Shrinking ozone hole not a trend, says scientists

By Stephen Withers
Thursday, 04 October 2007 02:25

Science - Climate

The ozone hole over Antarctica is much smaller than it was last year, but scientists say this does not indicate a new long-term trend.

The 2006 hole set a record at 40 million tonnes of ozone loss (based on measurements made by the ESA Envisat satellite), compared with 27.7 tonnes this year.

"Although the hole is somewhat smaller than usual, we cannot conclude from this that the ozone layer is recovering already," said Ronald van der A, a senior project scientist at the Royal Dutch Meteorological Institute.

"This year's ozone hole was less centred on the South Pole as in other years, which allowed it to mix with warmer air, reducing the growth of the hole because ozone is depleted at temperatures less than -78 degrees Celsius."

Prevailing winds during the southern winter keep the atmosphere above the Antarctic from mixing with warmer air from further north.

The ozone layer reduces the amount of ultraviolet light reaching the Earth's surface, and any thinning of the layer - as evidenced by polar holes - reduces its effectiveness, increasing the risk of skin cancer and cataracts.

The presence of halogens in the atmosphere encourages ozone to break down into oxygen, but their levels are declining following the ban on CFCs agreed in 1987.