or ozonosphere, region of the stratosphere containing relatively high concentrations of ozone, located at altitudes of 12–30 mi (19–48 km) above the earth's surface. Ozone in the ozone layer is formed by the action of solar ultraviolet light on oxygen.

The ozone layer prevents most ultraviolet (UV) and other high-energy radiation from penetrating to the earth's surface but does allow through sufficient ultraviolet rays to support the activation of vitamin D in humans. The full radiation, if unhindered by this filtering effect, would destroy animal tissue. Higher levels of radiation resulting from the depletion of the ozone layer have been linked with increases in skin cancers and cataracts and have been implicated in the decline of certain amphibian species.

In 1974 scientists warned that certain industrial chemicals, e.g., chlorofluorocarbons (CFCs) and to a lesser extent, halons and carbon tetrachloride, could migrate to the stratosphere. There, sunlight could free the chlorine or bromine atoms to form chlorine monoxide or other chemicals, which would deplete upper-atmospheric ozone. A seasonal decrease, or "hole," discovered in 1985 in the ozone layer above Antarctica was the first confirmation of a thinning of the layer. The hole occurs over Antarctica because the extreme cold helps the very high clouds characteristic of that area form tiny ice particles of water and nitric acid, which facilitate the chemical reactions involved. In addition, the polar winds, which follow a swirling pattern, create a confined vortex, trapping the chemicals. When the Antarctic sun rises in August or September and hits the trapped chemicals, a chain reaction begins in which chlorine, bromine (from the halons), and ice crystals react with the ozone and destroy it very quickly. The effect usually lasts through November. There is a corresponding hole over the Arctic that similarly appears in the spring, although in some years warmer winters there do not result in a major depletion of the ozone layer. A global thinning of the ozone layer results as ozone-rich air from the remaining ozone layer flows into the ozone-poor areas.

Minimum ozone levels in the Antarctic decreased steadily throughout the 1990s, and less dramatic decreases have been found above other areas of the world. In 2000 (and again in 2003) the hole reached a record size, extending over 10.8 million sq mi (28 million sq km), an area greater than that of North America. In 1987 an international agreement, the Montreal Protocol, was reached on reducing the production of ozone-depleting compounds. Revisions in 1992 called for an end to the production of the worst of such compounds by 1996, and CFC emissions dropped dramatically by 1993. Recovery of the ozone layer, however, is expected to take 50 to 100 years. Damage to the ozone layer can also be caused by sulfuric acid droplets produced by volcanic eruptions.

The Columbia Encyclopedia, Sixth Edition Copyright© 2004, Columbia University Press. Licensed from Lernout & Hauspie Speech Products N.V. All rights reserved.

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