The chlorine acts as a catalyst that is, it takes part in several chemical reactions, yet at the end emerges unchanged and able to react again. A single chlorine atom can destroy up to 100,000 ozone molecules in the stratosphere. Other pollutants, including nitrous oxide from fertilizers and the pesticide methyl bromide, also attack atmospheric ozone. Scientists are finding that under this assault the protective ozone layer in the stratosphere is thinning. In the Antarctic region, it vanishes almost entirely for a few weeks every year. Although CFC use has been greatly reduced in recent years, CFC molecules already released into the lower atmosphere will be making their way to the stratosphere for decades, and further ozone loss is expected. As a result, experts anticipate an increase in skin cancers, more cataracts (clouding of the lens of the eye), and reduced yields of some food crops. Humans are bringing about another global-scale change in the atmosphere: the increase in what are called greenhouse gases. Like glass in a greenhouse, these gases admit the sun's light but tend to reflect back downward the heat that is radiated from the ground below, trapping heat in the earth's atmosphere. This process is known as the greenhouse effect. Carbon dioxide is the most significant of these gases; there is 25 percent more carbon dioxide in the atmosphere today than there was a century ago, the result of our burning coal and fuels derived from oil. Methane, nitrous oxide, and CFCs are greenhouse gases as well. Scientists predict that increases in these gases in the atmosphere will make the earth a warmer place. They expect a global rise in average temperature somewhere between 1.0 ∘ and 3.5 ∘ C (1.8 ∘ and 6.3 ∘ F) in the next century. Average temperatures have in fact been rising, and the years from 1987 to 1997 were the warmest ten years on record. Most scientists are reluctant to say that global warming has actually begun because climate naturally varies from year to year and decade to decade, and it takes many years of records to be sure of a fundamental change.
