This year's hole in the Antarctic ozone layer is the worst on record, not only matching that of the year 2000 in surface area but registering the largest depletion ever measured of the naturally occurring gas that filters out cancer- and cataract-causing ultraviolet (UV) rays, the United Nations meteorological agency reported today.

“This year’s hole was caused by the continuing presence of peak levels of ozone destroying substances in the atmosphere combined with a particularly cold stratospheric winter,” the UN World Meteorological Organization (WMO) said of the phenomenon, which appears annually at the start of the southern hemisphere spring.

Large holes over the Antarctic are expected to reoccur over the next two decades before a clear decline in size and depth, and the Montreal Protocol and Vienna Convention phasing out ozone-destroying chemicals such as chlorofluorocarbons must be adhered to with the utmost vigilance, WMO spokesman Mark Oliver told a news briefing in Geneva.

The agency based its assessments on measurements taken by satellites of the United States National Aeronautics and Space Administration (NASA) and European Space Agency (ESA), validated by surface based observations of the WMO Global Atmosphere Watch (GAW) ozone network. NASA instruments showed that on 25 September the area of the hole reached 29.5 million square kilometres, compared to 29.4 million in September 2000. Each agency uses different instruments, giving slightly different values, and according to ESA, the hole reached 28 million square kilometres on 25 September, very close to its maximum for 2000, which peaked at 28.4 million.

The ozone mass deficit in 2006 was measured at 39.8 megatonnes on 1 October, higher than in 2000, which peaked at 39.6 megatonnes on 29 September. Mass deficit is the amount of ozone missing from a vertical column of air compared to a baseline measured many decades earlier before severe ozone depletion appeared. Scientists have become increasingly aware of possible links between ozone depletion and climate change. Increased atmospheric concentrations of greenhouse gases will lead to a warmer climate at the Earth’s surface. At altitudes where the ozone layer is found, the same increase is likely to lead to a cooling of the atmosphere, enhancing the chemical reactions that destroy ozone. At the same time, the amount of water vapour in the stratosphere has been increasing at the rate of about 1 per cent per year. A wetter and colder stratosphere means more polar stratospheric clouds, which is likely to lead to more severe ozone loss in both polar regions.