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SAN FRANCISCO — The Arctic ozone layer will not bounce back as quickly as expected from damage caused by ozone-depleting chemicals despite recent cutbacks in their use, scientists said Friday. 

The layer, monitored because ozone blocks cancer-causing ultraviolet radiation from the Earth’s surface, appears to have suffered damage from a combination of man-made pollutants and extremely low temperatures in the upper atmosphere. 

“The worry that we have is that in contrast with early years ... we have seen extremely low levels (of ozone),” said Paul Newman of NASA’s Goddard Space Flight Center. “This has given us concern as to why this is happening. How well do we understand the processes that are leading to these low levels?” 

Though the levels of some ozone-damaging pollutants have declined, extremely low temperatures in the upper atmosphere increased ozone loss above the North Pole from November 1999 through March 2000, the researchers said at the fall meeting of the American Geophysical Union. 

Compared with previous years, levels of ozone decreased up to 55 percent at an altitude of about 12 miles from Dec. 1, 1999 to March 15, 2000. Later studies showed a decrease of more than 60 percent through April. 

The cold temperatures in the stratosphere are believed to be the result of warmer temperatures at ground level. That, in turn, could be caused by greenhouse gases that are suspected of contributing to global warming. 

In the extremely cold temperatures above the Arctic, more polar stratospheric clouds form. They provide the surface on which normally benign chemicals turn into ozone-destroying compounds. 

As a result, if all ozone-destructive chemicals were completely banned, the speed of recovery will depend on other factors, such as the temperature and yet-to-be-determined factors. 

Researchers noted that they observed continued declines in chlorine, one of the destructive chemicals. In 1989, an international protocol called for the phase out of chlorofluorocarbons commonly used in aerosol sprays and refrigerants. 

The researchers, however, noticed a continued increase in bromides, another destructive chemical. These compounds, used in grain fumigation and termite control, are being phased out more slowly — even less so in developing countries. 

The findings suggest that full ozone recovery will probably not occur in 2050, as earlier predicted, said Dale Hurst of the National Oceanographic and Atmospheric Administration. 

And scientists are unsure of other factors that might delay it further. 

“This scenario shows the ozone layer recovery is significantly delayed. There are obvious uncertainties of predicting when ozone recovery will occur,” he said. 

Though the Arctic ozone layer is not as seriously depleted as the layer above Antarctica, researchers say millions of people could be exposed to high ultraviolet radiation if the thin layer should drift south over heavily populated areas of Europe and North America. 

The research presented Friday was part of the joint SAGE III Ozone Loss and Validation Experiment and the Third European Stratospheric Experiment on Ozone. Balloons, aircraft measurements and satellite observations were used. 

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On the Net: Solve home page: http://cloud1.arc.nasa.gov/solve