The results indicate the loss of Arctic ozone due to the presence of industrial chlorine and bromine in Earth's atmosphere may well be sensitive to subtle changes in stratospheric climate. Such ozone depletion leads to increased exposure to harmful, ultraviolet solar radiation at Earth's surface.

According to the study, the sensitivity of Arctic ozone to temperature is three times greater than predicted by atmospheric chemistry models. This leads to the possibility that decreases in stratospheric temperatures may have significantly larger impacts on future Arctic ozone concentrations than have been expected in the past.

Dr. Markus Rex of the Alfred Wegener Institute for Polar and Marine Research, Potsdam, Germany, led the study. It also included scientists from NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The researchers analyzed more than 2,000 balloon measurements collected over the past 12 years. They found the amount of ozone loss occurring in any given Arctic winter is closely related to the amount of air exposed to temperatures low enough to support the formation of polar stratospheric clouds. Reactions occurring on the surface of these clouds convert chlorine from unreactive forms to other forms that quickly deplete ozone.

Based on the relation between ozone loss and polar stratospheric cloud existence, the researchers found every degree Kelvin (equal to one Celsius degree) cooling of the Arctic results in an additional ozone destruction of five percent. This sensitivity is a factor of three larger than previously predicted by state-of-the-art, coupled climate- chemistry computer models.

The scientists found the coldest stratospheric winters, during which most of the ozone loss occurs due to greater polar stratospheric cloud formation, have gradually become significantly cooler during the past few decades.

"If stratospheric climatic conditions had not changed since the 1960s, Arctic ozone loss would be much less severe today, despite the increase in chlorofluorocarbons and bromine," Rex said.

"This study presents a new method of looking at a multi-year data set that enables us to relate year-to-year variations in the amount of ozone depletion to climate change," said co- author Dr. Ross Salawitch, a JPL research scientist.

"Results of this research will lead to substantially improved computer model simulations of this phenomenon and will provide an excellent method for analyzing data from satellites such as NASA's soon-to-be-launched Aura atmospheric chemistry laboratory."

Researchers are trying to understand why the Arctic stratosphere cools. It may be due to a number of factors: rising levels of greenhouse gases such as carbon dioxide; a feedback between ozone depletion and stratospheric temperature; and natural variability.

Higher amounts of greenhouse gases trap heat near Earth's surface, warming the surface and preventing the heat from reaching the stratosphere, thus cooling the upper atmosphere. However, climate models vary widely in their estimates of how much stratospheric cooling has occurred due to rising greenhouse gases over the past 40 years.

Stratospheric chlorine and bromine have begun to decline in response to the Montreal Protocol, a worldwide agreement signed in 1987 that limits the production of chlorofluorocarbons and other ozone depleting pollutants.

Scientists believe this indicates the cleansing process has begun, and eventually the ozone layer will recover, although chlorofluorocarbons can stay in the atmosphere for 50 to 100 years. The study suggests the healing process might be slowed, in the short term, by changes in stratospheric climate.

Tracking the predicted recovery of the ozone layer is a key science objective of NASA's Aura spacecraft. Aura is the latest in the Earth Observing System series and scheduled for launch in June.

Aura will study the atmosphere's chemistry and dynamics, providing data to help scientists better understand Earth's ozone, air quality and climate change.

Aura's chemistry measurements will follow up on records that began with NASA's Upper Atmospheric Research Satellite and will also continue the Total Ozone Mapping Spectrometer mission's goal of collecting comprehensive ozone data.

The paper was highlighted by the American Geophysical Union and published in Geophysical Research Letters, Volume 31, L04116.

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