Chlorofluorocarbons (CFCs)

British Antarctic Survey scientists first observed a large region of depleted stratospheric O₃ over Antarctica during the late 1970s, and dubbed it an "ozone hole." Springtime decreases in stratospheric ozone averaged 30–40% during the 1980s. By 1987, 50% of the ozone over the Antarctic continent was destroyed during the austral spring (September to December), and 90% was depleted in a 9–12 mi (15–20 km) wide zone over Queen Maud Land. Atmospheric scientists also observed an Arctic ozone hole, but it is smaller, more variable, and less depleted than its southern hemisphere counterpart. Rapid ozone depletion and the development of seasonal ozone holes is most pronounced at high latitudes where intensely cold, dark conditions and isolating atmospheric circulation promote ozone-destroying chemical reactions and inhibit synthesis of new ozone molecules during the darkest spring months. Stratospheric O₃ destruction by the secondary compounds of CFCs also occurs at lower latitudes, but the depletion is much slower because constant light and atmospheric mixing foster ozone regeneration. A seasonal thinning of the stratospheric ozone layer also occurs at lower latitudes when O₃-depleted air disperses from the poles in late spring.

In addition to accelerating the loss of stratospheric ozone, CFCs may also contribute to an intensification of the so-called "greenhouse effect," and to long-term global climate change. The greenhouse effect is a phenomenon by which an envelope of atmospheric gases and vapors like carbon dioxide and water vapor maintains the earth's average surface temperature at about 77°F (25°C) by trapping a portion of the heat emitted by the planet's surface. Insulation by the greenhouse gases keeps Earth's temperature approximately 33 degrees warmer than would be possible if the atmosphere was transparent to long-wave infrared energy. The greenhouse effect also permits existence of the large reservoirs of liquid water that sustain biological life on the planet. The pre-industrial concentration of greenhouse gases was chemically balanced to allow global cooling at a rate that maintained temperatures within an acceptable temperature range. However, environmental scientists are concerned that natural and synthetic radioactive gases emitted by human activities will slow the earth's cooling rate, and lead to global warming. CFCs are greenhouse gases; they are very efficient absorbers of infrared energy. On a per-molecule basis, CFC-11 is 3,000–12,000 times as efficient as carbon dioxide as a greenhouse gas, and CFC-12 is 7,000–15,000 times as effective. Atmospheric concentrations of total CFCs increased from almost zero several decades ago to about 0.5 ppb in 1992.