Stratospheric Ozone Depletion

There is now strong evidence that a consequence of human activities is the inadvertent accumulation of a variety of trace gases in the earth’s atmosphere. In the past decade, these global-scale changes in the chemical composition of the atmosphere have led to a very significant depletion of the stratospheric ozone layer over Antarctica in the southern spring months and also at northern latitudes. The ozone layer is important for the earth’s energy budget; also, it shields the earth’s surface from the sun’s ultraviolet radiation, which is damaging to biological systems. Ozone is continuously being generated in the stratosphere by the action of solar radiation on atmospheric oxygen, and it is destroyed by catalytic processes involving trace amounts of free radical species. In 1974 the release of chlorofluorocarbons (CFCs) of industrial origin was predicted to lead to the production of chlorine free radicals in the stratosphere, with the consequent catalytic destruction of ozone. Atmospheric observations, laboratory measurements, and modeling calculations have clearly established that this rapid decline in ozone is indeed caused by CFCs. There is now an international agreement in place, the Montreal Protocol, which led to the almost complete phase-out of these compounds in industrialized countries by the end of 1995.