Ozone Hole


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NOAA 96-62

Contact: Barbara McGehan                       IMMEDIATE RELEASE
         Dave Hofmann                          9/11/96


Ozone depletion over the South Pole this year could be worse than in 1995 by as much as 10 percent, according to an analysis of ozone trends from balloon measurements taken at the South Pole for the past ten years.

David J. Hofmann, director of the National Oceanic and Atmospheric Administration's Climate Monitoring and Diagnostics Laboratory (CMDL) in Boulder, Colo., wrote in the Sept. 12 issue of Nature that the variation in ozone depletion at the South Pole from one year to the next may be related to changing stratospheric wind patterns and the transport of air from the tropics to the poles.

"We are seeing increased ozone depletion at the South Pole, which we believe was caused by a change of wind in the tropics from westerly to easterly," Hoffman said. "In particular, ozone loss rates have been greater in years where the winds at very high altitudes have been easterly for several months early in the calendar year preceding the ozone hole period." This may cause more ozone-depleting halogens (chlorine-and bromine-containing molecules) to be transported from the tropics to the poles prior to formation of the winter vortex, causing greater ozone depletion the following spring. If this hypothesis is correct, Hofmann said, ozone depletion in September 1996 should again be high.

"The fact that ozone depletion over the South Pole may be worse in 1996 than in 1995 doesn't necessarily mean that overall ozone depletion is increasing. We're just starting to understand the interannual variabilities better," Hofmann said. Understanding the cause of this variability in ozone loss rate will be important for the detection of the expected recovery of the ozone hole early in the next century.

According to Hofmann, ozone depletion in the Antarctic has been especially severe since 1992, caused in part by aerosols from the Mt. Pinatubo volcanic eruption, which enhance the ozone-destroying ability of the man-made halogens. The effects from that eruption have largely subsided so that the 1995 ozone hole, although one of the longer lasting events in recent years, was not as deep as in 1993 or 1994. The ozone hole in 1995 was measured at 129 Dobson units during the late September-early October period, a slight improvement over the 119 Dobson units measured in 1994, and the 109 Dobson units measured in 1993. A Dobson unit is a measurement of the thickness of the ozone in a column of air from the ground to the top of the atmosphere.

However, Hofmann says, this is not a sign of recovery from ozone depletion in Antarctica. The ozone hole will continue to fluctuate for some years to come in spite of recent research that shows that chlorine and other ozone-depleting chemicals are starting to decline in the lower atmosphere. Based on measurements and models, Hofmann believes the level of ozone-depleting chemicals in the Antarctic stratosphere should peak in about the year 2000 and then begin a slow decrease. " Full recovery of the ozone hole is not expected until about 2050. But the more we understand the fluctuation and variabilities, the better we can monitor the recovery," Hofmann added.

For additional information, consult the CMDL home page on the Internet at http://www.cmdl.noaa.gov

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