The ozone layer, a thin veil of molecules in the stratosphere, absorbs the worst of the sun's ultraviolet (UV) rays before they reach the ground. Each ozone molecule comprises three oxygen atoms. The ozone layer's untimely demise is tied to chlorofluorocarbons (CFCs), the family of chemicals used as refrigerants and in the manufacture of a host of consumer products from foam insulation to computer chips. Over the years, some 20 million tons of CFCs have been released into the atmosphere, where, after a decade or so of random drift, they eventually rise to the stratosphere. There, intense sunlight causes their breakdown, releasing their destructive chlorine. CFCs are tremendously efficient ozone killers: one CFC-spawned chlorine atom can catalyze the destruction of as many as 100,000 ozone molecules.
Ozone destruction on a large scale was first observed over Antarctica in 1985. There, weather conditions and chlorine chemistry conspire to deplete the stratosphere of more than half of its ozone when the sun returns each spring, creating the infamous Antarctic ozone hole. Beneath the hole, UV intensities two or more times the normal level are common.
Substantial wintertime ozone losses have also occurred over temperate latitudes, where much of the world's population resides. Perhaps more importantly, these losses have been found to persist into spring and summer, when solar radiation is greater and UV levels are at their peak. Over northern mid-latitudes during the 1980s, summertime ozone levels dropped about 3 percent.
Healing the ozone layer will take time, but it is possible. When the chemistry of the stratosphere is in balance, oxygen molecules energized by radiation from the sun form ozone at the same rate that ozone is removed by naturally occurring chemical reactions. This equilibrium is unlikely to be regained any time soon. Returning to chlorine levels of 2.0 parts per billion (ppb), the level at which the Antarctic ozone hole first appeared, will probably take 70-80 years if we curtail CFC use on schedule.
As the ozone shield thins, greater amounts of biologically damaging UV radiation╤known as UV-B╤are expected to reach Earth's surface, leading to a variety of ill effects. Among humans, for example, the DNA damage that excessive UV-B causes is expected to increase the risk of skin cancer and cataracts.
The United Nations Environment Program estimates that sustained exposure to the extra UV associated with a 10 percent ozone loss╤a very real possibility within just a few years╤could eventually result in a 26 percent increase in the incidence of nonmelanoma skin cancers, or about 300,000 new cases per year worldwide. At the same time, about 1.6 million new cataract cases would be expected per year. Excessive UV exposure has also been linked to suppression of the human immune system, which could make people more likely to contract infectious diseases.
Studies have shown that UV radiation can impair photosynthesis and plant metabolism in a number of species, including many important food crops, such as soybeans, potatoes, beans, and sugar beets. Some research suggests that forest productivity (the overall health of the forest ecosystem) also may decrease as UV levels rise.
Marine ecosystems, which account for much of the productivity from photosynthesis at the base of the world's food webs, may be particularly vulnerable to damage by increased UV-B, which can penetrate beneath the ocean surface.
Adapted from The Information Please Environmental Almanac compiled by World Resources Institute, published by Houghton Mifflin.
Video: The Red, Green & Blue Company
Satellite: Nimbus 7
Data: Total Ozone Mapping Spectrometer (TOMS)
Visualization ⌐ 1993 University College London
Sources: Upper Atmosphere Research Satellite/Microwave Limb Sounder data
