WASHINGTON -- Researchers are worried that continued ozone loss over the Arctic, such as occurred in 1996 and 1997, may allow dangerous ultraviolet radiation to reach Europe and North America.
But it's too soon to declare a definite trend, they say, noting that the situation improved a bit last year.
Although it's considered a pollutant at ground level, ozone in the upper atmosphere blocks the sun's ultraviolet rays, which can cause sunburn, skin wrinkling, eye damage and even cancer.
Damage to that protective layer was first noticed in the colder Antarctic, where circular winds trap air, giving chemicals the chance to react with the ozone. Concern that the damage could spread prompted the ban on refrigerants and spray propellants that can destroy ozone.
But the chemicals already released remain in the air for years, and a significant reduction in the ozone layer over the North Pole was recorded in the winters of 1995-96 and 1996-97.
This threatens to "allow the penetration of enhanced UV radiation at northern mid-high latitudes," warn researchers in the latest issue of the Journal of Geophysical Research, published by the American Geophysical Union.
That potential threat could include heavily populated areas of Europe and North America, unlike the relatively small number of people endangered by the Antarctic ozone thinning.
Georg Hansen of the Norwegian Institute for Air Research in Tromso, Norway, and Martyn P. Chipperfield of Cambridge University in England studied the northern ozone layer.
"In particular, if Arctic (ozone) depletion persists later in the year, as in 1997, more UV will penetrate to the surface when the sun is higher in the sky in late spring and summer," Chipperfield said in an interview via electronic mail.
"There is the possibility that the Arctic may becoming more Antarctic-like," he said. "Certainly 1997 was nearer than other years."
However, Chipperfield added, "the winter polar vortex in the Arctic is very variable, and last year (1997-98) the vortex was warm, weak and there was little ozone loss. It is too soon to detect a definite trend."
Of particular concern, the researchers said, was the very long-lived northern low pressure zone of circular winds, called a vortex, located at between eight and 21 miles above the Earth's surface. The vortex is a key factor in providing the chemical conditions for ozone depletion.
The Antarctic is usually colder than the Arctic, causing more frequent stratospheric clouds of the type that lead to ozone depletion within the vortex. In 1996-1997, however, the Arctic vortex produced low temperatures and ozone depletion as great as that recorded in the Antarctic during the early 1980s, the researchers said.
Hansen and Chipperfield, using data collected from the Norwegian island of Andoya, reported that in 1995-1996 and 1996-1997, overall ozone depletion reached a maximum of 48 percent below the long-term average and up to 60 percent at the most affected altitude, 12 miles.
And the 1996-1997 vortex lasted exceptionally long, into early May, extending the period of reduced ozone levels. This led in turn to increased ultraviolet radiation in northern Europe.
Hansen and Chipperfield also found that in 1997, until the end of March, the well-known wintertime chemical action involving chlorine in stratospheric clouds was the principal cause of ozone depletion. After that, and into early May, further depletion was caused by chemical interactions of nitric oxide and nitrogen dioxide, which was made possible by the unusual persistence of the vortex, they said.
Reductions in the northern ozone layer have also been reported by scientists at the U.S. National Oceanic and Atmospheric Administration and National Aeronautics and Space Administration.