By PHILIP ENGLISH
Reach for your sunhat. This year's ozone hole over Antarctica is nearly the size of last year's record-breaking hole, which covered 30 million sq km of the frozen continent.
Scientists think that when the present ozone hole breaks up, its influence could have an impact on our summer as harmful ultraviolet radiation penetrates the atmosphere and hastens the onset of sunburn.
Last year, the ozone hole developed more quickly and grew to record-breaking size but disappeared early. It was gone by the end of November instead of mid-December or later. The impact on summer ultraviolet levels was low.
But this year, based on the phenomenon's past behaviour, scientists suspect there will be more danger of sunburn due to ultraviolet radiation.
The danger for New Zealand is brief, and comes when ozone-depleted portions of the hole disperse and dilute northwards as the southern polar region warms in the Antarctic spring and summer.
In December 1998, the ozone-depleted atmosphere over New Zealand led to warnings to cover up to protect ourselves from UV radiation usually blocked by ozone in the stratosphere.
Ozone holes have formed over Antarctica in late winter since the early 1980s. Lately, they have become larger and more intense.
The holes form as a combination of sunlight, low temperatures and chlorine pollution in the atmosphere reacts to destroy ozone inside the polar vortex. This is an air mass that circulates above Antarctica during winter and spring and isolates the continent from the rest of Earth's atmosphere.
Dr Brian Connor, of the National Institute of Water and Atmospheric Research laboratory in Lauder, Central Otago, said there was nothing to suggest this year's hole would break up quickly.
"I am speculating, but if it continues to be stable and large it is likely to break up relatively late ...
"That it is one of the larger holes of recent years and is quite stable and symmetrical would suggest that it may persist well into the summer, as it did in 1998. Therefore, it could have more of an effect on our summertime UV levels than last year's hole did."
Dr Connor said the Antarctic hole's impact on the atmosphere over New Zealand should not be seen as overly alarming.
"Our best estimate is that over the past 20 years, ozone depletion in general has caused summertime UV levels in New Zealand to increase by about 20 per cent.
"We believe that roughly half of that, or about 10 per cent, is due directly to this process where the ozone hole breaks down in early summer, and 10 per cent is due to worldwide background ozone depletion.
"That is the size of the effect. It is significant and important but not so dramatic that you need to stay indoors."
Dr Connor said that because of measures to counter pollution by ozone-depleting chemicals such as CFCs, ozone levels might recover in the next 50 years.
"It should start to improve some time in the next 10 to 15 years. We don't know just when ... We would expect the ozone hole to be gone in something like 50 years or possibly a little longer."
The Antarctic hole could take longer to recover because of global warming caused by greenhouse gas emissions.
Dr Connor said it was possible the size of ozone holes had peaked and was stabilising.
"There have been year-to-year fluctuations, but no obvious decreasing trend over the last three or four years. We could at this point be near the worst.
"Whether we are there yet or just round the corner or starting to improve isn't really known."
Intergovernmental Panel on Climate Change (IPCC)
United Nations Environment Program
World Meteorological Organisation
Framework Convention on Climate Change
Executive summary: Climate change impacts on NZ
IPCC Summary: Climate Change 2001