Gradual destruction of stratospheric ozone, which is attributed to the buildup of man-made compounds that deplete the ozone layer, continued during 1994. According to an executive summary of the 1994 Scientific Assessment of Ozone Depletion released jointly by the World Meteorological Organization and the United Nations Environmental Program, the rate of ozone destruction is expected to peak during the next few years and to begin decreasing early in the twenty-first century. The long-term decrease of total column ozone is presently 4-5 percent per decade at mid-latitudes in both hemispheres. Little negative trend in ozone is observed in the Tropics. [Ozone data are derived from data recorded by the SBUV instrument on the NASA Nimbus-7 satellite from 1979 to mid-1990 and by the SBUV/2 instrument on the NOAA-11 satellite from January 1989 to September 1994.]
In the Northern Hemisphere, ozone concentrations during February have decreased by an average of 3-6% since 1979 (Fig. 1.14), with positive (negative) anomalies dominating the early (late) part of the record (Fig. 1.15). Large negative anomalies in the Northern Hemisphere extratropics during 1992-1993 were related to increased stratospheric aerosols that resulted from the Mt. Pinatubo eruption in June 1991. During 1994 negative ozone anomalies decreased in magnitude in the Northern Hemisphere, as volcanic aerosol levels returned to near normal.
In the Southern Hemisphere middle and high latitudes, negative (positive) ozone anomalies also dominated during the latter (early) portion of the record, again indicating a substantial reduction in ozone concentrations since 1979. For example, October total ozone concentrations have decreased by nearly 100 Dobson Units (DU) over a large portion of Antarctica since 1979, which represents a 50% reduction in ozone in that region (Fig. 1.16). During 1994 ozone concentrations continued to decrease in the Southern Hemisphere extratropics and subtropics. The extremely low total column ozone values (near 100 DU) observed over Antarctica during September and October 1994 were similar to the record-setting values observed in 1992 and 1993.
Temperatures in the lower stratosphere are closely coupled to ozone concentrations through dynamics and photochemistry. Over the Antarctic region, extremely low temperatures (lower than -78oC) at the 50-mb level (approximately 19 km) contribute to the formation of polar stratospheric clouds (PSCs). PSCs enhance the production and lifetime of reactive chlorine, leading to ozone depletion. For most of the southern winter and spring of 1994, daily minimum temperatures at 50 mb over the south polar region (65oS to 90oS) were sufficiently low for polar stratospheric clouds to form (Fig. 1.17).
Climate and Global Change Issues - Trace Gases: Carbon Dioxide
Climate and Global Change Issues: Stratosphere Temperatures
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