Total column ozone data was obtained from the NASA Nimbus-7 SBUV instrument from 1979 through 1988, from the NOAA-11 SBUV/2 instrument from January 1989 through August 1994, and from the NOAA-9 SBUV/2 instrument beginning in September 1994.
Southern Hemisphere. Total column ozone concentrations exhibit a well-defined annual cycle in the Southern Hemisphere, with lowest values observed during September and October. It is during these two months that the well-known "ozone hole", denoted by total ozone concentrations less than 220 Dobson Units (DU), is most evident in the polar region. The ozone hole first began to appear in the 1980s.
Total ozone values over Antarctica during September and October 1995 (Fig. 14) were extremely low, with the minimum values only slightly higher than the record low values observed in 1993. The areal extent of very low ozone values in 1995 was as widespread over Antarctica as in the record low year of 1993. The monthly mean total ozone for October 1995 compared to the 1979-86 October mean values (Fig. 15) indicates decreases of 20% to 50% over a large portion of Antarctica. Conversely, small percent increases during October 1995 are evident over some areas of the tropics and midlatitudes, probably in association with the quasi-biennial oscillation (QBO).
Selected ozone profiles taken at the South Pole on 5 October 1995, the day of lowest total ozone amount at this location, are compared with profiles on 12 October 1993 and 23 August 1993 (Fig. 16). The October profiles show nearly complete destruction of ozone between 15 and 22 km. In addition, ozone depletion above 25 km, first noted in 1992, continued in 1995. This depletion of ozone at higher levels is believed to be related to increases in chlorine.
Temperatures in the lower stratosphere are closely coupled to ozone through dynamics and photochemistry. Extremely low stratospheric temperatures (less than 78°C) over the Antarctic region are believed to lead to ozone depletion, in that they contribute to the presence of polar stratospheric clouds. These clouds enhance the production and lifetime of reactive chlorine, which leads to ozone depletion.
Daily minimum temperatures over the polar region (65°S-90°S) at 50 hPa (approximately 19 km) (Fig. 17) indicate that temperatures were substantially below normal during the Southern Hemisphere winter and spring seasons. These temperatures were sufficiently low to allow for polar stratospheric clouds and enhanced ozone depletion. Temperature anomalies for the 100-50 hPa layer, derived from radiosonde data (Fig. 18), also indicate record low temperatures during September_November 1995. These low values are consistent with an overall trend toward colder temperatures in the lower stratosphere observed since the late 1960s.
Northern Hemisphere. Total column ozone values in the Northern Hemisphere are generally lowest during the January-March period. In the middle and high latitudes, total column ozone values during January-March 1995 were 10%-20% lower than was typically observed during the late 1970s and early 1980s. In these regions, a decrease of 2%-4% per decade in total ozone is evident.
Monthly mean total ozone amounts for March 1995 (Fig. 19) indicate very low ozone totals over the polar region extending from north-central Siberia to northern Greenland. In these regions, the total ozone concentration is more than 40% below the values observed in 1979 (Fig. 20). However, it should be noted that regional variations in total ozone in the Northern Hemisphere are highly variable from one year to the next, and tend to be strongly associated with the existing planetary-scale circulation features.
The atmospheric carbon dioxide (CO2) measurements made at Mauna Loa Observatory, Hawaii, since 1958 provide strong evidence for human alteration of the environment (Fig. 21). The data through 1973 are from Keeling et al. (1982), while data since 1973 are from the NOAA program (Thoning et al. 1989).
Mauna Loa Observatory, located at an elevation of 3350 m on the flank of Mauna Loa volcano, is an ideal site for carbon dioxide measurements. There is no nearby vegetation and the prevailing nighttime downslope winds give a representative sampling of mid-tropospheric air from the central North Pacific Ocean. As such, the record is a reliable indicator of long-term carbon dioxide growth.
The average ozone concentration increase at Mauna Loa during the 1980s was about 1.4 ppm per year, but with significant year-to-year variability in this growth rate. In 1992-93, the growth rate decreased to near 0.5 ppm per year. However, it increased to more than 2 ppm per year during the last year. Contributing factors to these variations in growth rate include ENSO and the natural exchange of carbon dioxide by the oceans and/or the terrestrial biosphere with the atmosphere. The global temperature decrease resulting from the eruption of Mt. Pinatubo in mid-1991 may also have contributed to the slower growth rate during 1992-93.
Globally-averaged methane mixing ratios are collected approximately weekly from various sites in the NOAA/CMDL cooperative air sampling network (Dlugokencky et al. 1994). Air sampling sites are distributed between 90°S and 82°N. The average increase in the globally-averaged methane mixing ratio over the period 1983-93 is approximately 0.6% per year, when referenced to the middle of the sample record (Fig. 22). The growth of methane over the past few years has slowed, probably due to a change in the anthropogenic source (Dlugokencky et al. 1994). However, the strong decrease in growth observed during 1992 and 1993 may have been associated with the eruption of Mt. Pinatubo in June 1991.
Increased methane affects the Earth's radiation balance and the chemistry of the atmosphere. While the major sources of methane have been identified, their absolute contributions to the global methane budget remain poorly quantified. Until a better understanding of the methane budget is realized, the exact causes of the observed increase will remain uncertain.
