Global warming thins Arctic ozone layer

Scientists announced this week they have found the most conclusive evidence yet that greenhouse gases are causing a thinning of the Arctic ozone layer.

For years, scientists have known that there is a hole in the Antarctic ozone layer - which protects the earth from solar ultraviolet radiation. But atmospheric conditions in the Northern Hemisphere have prevented the development of a similar situation there.

Now, members of an international research team say greenhouse gases such as carbon dioxide have played a key role in changing these conditions, leading to the destruction of Arctic ozone. While this process has not created a hole over northern latitudes, it's causing concern about dangerously low levels of ozone over populated areas, as well as how ozone will recover.

"Something funny is happening in the Arctic," said Brian Toon of the University of Colorado at Boulder, addressing a meeting of the American Geophysical Union in Washington. "We didn't expect it. It involves greenhouse gases."

The ozone loss is tied to clouds. Until now, the Arctic winter stratosphere has been too warm for clouds to last long. This helps the ozone, because without clouds, ozone-destroying compounds such as chlorine and bromine form compounds that have a limited effect.

But if clouds are present, reactions on cloud particles convert these compounds back into ozone-eating forms. This cloud-assisted process opens the Antarctic hole.

The Arctic stratosphere, however, has not been quite cold enough to sustain these clouds. Theorists have suggested that climate change driven by greenhouse gases could change that: While the heat-trapping gases warm the surface, they cool the stratosphere.

Now, this speculation appears to have become fact. The team found unexpected ozone loss this spring when stratospheric clouds lasted longer than ever before.

Team members say they are "astounded" at what they found.

Michael Kurylo, upper atmosphere research program manager at NASA headquarters, explained that this "unexpected interaction ... changes the chemistry of the stratosphere."

The discovery will change predictions about how the ozone layer will recover now that ozone-depleting substances such as chlorofluorocarbon refrigerants and firefighting halons are being phased out, says Dr. Kurylo. These compounds migrate to the stratosphere where they release ozone-eating chlorine and bromine.

The ozone layer should recover as the chlorine and bromine residues dissipate in the stratosphere. But Kurylo said that the unexpected persistence of the polar stratospheric clouds will slow that recovery over large parts of the Northern Hemisphere.

The process is not creating a northern ozone hole. But Kurylo said it is destroying enough ozone in polar regions "to give us pause."

As polar air with lower levels of ozone spreads, it dips down into middle latitudes, Dr. Toon notes. This reduces the ozone shield over many of the heavily populated regions of the Northern Hemisphere.

New sensors on satellites to be launched over the next five years will give scientists powerful tools to monitor the ozone layer and watch for any public danger.

This is an international concern. More than 200 scientists and support staff from Canada, Europe, Russia, and the United States took part in the recent research program, known by the code name SOLVE. They studied the Arctic stratosphere from the ground, and from aircraft, balloons, and satellites.

This gave them an unprecedented view of what was happening 13 miles over the Arctic from December 1999 through March 2000.

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