Science: The Antarctic Ozone Hole

The Antarctic Ozone Hole was discovered by the British Antarctic Survey from data obtained with a ground-based instrument from a measuring station at Halley Bay, Antarctica, in the 1981-1983 period. They reported the October ozone loss in 1985. Satellite measurements then confirmed that the springtime ozone loss was a continent-wide feature.

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Research conducted during the National Ozone Expeditions to the U.S. McMurdo Station in 1986 and 1987, and NASA stratospheric aircraft flights into the Antarctic region from Chile in 1987 showed conclusively that the ozone loss was related to halogen (chlorine)-catalyzed chemical destruction which takes place following spring sunrise in the Antarctic polar region. The chlorine is derived from manmade chlorofluorocarbons (CFCs) which have migrated to the stratosphere and have been broken down by solar ultraviolet light, freeing chlorine atoms.

The ozone hole is formed each year in the Southern Hemisphere spring (September-November) when there is a sharp decline (currently up to 60%) in the total ozone over most of Antarctica. During the cold dark Antarctic winter, stratospheric ice clouds (PSCs, polar stratospheric clouds) form when temperatures drop below -78C. These clouds are responsible for chemical changes that promote production of chemically active chlorine and bromine. When sunlight returns to the Antarctic in the Southern Hemisphere spring, this chlorine and bromine activation leads to rapid ozone loss, which then results in the Antarctic ozone hole. Although some ozone depletion also occurs in the Arctic during the Northern Hemisphere spring (March-May), wintertime temperatures in the Arctic stratosphere are not persistently low for as many weeks which results in less ozone depletion.

Owing to regulations on the production and use of certain ozone-destroying chlorinated compounds, which went into effect in January 1996, the atmospheric concentration of some of these man-made substances has begun to decline. Chlorine/bromine should reach maximum levels in the stratosphere in the first few years of the 21st century, and ozone concentrations should correspondingly be at their minimum levels during that time period. It is anticipated that the recovery of the Antarctic Ozone Hole can then begin. But because of the slow rate of healing, it is expected that the beginning of this recovery will not be conclusively detected for a decade or more, and that complete recovery of the Antarctic ozone layer will not occur until the year 2050 or later. The exact date of recovery will depend on the effectiveness of present and future regulations on the emission of CFCs and their replacements. It will also depend on climate change in the intervening years, such as long-term cooling in the stratosphere, which could exacerbate ozone loss and prolong recovery of the ozone layer.

Although increasing greenhouse gas concentrations in the atmosphere may result in warmer surface temperatures, colder temperatures are expected to occur in the stratosphere. In fact, temperatures in the lower stratosphere, as measured by NOAA's Microwave Sounding Unit, have cooled during the past 22-years, the length of the satellite record. Colder stratospheric temperatures can enhance ozone loss through their affect on the formation of polar stratospheric clouds which in turn promote chlorine-caused ozone destruction.