WASHINGTON -- A hole in the earth's protective ozone layer did not develop this past winter over the Northern Hemisphere, despite an alarming buildup of ozone-destroying chemicals over parts of the United States, Canada and Europe, federal scientists said yesterday.
But satellite and high-altitude airplane measurements over the pastseven months still detected a "significant" thinning of the ozone layer that is likely to worsen over the next decade, scientists said.
An early warming in the Arctic region in February and March counteracted the buildup of ozone-destroying chemicals in the upper atmosphere that had been detected only a month before, said Dr. James G. Anderson, a chemist from Harvard University who participated in the NASA-directed study.
Measurements of record levels of chlorine and bromine compounds, two chemicals largely responsible for ozone destruction, prompted scientists for NASA and the National Oceanic and Atmospheric Administration to warn that an ozone hole similar to the one seen over Antarctica could develop over the Northern Hemisphere as early as this winter.
The expected severe ozone losses did not materialize, however, because warming temperatures over the Arctic prevented the formation in the upper atmosphere of ice clouds that lead to the destruction of ozone.
Though it can cause breathing problems at ground level, ozone in the upper atmosphere shields people and plants on earth from the sun's harmful ultraviolet radiation.
Ozone depletion is caused by complex chemical reactions involving chlorine and bromine compounds in the upper atmosphere. The buildup of those compounds has been tied to the widespread use of chlorofluorocarbons in refrigeration, electronics and other industries.
Ozone losses of about 20 percent were measured in the Arctic last winter, while ozone levels in the northern mid-latitudes were 10 to 15 percent lower, NASA scientists said.
"It is not an ozone hole," said Dr. Anderson. "Is it a significant ozone loss? Yes, it is."
Overall, the total ozone measured over the Northern Hemisphere this past winter was "as low as we've ever seen it," said Dr. Mark Schoeberl, head of atmospheric chemistry at NASA's Goddard Space Flight Center in Greenbelt.
Ozone builds up each winter in the Arctic region, but for unknown reasons this year's buildup was unusually small, scientists said.
Given the continuing buildup of chlorine and bromine in the atmosphere, ozone losses of 25 percent to 30 percent might be expected if the Arctic winter is as cold and long-lasting as it has been in five of the last 10 years, Dr. Anderson said.
Though ozone levels have declined enough for a hole to form over the Antarctic each of the past three years, a similar "hole" in the ozone layer is less likely over the Arctic because it is not as cold and wind patterns there are not as stable.
More troubling is the apparent erosion of the ozone layer over more populous regions south of the Arctic, where the thinning tends to last into spring and early summer, said Dr. Michael Kurylo, a NASA scientist.
The eruption of Mount Pinatubo in the Philippines last year, which spewed sulfurous clouds into the atmosphere, may have enhanced ozone losses slightly in the Northern Hemisphere, the scientists said. But the volcano's impact is short-lived and overshadowed by the buildup of man-made ozone-depleting chemicals, they said.
The latest findings reinforce international efforts to hasten the elimination of chlorofluorocarbons worldwide, said Dr. Robert Watson, the NASA study director. The United States and other nations have agreed to phase out CFC production by 1996, five years sooner than agreed to previously.