Ozone hole

Greenhouse effect

is a warming of the lower atmosphere and surface of a planet by a complex process involving sunlight, gases, and particles in the atmosphere. On the earth, the greenhouse effect began long before human beings existed. However, recent human activity may have added to the effect. The amounts of heat-trapping atmospheric gases, called greenhouse gases, have greatly increased since the mid-1800's, when modern industry became widespread. Since the late 1800's, the temperature of the earth's surface has also risen. The greenhouse effect is so named because the atmosphere acts much like the glass roof and walls of a greenhouse, trapping heat from the sun.

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Ozone hole
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Ozone hole is a region of the atmosphere over Antarctica where a protective layer of ozone gas, a form of oxygen, becomes less concentrated every spring. The ozone layer blocks harmful ultraviolet rays from the sun, protecting life on Earth.

The ozone layer is roughly 10 miles (16 kilometers) thick. The bottom of the layer is about 6 miles (10 kilometers) above the North and South poles. When the gas becomes less concentrated, those dimensions remain about the same, but the number of ozone molecules in the layer decreases. Scientists say that the layer is depleted.

Scientists monitoring the atmosphere over Antarctica first noticed ozone depletion in the late 1970's. In 1985, researchers led by the British atmospheric scientist Joseph C. Farman showed that the ozone hole had grown since the 1960's. That is, over the years, the area of low concentrations of ozone had become larger. In addition, the ozone concentrations had become smaller.

A combination of cold weather and chemical activity creates an ozone hole every year. The cold weather creates ice surfaces on which certain chemical reactions can take place. The chemical activity begins when chemicals called chlorofluorocarbons (CFC's) break apart, creating molecules that take part in the reactions. See CHLOROFLUOROCARBON.

Most of Antarctica receives little or no sunshine from April through August, so the air becomes extremely cold. In addition, a powerful wind known as the polar jet prevents the cold air from mixing with warmer air to the north. As a result, ice clouds form in the ozone layer.

CFC molecules rise to the upper atmosphere, where ultraviolet rays break them up into smaller molecules. The smaller molecules, in turn, take part in chemical reactions on ice particles in the clouds. Those reactions create molecules that destroy ozone in the presence of sunlight.

Between August and October of each year, the number of ozone molecules in the ozone layer over Antarctica decreases by about two-thirds. The reactions that deplete the layer stop around the end of November-late spring in Antarctica. The polar jet also weakens, helping to return ozone concentrations to normal.

Ozone depletion also occurs over the Arctic. However, depletion there is usually less severe because Arctic air is not as cold as Antarctic air.

CFC's do not occur in nature. Instead, they are industrially produced. They were once widely used as refrigerants and as propellants in aerosol spray cans. By international agreement, most countries have halted the production of CFC's. But an ozone hole occurs every year because of CFC molecules that remain in the atmosphere.

Contributor: Linnea M. Avallone, Ph.D., Professor, Laboratory for Atmospheric and Space Physics, University of Colorado.

Source : World Book 2005.