c. Trace gases

1) Ozone

Total column ozone data were 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, from the NOAA-9 SBUV/2 instrument from September 1994 through June 1997, and from the NOAA-14 SBUV/2 beginning July 1997. Data from the SBUV instruments are only available during daylight viewing conditions; therefore no data are available over polar latitudes during winter. Other sources of ozone data include Dobson spectrophotometer readings and measurements from balloon-borne ozonesondes, both obtained from the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL).

(i) Continental United States

CMDL operates a network of 8 Dobson ozone spectrophotometers across the contiguous United States, in addition to instruments at Barrow, Alaska, Mauna Loa, Hawaii, and American Samoa. These instruments provide daily total column ozone amounts (weather and sun permitting), and many Dobson stations have been in operation since the early 1960s. Over the continental United States a large annual cycle in total ozone amounts (Fig. 11a ) is evident, resulting from stratospheric ozone transport processes which cause a winter-spring maximum and summer-fall minimum. Since 1980 the average total ozone amount has decreased by about 5%, due mainly to the chemical destruction of stratospheric ozone in reactions with anthropogenic halocarbon compounds. The largest decrease in total ozone occurred from 1979 through 1993 (Fig. 11b). From 1994 to 1998 total ozone concentrations over the continental United States have remained fairly constant, at levels approximately 5% below the 1980 average.

(ii) Southern Hemisphere

Total column ozone in the Southern Hemisphere also exhibits a well-defined annual cycle, with the lowest values typically observed over Antarctica during September-November. It is in this region that the "ozone
hole" is located, denoted by total column ozone concentrations of less than 220 Dobson Units (DU). The ozone hole is usually well-developed in September and typically reaches its maximum areal extent in late-September and early October. It then usually persists well into November and generally disappears by December. The ozone hole first began to appear over the Antarctic region in the early 1980s (Farman et al. 1985). During the past 20 years (Fig. 12 ), the average October-November size of the ozone hole area has increased, from a small area of 1.5 x 106 km2 in 1982 to a record area of 16.7 x 106 km2 in 1998. This value surpasses the previous record of 14.5 x 106 km2 set in 1996.

Daily measurements for the period 1987-98 show a record areal extent of the ozone hole during most of October-November 1998 (Fig. 13a ), with a maximum extent of more than 25 x 106 km2 (an area larger than North America) observed during late September. The 1998 ozone hole also persisted into mid-December, which is longer than has been observed in any previous year. This duration is nearly one month longer than was observed in 1997, when the ozone hole disappeared by mid-November.

A further inspection of ozone concentrations during 1998, obtained from a sounding at the South Pole on 3 October 1998 (Fig. 14 ), shows the complete absence of ozone between 15 and 21 kilometers, which is comparable to the vertical extent of ozone depletion observed in 1997. These conditions contrast with the "pre-ozone hole" period (1967-71) when no ozone depletion was detected between 15 and 20 km, and also with conditions earlier in 1998 before the development of the ozone hole. During 1998, the lowest daily total column ozone concentration within this region was 98 DU. This value is second to the record low concentration of 86 DU measured in 1993.

Total ozone concentrations are closely coupled to lower stratospheric temperatures through photochemistry. Extremely low stratospheric temperatures (below -78C) contribute to the formation of polar stratospheric clouds (PSCs), which enhance the production and lifetime of reactive chlorine, thereby leading to ozone depletion (WMO/UNEP 1994). During most of the Southern Hemisphere winter and spring of 1998, the minimum temperatures in the polar region reached or exceeded previous record low values (Fig. 15) and were again sufficiently low to allow for enhanced ozone depletion.

The areal extent of the ozone hole is also closely related to the polar vortex (Fig. 13b ), which isolates and concentrates the chemicals that destroy ozone at low temperatures. During most of September-December 1998, this stratospheric polar vortex was larger and persisted longer than has been observed previously in the historical record.

A Scientific Assessment by the World Meteorological Organization (WMO) and UNEP states that the abundance of ozone-depleting substances in the stratosphere is expected to peak by the year 2000. Even though international actions are working well to reduce the use and release of ozone depleting substances, chemicals already in the atmosphere will continue ozone depletion into the 21st century. Changing atmospheric conditions and natural ozone variability are expected to complicate the task of detecting the start of the ozone layer recovery. Based on an analysis of 10 years of ozone vertical profile measurements, Hofmann et al. (1997) estimate that the recovery of the Antarctic ozone hole may not be conclusively detected until the year 2008.

2) Carbon dioxide

The Mauna Loa Observatory is located at an elevation of 3350 m on the flank of Mauna Loa volcano and 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. Thus, the CO2 record based on measurements at this observatory is taken as a reliable index of long-term carbon dioxide growth. This record is the longest of its kind in existence and shows a continued increase of CO2 through 1998 (Fig. 16). This result indicates that fossil fuel consumption and deforestation continue to add CO2 to the atmosphere at a rate faster than uptake by the oceans and biosphere.

The increase in CO2 concentration has averaged approximately 1.4 ppm yr-1 during the 1980s and 1990s but with significant year-to-year variability in the growth rate. The growth rate decreased to 0.5 ppm yr-1 during the aftermath of the Mt. Pinatubo eruption in 1991 and subsequently increased to more than 2 ppm yr-1 during 1995 before dropping back to near 1.4 ppm-1 in 1996. The growth rate then increased during 1998 to more than 3 ppm yr-1, which is the highest rate observed since measurements began in 1957. Contributing factors to these interannual variations in growth rate are 1) temperature and precipitation over large areas, which influence the growth of plants and the decay of soil organic matter, and 2) variations in the partial pressure of CO2 in surface seawater. The large increase in the growth rate during 1998 may have been related to the record warmth experienced during the year.

3) Methane

The amount of methane in Earth's atmosphere has more than doubled since pre-industrial times (Etheridge et al. 1992). This increase is responsible for approximately 20% of the estimated change in direct radiative forcing of Earth's climate due to anthropogenic greenhouse gas emissions (Myhre et al. 1998). Recent measurements by CMDL indicate that although methane concentrations continue to increase, the rate of increase has slowed considerably since the early 1990s (Fig. 17). If global methane and hydroxide (OH) emissions remain constant, then globally averaged methane concentrations are projected to increase to approximately 1800 ppb over the next few decades (1997 values are 1730 ppb), with little further change expected in its contribution to the greenhouse effect (Dlugokencky et al. 1998).

4) Global chlorofluorocarbons (CFCs)

The concentrations of two types of ozone depleting CFCs and three types of ozone depleting chlorinated solvents are monitored by CMDL at 8 locations from Alert, Canada and Point Barrow, Alaska to the South Pole (Fig. 18). Each of these five substances show markedly reduced growth rates from those observed in the early 1990s, with some species showing significant declines in concentration since 1994. These trends are related directly to an end of production of these substances on January 1, 1996 in the developed countries. However, the production of these chemicals continues in a few lesser developed countries.

The atmospheric growth rate of CFC-12 is decreasing with time as a result of emission reductions, although concentrations of this long lifetime atmospheric gas have not yet peaked. CFC-12 was used in pre-1993 auto air conditioners, as an aerosol propellant, and in refrigerators. The accumulation of CFC-11 in the atmosphere peaked during 1993_94 and has been decreasing since that time. CFC-11 was used as a cell-blowing agent for the manufacture of foams, in large air conditioning systems, and in refrigeration. Atmospheric trends of the shorter lifetime chlorinated solvents methyl chloroform (CH3CCl3), carbon tetrachloride (CCl4), and CFC-113 (CCl3F-CClF3) have all been decreasing at various rates during the 1990s. The concentration of CH3CCl3 has decreased most rapidly and has been cut in half between 1993 and 1998. This chemical was used as a metal degreaser in manufacturing. The concentration of CCl4 has decreased slowly since 1991. This chemical was used in dry cleaning and in the production of CFCs. The concentrations of CFC-113 has been slowly decreasing since 1995. This chemical was used as a degreaser solvent in the manufacture of circuit boards.

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