Government and university scientists say that even though ozone depleting
substances are beginning to decrease in the atmosphere, it will be some time yet before scientists are able to observe whether the ozone layer itself is getting back to normal.

A new study published in the September 16 issue of the Journal of Geophysical Research estimates that it will be several decades before an increase in the amount of total overhead ozone (called "total column ozone") will be detectable.

Elizabeth C. Weatherhead, a University of Colorado scientist at NOAA's Air Resources Laboratory in Boulder, Colo., and colleagues, analyzed predictions from NASA/Goddard's two dimensional chemical model, along with predictions from nine other chemical models used in the World Meteorological Organization's 1998 ozone assessment, to estimate the time required to detect predicted trends of ozone recovery in different areas of the world. Results indicate that recovery of total column ozone is likely to show up earliest in the Southern Hemisphere near New Zealand, southern Africa and southern South America.

"We should expect to be able to detect recovery in most regions of the world within the next 15 to 45 years," Weatherhead said. "That's based on full compliance with the Montreal Protocol and its amendments and no other complicating factors such as major volcanic eruptions or enhanced stratospheric cooling," she continued.

Although there are other ways to detect signs of ozone recovery, for example, recovery of the ozone layer at a particular altitude above the earth, total column ozone measurements provide a complete picture of how much ozone is present over a region. The total column ozone amount represents the number of ozone molecules in an imaginary tube 1 centimeter on a side, stretching upward from the surface to the top of the atmosphere. Most of this ozone is located high in the atmosphere, between 20 and 30 kilometers (12 to 18 miles) in altitude. The ozone in this layer absorbs ultraviolet radiation from the sun, preventing a portion of the radiation from reaching the earth's surface where it can be harmful to plants and animals.

Weatherhead says it is crucial to detect not just a decrease in ozone depleting substances but a recovery in total column ozone amounts. "Statistical detection of ozone layer recovery will be an important step toward verification that all relevant processes in ozone destruction have been identified and that appropriate measures have been taken to assure the ozone layer's health. We can't really expect much of an improvement in the UV levels reaching the biosphere until we can detect an increase in total column ozone."

The work was co-sponsored by the Department of Energy and the U.S. Environmental Protection Agency and builds off measurements and work from the NASA/Goddard Space Flight Center and its scientists.

Contacts: Dr. Betsy Weatherhead - University of Colorado/NOAA - (303) 521-4040
Dr. Charles Jackmon - NASA/Goddard - (301) 286-8397
Dr. John Frederick, University of Chicago - (773) 702-3237