This year's ozone hole surrounding the Southern Hemisphere's pole is shaping up to be one of the largest ever, having already surpassed the size of last year's, according to the World Meteorological Organization.

The WMO says that despite a relatively late start, the ozone hole has grown rapidly during the past couple of weeks and has now passed the maximum size attained in 2007.

According to the WMO, the hole in the ozone layer over Antarctica currently covers an area of 27 million square kilometres, compared to a maximum of 25 million square kilometres last year. It says that while the hole continues to grow, it is still too early to determine how large it will be, before it breaks apart in mid-December.

'This year's hole is shaping up to be a big one like 2006, which was one of the biggest.' —Paul Fraser, scientist

Paul Fraser, an atmospheric scientist with Australia's Commonwealth Scientific and Industrial Research Organization, says that while this year's ozone hole is large, it isn't unexpected.

"Last year's hole was relatively small," says Fraser. "Most of the holes over the past few years have been large."

He added that, "This year's hole is shaping up to be a big one like 2006, which was one of the biggest."

The hole in the ozone layer over Antarctica in 2006 measured 28 million square kilometres.

Driven by weather

Ozone-depleting chemicals, such as chlorofluorocarbons, were phased out under the 1987 Montreal Protocol, but continue to linger in the atmosphere.

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The hole in the ozone layer already covers an area of 27 million square kilometres. ((National Oceanic and Atmospheric Administration))

Fraser says these chemicals are currently decreasing at a rate of approximately 1 per cent per year. Despite this, the size of the ozone hole is still determined mainly by the stratospheric weather.

"The size is driven largely by the temperature and shape of the polar vortex — the air mass in the stratosphere in which the ozone hole forms," says Fraser.

"When it's exceptionally cold and symmetrical then you'll have a very deep ozone hole."

He added that it's unlikely there will be statistically significant changes in the ozone hole driven by those small changes in ozone-depleting chemicals for the next five to 10 years.

Experts predict that the ozone layer will fully recover sometime in the second half of this century, but this is complicated by other atmospheric processes, such as an increase in the level of greenhouse gases.

"Not only do we have to predict the supply of chemicals, but we also have to predict the stratospheric temperatures," says Fraser.

"In the stratosphere, greenhouse gases cause cooling. So in the polar regions they enhance the ability of chemicals to destroy ozone, while the accumulation of greenhouse gases will hasten ozone recovery around mid-latitudes.... So the role of greenhouse gases in ozone depletion is quite complicated."