How strong is the evidence that man-made chemicals cause Ozone depletion?
The first hypotheses that human activities could damage the Ozone layer were published in the early 1970s. For some years afterwards, it remained uncertain whether Ozone depletion would actually happen, and if so, whether human activities could be to blame. Initially, some thought that emissions of nitrogen oxides from high-flying supersonic aircraft were the main threat. Other argued that man-made chemicals could make only a tiny difference compared with natural sources of potentially Ozone-depleting chemicals, such as volcanoes. This conclusion has been further supported by improved scientific understanding of the chemical mechanisms of Ozone depletion. Volcanic eruptions can has-ten the rate of Ozone depletion, but their effects are relatively short-lived. In 1991, the eruption of Mount Pinatubo in the Philippines injected some 20 million tones of sulphur dioxide into the atmosphere, which contributed to record levels of Ozone depletion in 1992 and 1993. In the atmosphere, the sulph
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