Q

FIGURE 14.15 Rate of growth (ppm per year) of atmospheric C02 at Mauna Loa, Hawaii, from 1958 to 1994 (from Keeling and Worf as reported in IPCC, 1996).

Year

FIGURE 14.15 Rate of growth (ppm per year) of atmospheric C02 at Mauna Loa, Hawaii, from 1958 to 1994 (from Keeling and Worf as reported in IPCC, 1996).

on atmospheric circulation processes (e.g., see Rind et al., 1990; and Rind and Lacis, 1993).

Like C02, methane is emitted by both natural and anthropogenic processes. While the major sources are thought to have been identified, there is some uncertainty in the absolute magnitudes of their contributions as well as the factors that affect these (Cicerone and Oremland, 1988; Fung et al., 1991). Table 14.1 shows one estimate of methane sources during the mid-1980s, expressed in units of teragrams (fO12 g) of carbon per year (Crutzen, 1995). Approximately 60 + 10% of the total emissions (370 out of a total of 630 Tg per year) is estimated to be associated with human activities. Of the 370 Tg of C per year, approximately 35% is due to losses during natural gas and oil production and distribution and coal mining. This estimate is reasonably consistent with measurements of the 14C content of atmospheric methane, since fossil fuel derived methane is depleted in l4C due to the long time frame for the fuel formation (Lowe et al., 1988; Wahlen et al., 1989).

The next largest source, approximately 30% of the 370 Tg of C per year, is due to emissions from domesticated ruminant livestock (e.g., see Johnson et al., 1994) and from the decay of animal wastes. Emissions from rice fields (e.g., see Cicerone and Shetter, 1981; Cicerone et al., 1983, 1992; and Tyler et al., 1994) appear to comprise about 20%. The remainder of the 370 Tg of C per year is believed to be due about equally to emissions from sanitary landfills (e.g., see

Bogner and Spokas, 1993) and from biomass burning (e.g., see Hao and Ward, 1993). It should be noted that the emissions in some cases, for example rice fields and landfills, represent the net flux of emission and micro-bially mediated oxidation processes so that both need to be understood in assessing the methane budget (e.g., Reeburgh et al., 1993; Bogner and Spokas, 1993).

There were relatively few measurements of atmospheric methane concentrations prior to about 1980, except for a set from 1963 to f970 by Stephens and co-workers (Stephens and Burleson, 1969; Stephens, 1985), which were in the 1.37-1.57 ppm range. The current global mean concentration of methane is f.72 ppm, with higher concentrations in the Northern than

TABLE 14.1 Estimated Methane Sources during the Mid-1980s u

Emissions

Natural 260 + 30

Anthropogenic 370 ± 40

Gas leakage and oil production 85-105

Coal mining 25-45

Rice fields 20-150

Ruminants 65-100

Biomass burning 20-60

Animal wastes 20-40

Sanitary landfills 20-60

" From Crutzen (1995). b Tg of C = 1012 g of C.

FIGURE 14.16 Averaged 3-D methane concentrations in the marine boundary layer. Lines are guides for the eye (adapted from Dlugokencky et al., 1994a).

in the Southern Hemisphere. Figure 14.16 shows the latitudinal distribution as a function of time from 1983 to 1992 (Dlugokencky et al., 1994a). The hemispheric distribution is clearly seen, as is the seasonal cycle due to changes in oxidation by OH and in methane emission sources. Even in this relatively short time span, the increase in the atmospheric concentrations is evident. Unlike C02, there does not appear to be a discernible trend in the amplitude of the seasonal cycles (Dlugokencky et al., 1997).

Ice core data show that the concentration prior to about the year 1750 was ~700 ppb, less than half of the current global average. The increase appears to have begun in the 1750-1800 period (e.g., see Khalil and Rasmussen, 1987, 1994b; Blunier et al., 1993; and Etheridge et al., 1998). Figure 14.17, for example, shows the concentrations of atmospheric methane for the past approximately 1000 years (Etheridge et al., 1998). The increase in concentration to the present value is well outside the variations of ~70 ppb observed prior to 1750. It is interesting that the increase appears to have begun prior to significant industrial activity, but parallels the increase in population growth in China; Blunier and co-workers (1993) suggest that this may be due to associated changes in emissions from rice fields.

The rate of increase of atmospheric CH4 has been variable. From 1978 to 1987, the average growth rate was approximately 16 ppb per year (Blake and Rowland, 1988). However, the global growth rate slowed during the latter part of the 1980s (Steele et al., 1992; Dlugokencky et al., 1994a, 1994b, 1998). Figure 14.18

shows globally averaged methane concentrations (Fig. 14.18a) as well as the growth rate for CH4 for latitudes from 82°N to 90°S from 1984 to 1996 (Fig. 14.18b) (Dlugokencky et al., 1998). Dlugokencky and co-workers suggest that the apparent leveling off of CH4 may not reflect a change in sources or sinks but rather reflect an approach to steady state. The growth rates in the late 1980s were lower, but there was a sharp rise in 1991 immediately after the Mount Pinatubo volcanic eruption, followed by a sharp decrease to temporarily

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