Saving Earth's Shield

If countries hadn't taken action to protect stratospheric ozone, the atmosphere would have significantly more potential to trap heat than it does now, states a new U.S. government report.

Synthetic chemicals such as chlorofluorocarbons that deplete stratospheric ozone, which protects Earth's surface from the sun's harmful ultraviolet rays, are also powerful greenhouse gases. For example, a single molecule of CFC-12 has the global-warming potential of 8,500 molecules of CO₂, the greenhouse gas most widely emitted through human activity.

Governments moved to restrict ozone-depleting substances under the 1987 United Nations treaty known as the Montreal protocol. But if the manufacture and use of ozone-depleting compounds hadn't been curtailed, emissions of these substances would have contributed significantly to the buildup of heat-trapping gases in the atmosphere, according to the report, which was released last week by the U.S. Climate Change Science Program.

Because of the Montreal protocol, "the world has already taken some action to reduce greenhouse gases," says A. R. Ravishankara, lead author of the report. Ravishankara is director of chemical sciences at the National Oceanic & Atmospheric Administration's Earth System Research Laboratory, in Boulder, Colo.

Without the Montreal protocol, levels of ozone-depleting chemicals in the atmosphere by 2010 would likely be 50% higher than now predicted, the report says.

The report, "Trends in Emissions of Ozone-Depleting Substances, Ozone Layer Recovery, and Implications for Ultraviolet Radiation Exposure," is available at climatescience.gov.