atmosphere," said Dr. F. Sherwood Rowland of the University of California at Irvine. "It was not predicted. The questions now are will it spread and how rapidly will it spread."
Members of the British Antarctic Survey, which has monitored ozone in Antarctica since 1957, first reported the phenomenon last May in the journal Nature. The seasonal dip, they said, did not show up in their measurements until 1973.
The findings sent NASA scurrying for reams of ozone data gathered by the Nimbus satellite, which was launched in 1978. The data showed the same phenomenon.
NASA reported the data at a meeting of scientists this month at the World Resources Institute, a Washington-based policy research organization. Institute associate Alan Miller called the findings "pretty stunning," particularly in light of the fact that none of the current computerized forecasting methods predicted an ozone drop of that magnitude anywhere on the globe.
"Our best models cannot explain the ozone hole," he said. "This is the lowest ozone ever measured above the surface of the Earth."
Scientists are not prepared to say what is causing the phenomenon. Theories range from chemical pollutants to methane releases from marine organisms. Some scientists have attributed it to the 1982 eruption of El Chichon in Mexico or to the cycle of sunspots, which can affect the creation of new ozone in the stratosphere.
"We don't know how long the hole has been there, whether it's growing or what's causing it," said Gordon MacDonald, an atmospheric scientist with the Washington-based MITRE Corp., an engineering consulting firm.
Scientists have hypothesized that any decrease in ozone levels would be most pronounced in Antarctica, partly because atmospheric changes tend to be intensified at the Earth's poles and partly because of unique meteorological conditions there.
The seasonal depletion that produces the "hole" occurs at a time of year when atmospheric conditions over Antarctica create a massive "vortex," or whirlwind, that keeps the same mass of air swirling over the ice caps for several months in darkness and bitter cold.
"It is in this polar vortex that something has happened in the last decade or so that wasn't operating in the '50s and '60s," Rowland said.
The British team reported that the annual ozone dip cannot be explained by any change in atmospheric conditions and "chemical causes must be considered."
In Rowland's view, the most likely culprit is the class of chemicals called chlorofluorocarbons, used as aerosol propellants and refrigerants. He was among the first to link chlorofluorocarbons to the destruction of ozone in the stratosphere, the segment of the atmosphere that extends from about nine to 30 miles above the Earth's surface.
The United States banned use of chlorofluorocarbons as aerosol propellants in 1977, but global use of the chemicals is growing. Other nations permit aerosols containing chlorofluorocarbons, and it is in wide use in the United States and elsewhere for refrigeration, air conditioning and plastic foam products.
MacDonald said ozone levels may also be affected by bromine compounds -- used in fire extinguishers -- as well as other chemicals and methane, a gaseous hydrocarbon that can be either manmade or produced naturally through the decomposition of vegetation.
Whatever its cause, the mysterious hole has attracted attention from dozens of atmospheric scientists across the globe as well as calls for additional research.
"We may be seeing things in the Southern Hemisphere ahead of time to show what the future will be in the Northern Hemisphere," MacDonald said.