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Northern chill produces Arctic ozone hole

Cold snap A prolonged cold spell in the northern hemisphere has caused the largest recorded drop in ozone over the Arctic, a new international study has found.

While the area has seen severe depletion of ozone levels in the past, this is the first time it has occurred at such a magnitude to be considered an ozone hole, says study co-author Dr Michelle Santee from NASA's Jet Propulsion Laboratory.

"Ozone depletion was about twice that in both 1996 and 2005 - and those were the previous record setting Arctic ozone loss years," says Santee.

The findings, gathered from NASA satellite measurements of polar stratosphere cloud and ozone sonde measurements of gas formation and over polar regions, appears in today's issue of Nature.

Loss of ozone in the stratosphere occurs when temperatures are low enough for nitric acid and water to condense in the stratosphere and form clouds, on which chlorine compounds are converted into ozone-depleting gases.

Temperatures in Antarctica are low enough to cause the production of ozone-depleted gases and almost complete destruction of the ozone layer over the continent for four to five months each southern winter/spring.

But usually temperatures in the Arctic winter/spring are too warm to form as many clouds. So less chlorine is produced and it only stays converted for a shorter period of time.

However, this year Arctic temperatures in the lower stratosphere were continuously cold from December until April, says Santee.

"This year in the Arctic temperatures did get cold, but more importantly they stayed cold for a really, really long time."

Santee says at its greatest point between February and April the low ozone region covered about two million square kilometres, and shifted over populated areas in Russia and central Asia.

This is around 60 per cent of the size of the hole that covers Antarctica in the southern winter/spring.

"Although the degree of ozone loss this year approached that of some early Antarctic ozone holes, it was over a much smaller area than what the current Antarctic ozone hole is," says Santee.

Ongoing legacy of CFCs

Concern about the emergence of an Antarctic ozone hole in the 1980s led to an international phase out of chlorofluorocarbons (CFC) under the Montreal Protocol in 1989.

But CFCs still linger in the stratosphere.

"They will linger for some decades to come. Some of these are very long-lived and it's just going to take a while before their presence is completely gone," says Santee.

This means that under the right conditions an ozone hole may form again in the Arctic, but Santee says scientists can't predict when.

"So long as chlorine remains high in the stratosphere, whenever we get meteorological conditions in the stratosphere such as we did this year with low temperatures and a prolonged period of low temperatures, we will be vulnerable to this kind of ozone loss," she says.