Hole fails to faze experts

The deepest ozone hole over the Antarctic for several years has failed to dent scientists' confidence that the ozone layer is recovering.

The massive area of ozone-depleted air is starting to dissipate and will head north and break up into smaller pools over the next few weeks.

Atmospheric chemist Greg Bodeker, of Bodeker Scientific in Alexandra, said there had been a "steady but slow" improvement in recent years in the size and depth of the annual spring ozone hole in response to the Montreal Protocol on ozone-depleting substances.

However, this season's ozone hole had bucked that trend.

The hole had started developing about a week later than usual, in mid-August, and by the third week of September the minimum ozone over the Antarctic had fallen to about 115 Dobson Units (DU).

The minimum ozone recorded during the season was 96 DU in late September.

Bodeker said only the record-low ozone in 2006 (84 DU) has been lower in recent years.

He said the largest area covered by the ozone hole during the season was 24.5 million square kilometres about mid-September, down from the 29.8 million sq km record of 2000.

"Perhaps the story here, which appears to be not well-perceived by the public, is that the ozone layer is recovering and the hole is recovering," he said.

"The ozone holes we are seeing now are not as bad as we saw in 1995-96 through to 2000, they are a bit weaker.

"They are starting to indicate some perceptible response to the Montreal protocol and that is a good piece of news."

Last year's ozone hole covered 27 million sq km at its peak, with a minimum ozone level of 100DU.

Bodeker said chemistry climate models projected the near-complete recovery of the ozone hole would not be before the latter part of the century.

"It is not inconceivable that we could have a year that is just incredibly cold in the stratosphere over a very large region of Antarctica and we could still have a record ozone hole.

"The analogy is the weather – as we move from September into December it's not like each day simply gets a bit warmer than the previous one.

"The same thing has happened in Antarctica – chlorine and bromine peaked in 2000 and have been declining since, but there's still all this natural variability."

WHAT IS THE OZONE HOLE?

By the late 1970s, large emissions of man-made gases containing chlorine – from chlorofluorocarbons, bromine and nitrogen oxides – began affecting the ozone layer.

These gases, many from the refrigeration industry, help chain reactions that break down ozone molecules and deplete the amount of ozone in the stratosphere, the second layer of the atmosphere.

During the spring, severe ozone depletion occurs over the Antarctic as temperatures reach their coldest in the stratosphere and sunlight returns and stirs up the atmosphere.

The "hole" is confined to Antarctica and never extends over New Zealand, although smaller pools of ozone-depleted atmosphere have spun across the country.