In the 10 years from 1981 to 1991, the start of the plant growth cycle that accompanies the spring thaw appeared to leap ahead by an average of eight days. Ozone, found in the atmosphere between the earth's surface and altitude of 37 miles, absorbs much of the sun's radiation. But it is depleted through complex chemical reactions, some from man-made chlorofluorocarbons used in refrigeration, electronics and insulating materials. The ozone depletion has been sharply reduced in recent years through international agreements curbing chlorofluorocarbon emissions. The rate of increase has been about 4 percent a decade for areas near the United States-Canadian border, and 6.8 percent for Britain, Germany, Russia and Scandinavia.
Scientists consider the risks of further increases in ultraviolet radiation to be serious.
It also harms some crops and interferes with marine life. But the United Nations agency said the decline in the earth's protective layer over roughly one-third of the Northern Hemisphere did not constitute an ozone hole, as exists over Antarctica. Fahrenheit facilitated the formation of polar stratospheric clouds that in the presence of chlorine and bromine compounds, brought about the severe ozone destruction, the meteorological agency said. Dr. Hansen has been one of only a few scientists to maintain steadfastly that a century-long global warming trend is being caused mostly by humaninfluence , a belief he reiterated yesterday. The hole in the earth's ozone shield, which filters out cancer-causing ultraviolet rays, covered an area twice the size of Europe at its seasonal peak in October and grew at an unprecedented rate in 1995, the United Nations said today.