Smaller ozone hole good news for NZ
By MICHAEL FIELD - Fairfax Media | Thursday, 4 October 2007

The spring ozone hole over Antarctica - which can led to lower ozone levels over New Zealand - has dramatically shrunk this year, the European Space Agency (ESA) said today.

The ozone hole can lead to increased cancer causing UV rays reaching the Earth's surface.

As the annual Antarctic ozone hole disperses around late November and December, it can led to lower ozone protection over New Zealand.

In a statement today the ESA says its Envisat satellite is recording that this year's ozone hole has shrunk 30 per cent as compared to last year's record size.

This year's ozone loss peaked at 27.7 million tonnes, compared to the 2006 record ozone loss of 40 million tonnes.

Scientists say this year's smaller hole - a thinning in the ozone layer over the South Pole - is due to natural variations in temperature and atmospheric dynamics (illustrated in the time series to the right) and is not indicative of a long-term trend.

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

"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."

During the southern hemisphere winter, the atmospheric mass above the Antarctic continent is kept cut off from exchanges with mid-latitude air by prevailing winds known as the polar vortex. This leads to very low temperatures, and in the cold and continuous darkness of this season, polar stratospheric clouds are formed that contain chlorine.

As the polar spring arrives, the combination of returning sunlight and the presence of polar stratospheric clouds leads to splitting of chlorine compounds into highly ozone-reactive radicals that break ozone down into individual oxygen molecules. A single molecule of chlorine has the potential to break down thousands of molecules of ozone.

Ozone is a protective layer found about 25 kilometres above us mostly in the stratospheric stratum of the atmosphere that acts as a sunlight filter shielding life on earth from harmful ultraviolet rays. Over the last decade the ozone layer has thinned by about 0.3 percent per year on a global scale, increasing the risk of skin cancer, cataracts and harm to marine life.

The thinning of the ozone is caused by the presence of ozone destructing gases in the atmosphere such as chlorine and bromine, originating from man-made products like chlorofluorocarbons, which are on the decline as they are banned under the Montreal Protocol, signed in 1987.

From here .