updated 9/16/2005 4:16:55 PM ET 2005-09-16T20:16:55

The hole in the ozone layer this year will probably be slightly smaller than the all-time largest of 2003, in line with the general trend of a gradual healing of the ozone layer's depletion, a U.N. agency said Friday.

"The size of this year's ozone hole is approaching an all-time high, but it will probably not break any records," said Geir Braathen, an ozone specialist at the World Meteorological Organization.

At present, the hole over Antarctica is about 27 million square kilometers (10 million square miles) and the WMO expects it to increase to about 28 million square kilometers (10.8 million square miles) — a notch below its 2003 peak at about 29 million square kilometers (11.2 million square miles).

Based on recent patterns, Braathen expects the ozone holes to hover around this year's size for a few more years before they begin to shrink. Some scientists predict it will take about 50 years for the ozone hole to stop forming.

Chlorofluorocarbons, or CFCs, once widely used in spray cans and refrigerators, deplete the earth's protective layer. The hole has been forming in the extremely low temperatures that mark the end of Antarctic winter every year since the mid-1980s. Generally the hole is biggest around late September, while the so-called ozone value does not bottom out until mid-October.

The ozone layer keeps out ultraviolet radiation, which is dangerous to humans and animals. Less protection could increase risks of skin cancer and cataracts and affect biodiversity, scientists say.

Nevertheless, Braathen warned it was too early to "sound the all-clear," and said global warming had started to reverse some of the positive developments.

"Global warming might actually delay the healing of the ozone layer or altogether worsen the issue," he said.

Braathen said agencies like his must continue to closely monitor the ozone layer via satellites and ground stations. Signatories to the anti-CFC treaty known as the Montreal Protocol, particularly developing countries that had less strict phase-out schedules, should respect the accord, he stressed.

"One should not say the problem is solved and lean back," he warned. CFCs can be replaced with more ozone-friendly chemicals.

More than 180 nations have signed the Montreal Protocol, which went into effect in 1997.

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