More polar stratospheric clouds than anticipated are forming high above the North Pole, causing additional ozone loss in the sky over the Arctic, according to Dr. Azadeh Tabazadeh, lead author of the paper and a scientist at NASA's Ames Research Center in California's Silicon Valley. The stratosphere comprises Earth's atmosphere from about 9 to 25 miles (about 15 to 40 kilometers) altitude and includes the ozone layer.

"Polar stratospheric clouds provide a 'double-whammy' to stratospheric ozone. They provide the surfaces which convert benign forms of chlorine into reactive, ozone-destroying forms, and they remove nitrogen compounds that act to moderate the destructive impact of chlorine," said Dr. Phil DeCola, Atmospheric Chemistry Program Manager at NASA Headquarters, Washington, D.C.

"The Arctic has become colder and more humid, conditions that promote formation of more polar stratospheric clouds that take part in polar ozone destruction. The main conclusion of our study is that if this trend continues, Arctic clouds will remain longer in the stratosphere in the future," Tabazadeh said.

"An ozone hole forms every spring over the Antarctic in the Southern Hemisphere which is colder than the Arctic," said Tabazadeh. "The Arctic has been getting colder and is becoming more like the Antarctic; this could lead to more dramatic ozone loss in the future over the Northern Hemisphere, where many people live."

Researchers used data from NASA's Upper Atmosphere Research Satellite to analyze cloud data from both the north and south polar regions for the study. "What we found from the satellite was that polar stratospheric clouds currently last twice as long in the Antarctic as compared to the Arctic," Tabazadeh said.

"However, our calculations show that by 2010 the Arctic may become more Antarctic-like' if Arctic temperatures drop further by about 5 to 7 degrees Fahrenheit (a drop of about 3 to 4 degrees Celsius)," she said.

When Arctic polar stratospheric clouds last longer, they can precipitate, removing nitrogen from the upper atmosphere, which increases the opportunity for chlorine compounds to destroy ozone more efficiently. The polar stratospheric clouds involved in the reactions contain nitric acid and water, according to researchers who discovered these clouds in 1986.

"Data from the Microwave Limb Sounder on UARS have provided the first opportunity to observe nitric acid throughout the Arctic and the Antarctic over a period of many years," said Dr. Michelle Santee, a scientist at NASA's Jet Propulsion Laboratory, Pasadena, CA, who is a co-author of the Science paper.

"The continued presence of nitric acid in the Arctic winter -- which is not the case in the Antarctic -- helps to moderate ozone loss by reducing the amount of reactive chlorine, but this could change in the future," she added.

More than a decade ago, scientists determined that human- made chlorine and bromine compounds cause most ozone depletion. Manufacturers made the chlorine compounds, chloroflourocarbons or "CFCs," for use as refrigerants, aerosol sprays, solvents and foam-blowing agents.

Fire fighters used bromine-containing halogens to put out fires. Manufacture of CFCs ceased in 1996 in signatory countries under the terms of the Montreal Protocol and its amendments.

The Montreal Protocol bans CFC emissions. As a result, the chlorine concentration in the upper atmosphere is already starting to decline, according to Tabazadeh.

"Scientists used to believe that as chlorine levels decline in the upper atmosphere, the ozone layer should slowly start to recover. However, greenhouse gas emissions, which provide warming at the Earth's surface, lead to cooling in the upper atmosphere. "This cooling promotes formation of the kind of polar stratospheric clouds that contribute to ozone loss. Several recent studies, including this one, show that ozone recovery is more complex and will take longer than originally thought," she explained.