Thinning of the Ozone Layer
Another air pollution problem is the thinning of the ozone layer of the atmosphere, a fragile, invisible layer about 10-30 miles above the earth’s surface that shields the planet from the sun’s hazardous ultraviolet (UV) rays. Since the mid-1980s, scientists have observed the seasonal appearance and growth of a hole in the ozone layer over Antarctica. More recently, thinning over other areas has been noted.
The ozone layer is being destroyed primarily by chloro-fluorocarbons (CFCs), industrial chemicals used as coolants in refrigerators and air conditioners, as foaming agents in some rigid foam products, as propellants in some kinds of aerosol sprays (most of which were banned in 1978), and as solvents. When CFCs rise into the atmosphere, winds carry them toward the polar regions. During winter, circular winds form a vortex that keeps the air over Antarctica from mixing with air from elsewhere. CFCs react with airborne ice crystals, releasing chlorine atoms, which destroy ozone. When the polar vortex weakens in the summer, winds richer in ozone from the north replenish the lost Antarctic ozone.
Thinning of the Ozone Layer Photo Gallery
The largest and deepest ozone hole on record, measured at 11.5 million square miles (29.9 million square kilometers), occurred on September 6, 2000. In 2012, the maximum size of the ozone hole was 8.2 million square miles (21.2 million square kilometers), or the area of the United States, Canada,
and Mexico combined. The Antarctic ozone layer likely will not return to its early 1980s state until about 2065, because of the long lifetimes of ozone-depleting substances in the atmosphere. Because of an international agreement that regulates the production of ozone-depleting chemicals, overall atmospheric ozone is no longer decreasing. The gradual recovery is masked by annual variations caused by weather fluctuations over Antarctica.
Without the ozone layer to absorb the sun’s UV radiation, life on Earth would be impossible. (UV radiation levels under the Antarctic hole were high enough to cause sunburn within 7 minutes.) The potential effects of increased longterm exposure to UV light for humans include skin cancer, wrinkling and aging of the skin, cataracts and blindness, and reduced immune response. The United Nations Environment Programme predicts that a drop of 10% in overall ozone levels would cause a 26% rise in the incidence of nonmelanoma skin cancers. Some scientists blame ozone loss for many cases of melanoma.
UV light may interfere with photosynthesis and cause lower crop yields; it may also kill phytoplankton and krill, the basis of the ocean food chain. And because heat generated by the absorption of UV rays in the ozone layer helps create stratospheric winds, the driving force behind weather patterns, a drop in the concentration of ozone could potentially alter the earth’s climate systems.
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