章篇結構21-25： Each year since the late 1970s, much of the protective layer of ozone above Antarctica has disappeared during September, creating what is popularly known as the ozone hole. How did researchers discover this hole in the earth’s atmosphere and the dangerous consequences of the ozone loss?It is now known that a man-made class of chemicals, chlorofluorocarbons (CFCs), widely used until recently in spray cans, is responsible for much of our ozone reduction. ___1___ Investigators did not set out to determine whether human activity affects basic questions about the nature of the earth’s atmosphere.It was not until the 1920s that GMB Dobson developed a machine that could measure overall amounts of ozone and started a program for the regular measurement of atmospheric ozone levels. ___2___ They were first detected in the early 1970s by the British scientist James Lovelock. He correctly concluded that the gas was carried by large-scale wind motions. ___3___Learning Lovelock’s finding in 1972, an American chemist, F. Rowland, although unaware that CFCs could harm the environment, was prompted to wonder about the ultimate fate of these elements. Joined by M. Molina, he showed that CFCs remain in the lower atmosphere for decades, while at altitudes above 18 miles, the high energy radiation from the sun hits directly on the CFC molecules, breaking them apart into chlorine atoms and others. ___4___ In 1974, Rowland and Molina made a disturbing prediction: if industry continues to release a million tons of CFCs into the atmosphere each year, atmospheric ozone would drop by 7 to 13 percent.___5___ Although it will take a long time for the ozone to return to 1970s levels, thanks to the research that led to early recognition of the problem and the steps that have been taken to solve it, the possible consequences are much less severe than they would otherwise have been.
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