National Indicators:
Status of Stratospheric Ozone

Human activities that result in the emission of gases that contain chlorine and bromine have resulted in the depletion of stratospheric ozone. The stratospheric ozone layer is important because it shields the Earth from harmful ultraviolet (UV)-B radiation. Reduced levels of stratospheric ozone allow more UV-B radiation to reach the earth, and this radiation is known to be harmful to certain biological systems and is believed to be related to an increased incidence of skin cancer in humans. A continuing increase in UV-B radiation over the long term could potentially affect the health of all forms of life.

The total atmospheric column ozone for the United States is measured at a number of U.S. stations with Dobson spectrophotometers by the National Oceanic and Atmospheric Administration (NOAA), U. S. Department of Commerce (DOC). Figure 4.24 shows that the average monthly total column ozone over the United States has declined by about 7% since 1979 (based on measurements at Fresno, California; Boulder, Colorado; Nashville, Tennessee; and Wallops Island, Virginia).

The largest ozone depletions related to chlorine and bromine emissions have been observed in Antarctica during the austral spring. Figure 4.25 shows that more than 50% of the springtime ozone at the South Pole has been lost compared to pre-1970 values.

Regulations on the emission of certain ozone-depleting gases became effective on January 1, 1996. NOAA measurements indicate that the total ozone depleting potential of these gases reached a peak at the surface in 1994, presumably due to steps taken in anticipation of the regulations. Based on this data, the concentrations of these ozone-depleting gases in the stratosphere are expected to peak by the turn of the century.

It is likely that the first evidence of stratospheric ozone recovery will be seen at the South Pole since the springtime depletion there is very large compared to natural variability. Based on atmospheric chemistry models, the implementation of international agreements, and current regulations, the beginning of stratospheric ozone recovery is expected to occur early in the 21^th century. This recovery will be harder to detect in the mid-latitudes--for example, over the United States--since the depletion there is of the same order of magnitude as natural variability.

If the stratospheric ozone layer does begin to recover, as expected, it will demonstrate the success of the scientific community, international policy makers, the private sector, and the public in protecting the environment, while also addressing the related economic concerns through the development and phase-in of ozone-friendly substitutes.

Reference:

• NOAA Climate Monitoring and Diagnostics Laboratory (CMDL), Summary Review (Bi-Annual); NOAA Climate Prediction Center, Northern Hemisphere Winter Summary.

http://www.sdi.gov/indicators/lc_ozone.htm Last Modified: May 14, 2002