As spring dawns in the Antarctic, scientists are finding a larger-than-expected hole in the ozone layer, leading them to conclude that the Earth will have to live with less of its protective shield for the next several decades.
Over the past 12 years, scientists have seen the size of the seasonal hole vary in a regular pattern. This was a year in which they expected it to shrink.
But this week the hole became as big as it was last year and four out of the five previous years, according to Richard Stolarski, a research scientist at the National Aeronautics and Space Administration's Goddard Space Flight Center.
"The pictures of 1991 look just like the years before. . . . It seems to be the normal state," he said.
Last year, twice as much ultraviolet radiation as ever before pierced the stratosphere and reached Antarctica through a hole the size of North America, according to research by John Frederick at the University of Chicago.
As the ozone layer depletes and ultraviolet radiation increases, the number of skin cancer and cataract cases is expected to rise worldwide.
The effects on the region's fish and wildlife are still unknown, Dr. Frederick said.
Scientists have predicted that the increase in ultraviolet radiation would reduce the amount of phytoplankton in the oceans, upsetting the balance of its marine life.
Man-made chemicals called chlorofluorocarbons have been blamed for the depletion. The chemicals, which are used in car air-conditioners, refrigerators and insulation, are released into the atmosphere and break down into chlorine.
The cold, wind, ice crystals and sunlight of the Antarctic springs in September and October provide the perfect environment for the photochemical reaction that reduces ozone. Chlorine gobbles up the ozone, causing a hole that becomes biggest the first week in October and gradually closes up in December.
What becomes clear from the new data is that chlorine is building up in the atmosphere, said F. Sherwood Rowland, a chemist at the University of California at Irvine and a leading authority on ozone depletion. The natural year-to-year variation in ozone levels has been upset.
What that means for the atmosphere over the Northern Hemisphere, including sections of the United States, is that ozone depletion will probably continue to get worse until 2010, when Dr. Rowland predicts that the atmosphere might begin to repair itself.
That is because many nations, including the United States, have agreed to phase out CFCs by 2000. It will take years, however, for the chemicals already in the atmosphere to break down.
Dr. Stolarski said that this year's data make him wonder whether the North Pole might begin experiencing a similar hole, even though conditions there are not as ripe for such a dramatic depletion. "This really makes it imperative that we understand the limits under which this occurs," he said.
Predicting how the atmosphere would react to an increase in chlorine has been stumping scientists for some time. No scientific studies in the 1970s predicted that a hole would appear over Antarctica.
For several years at Goddard, Dr. Stolarski said, there was an annual office pool on how big the depletion would be. The technicians, not the Ph.D.s, usually won, he said.
PTC However, scientific opinion does not generally question that ozone depletion is caused by CFCs or that the depletion will continue to worsen.
"The news means that the pressure for a faster phaseout should be increased. The natural infrastructure is in trouble, and we have to get very serious about repairing it," said Rafe Pomerance, a senior associate at the World Resources Institute in Washington, D.C.