By Environment Correspondent Alex Kirby
Scientists say they now think it possible that levels of ozone depletion in the Northern Hemisphere might approach those seen over Antarctica.
In the Southern Hemisphere, where damage to the Earth's protective ozone layer was first detected, circumpolar winds trap the air, allowing chlorine and bromine to react with the ozone molecules.
It was the discovery of what was happening high above Antarctica that prompted the international agreement to phase out chlorofluorocarbons (CFCs) and other ozone-depleting chemicals.
The problem is that the chemicals survive for years in the stratosphere, so the damage is continuing even though the source of it is being tackled. A significant reduction in the ozone over the North Pole was detected in the winters of 1995-96 and 1996-97.
A threat to all of life
That could be more immediately threatening to human populations, simply because far more people live in relatively high northern latitudes than in similar regions in the Southern Hemisphere.
But the increased ultra-violet (UV) solar radiation which the damaged ozone allows through to the surface of the earth is a threat to all living things. In humans and other developed species it can cause eye cataracts and skin cancer.
It can adversely affect forests and crops, and is also linked with damage to plankton, the microscopic creatures at the bottom of the marine food chain.
An article in the Journal of Geophysical Research, published by the American Geophysical Union, says the Arctic damage threatens to "allow the penetration of enhanced UV radiation at northern mid-high latitudes".
The authors are Georg Hansen, of the Norwegian Institute for Air Research, and Martyn Chipperfield of Cambridge University.
They say there is particular concern over the very long-lived northern low pressure zone of circumpolar winds, called a vortex, found at between 13 and 34 km (8 and 21 miles) above the Earth's surface. The vortex is critical to providing the right chemical conditions for ozone depletion.
An ominous parallel
The Antarctic is usually colder than the Arctic, which is why ozone loss there is more marked. But the article says that in 1996-97 the Arctic vortex produced low temperatures and ozone loss as great as that recorded in the Antarctic during the early 1980s.
That year, and the year before, overall Arctic ozone depletion reached a maximum of 48% below the long-term average, and up to 60% at the worst-affected altitude, 19 km (12 miles) up.
In 1997 the vortex persisted for an unusually long time, into early May. In the last two months of that year the damage to the ozone was caused not just by chlorine, but also by nitric oxide and nitrogen dioxide, because the vortex lasted so long.
Martyn Chipperfield said it was possible that the Arctic was becoming more like the Antarctic. "Certainly 1997 was nearer than other years", he said.
But he added: "The winter polar vortex in the Arctic is very variable, and in 1997-98 it was warm and weak, and there was little ozone loss. It is too soon to detect a definite trend."
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