Ozone Depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% per decade in the total volume of ozone in Earth's stratosphere (the ozone layer), and a much larger springtime decrease in stratospheric ozone over Earth's polar regions. The latter phenomenon is referred to as the ozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polar tropospheric ozone depletion events.

The details of polar ozone hole formation differ from that of mid-latitude thinning, but the most important process in both is catalytic destruction of ozone by atomic halogens. The main source of these halogen atoms in the stratosphere is photodissociation of man-made Halocarbon refrigerants (CFCs, freons, halons). These compounds are transported into the stratosphere after being emitted at the surface. Both types of ozone depletion were observed to increase as emissions of Halocarbons increased.

CFCs and other contributory substances are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280-315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern leading to adoption of the Montreal Protocol that bans the production of CFCs, halons as and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in skin cancer, cataracts, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion.

Earth's First Arctic Ozone Hole Recorded   Live Science - October 3, 2011
The high atmosphere over the Arctic lost an unprecedented amount of its protective ozone earlier this year, so much that conditions echoed the infamous ozone hole that forms annually over the opposite side of the planet, the Antarctic, scientists say.

"For the first time, sufficient loss occurred to reasonably be described as an Arctic ozone hole," write researchers in an article released Oct. 2 by the journal Nature. Some degree of ozone loss above the Arctic, and the formation of the Antarctic ozone hole, are annual events during the poles' respective winters. They are driven by a combination of cold temperatures and lingering ozone-depleting pollutants.

The reactions that convert less reactive chemicals into ozone-destroying ones take place within what is known as the polar vortex, an atmospheric circulation pattern created by the rotation of the Earth and cold temperatures. This past winter and spring saw an unusually strong polar vortex and an unusually long cold period.

This year's record vortex persisted over the Arctic from December to the end of March, and the cold temperatures extended down to a remarkably low altitude, the researchers write. At altitudes of about 11 to 12 miles (18 to 20 kilometers), more than 80 percent of the ozone present in January had been chemically destroyed by late March.

The same dynamics create the infamous ozone hole over Antarctica. But above the South Pole, ozone is essentially completely removed from the lower stratosphere ever year. Above the North Pole, however, ozone loss is highly variable and has, until now, been much more limited, writes the international research team led by Gloria Manney of the California Institute of Technology.

Countries agreed to end their production of the substances ultimately responsible for destruction of the ozone in 1987 with the Montreal Protocol. However, these pollutants, including chlorofluorocarbons, still linger in the atmosphere. Ozone loss is expected to improve in the coming decades as atmospheric levels of these chemicals decline.

On the Earth's surface, ozone is a pollutant, but in the stratosphere it forms a protective layer that reflects ultraviolet radiation back out into space. Ultraviolet rays can damage DNA and lead to skin cancer and other problems.

Global warming is implicated in the loss of Arctic ozone because greenhouse gases trap energy lower down, heating up the atmosphere nearer the ground but cooling the stratosphere, creating conditions conducive to the formation of the reactive chemicals that break apart the three-oxygen molecules of ozone.

North vs. South Poles: 10 Wild Differences   Live Science

First North Pole Ozone Hole Forming?   National Geographic - March 23, 2011

"Put on your sunscreen" - damaging air mass could drift far south. Spawned by strangely cold temperatures, "beautiful" clouds helped strip the Arctic atmosphere of most of its protective ozone this winter, new research shows. The resulting zone of low-ozone air could drift as far south as New York, according to experts who warn of increased skin-cancer risk.