Working Group I: The Scientific Basis

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The atmospheric abundance of CH4 has increased by about a factor of 2.5 since the pre-industrial era (see Figure 4.1a) as evidenced by measurements of CH4 in air extracted from ice cores and firn (Etheridge et al., 1998). This increase still continues, albeit at a declining rate (see Figure 4.1b). The global tropospheric CH4 growth rate averaged over the period 1992 through 1998 is about 4.9 ppb/yr, corresponding to an average annual increase in atmospheric burden of 14 Tg. Superimposed on this long-term decline in growth rate are interannual variations in the trend (Figure 4.1c). There are no clear quantitative explanations for this variability, but understanding these variations in trend will ultimately help constrain specific budget terms. After the eruption of Mt. Pinatubo, a large positive anomaly in growth rate was observed at tropical latitudes. It has been attributed to short-term decreases in solar UV in the tropics immediately following the eruption that decreased OH formation rates in the troposphere (Dlugokencky et al., 1996). A large decrease in growth was observed, particularly in high northern latitudes, in 1992. This feature has been attributed in part to decreased northern wetland emission rates resulting from anomalously low surface temperatures (Hogan and Harriss, 1994) and in part to stratospheric ozone depletion that increased tropospheric OH (Bekki et al., 1994; Fuglestvedt et al., 1994). Records of changes in the 13C/12C ratios in atmospheric CH4 during this period suggest the existence of an anomaly in the sources or sinks involving more than one causal factor (Lowe et al., 1997; Mak et al., 2000).

There is no consensus on the causes of the long-term decline in the annual growth rate. Assuming a constant mean atmospheric lifetime of CH4 of 8.9 years as derived by Prinn et al. (1995), Dlugokencky et al. (1998) suggest that during the period 1984 to 1997 global emissions were essentially constant and that the decline in annual growth rate was caused by an approach to steady state between global emissions and atmospheric loss rate. Their estimated average source strength was about 550 Tg/yr. (Inclusion of a soil sink term of 30 Tg/yr would decrease the lifetime to 8.6 years and suggest an average source strength of about 570 Tg/yr.) Francey et al. (1999), using measurements of 13CH4 from Antarctic firn air samples and archived air from Cape Grim, Tasmania, also concluded that the decreased CH4 growth rate was consistent with constant OH and constant or very slowly increasing CH4 sources after 1982. However, other analyses of the global methyl chloroform (CH3CCl3) budget (Krol et al., 1998) and the changing chemistry of the atmosphere (Karlsdottir and Isaksen, 2000) argue for an increase in globally averaged OH of +0.5%/yr over the last two decades (see Section 4.2.6 below) and hence a parallel increase in global CH4 emissions by +0.5%/yr.

The historic record of atmospheric CH4 obtained from ice cores has been extended to 420,000 years before present (BP) (Petit et al., 1999). As Figure 4.1e demonstrates, at no time during this record have atmospheric CH4 mixing ratios approached today’s values. CH4 varies with climate as does CO2. High values are observed during interglacial periods, but these maxima barely exceed the immediate pre-industrial CH4 mixing ratio of 700 ppb. At the same time, ice core measurements from Greenland and Antarctica indicate that during the Holocene CH4 had a pole-to-pole difference of about 44 ± 7 ppb with higher values in the Arctic as today, but long before humans influenced atmospheric methane concentrations (Chappelaz et al., 1997; Figure 4.1d). Finally, study of CH4 ice-core records at high time resolution reveals no evidence for rapid, massive CH4 excursions that might be associated with large-scale decomposition of methane hydrates in sediments (Brook et al., 2000).

The feedback of CH4 on tropospheric OH and its own lifetime is re-evaluated with contemporary CTMs as part of OxComp, and results are summarised in Table 4.3. The calculated OH feedback, ln(OH) / ln(CH4), is consistent between the models, indicating that tropospheric OH abundances decline by 0.32% for every 1% increase in CH4. The TAR value for the sensitivity coefficient s = ln(LT) / ln(CH4) is then 0.28 and the ratio PT/LT is 1.4. This 40% increase in the integrated effect of a CH4 perturbation does not appear as a 40% larger amplitude in the perturbation but rather as a lengthening of the duration of the perturbation to 12 years. This feedback is difficult to observe, since it would require knowledge of the increase in CH4 sources plus other factors affecting OH over the past two decades. Unlike for the global mean tropospheric OH abundance, there is also no synthetic compound that can calibrate this feedback; but it is possible that an analysis of the budgets of 13CH4 and 12CH4 separately may lead to an observational constraint (Manning, 1999).

Table 4.3: Methane lifetime and feedback on tropospheric OHa for the 1990s.
CTM lifetime vs. OH(yr)b ln(OH)/ln(CH4) s=ln(LT)/ln(CH4) PT/LT
IASB 8.1 -0.31 +0.27 1.37
KNMI 9.8 -0.35 +0.31 1.45
MPIC 8.5 -0.29 +0.25 1.33
UCI 9.0 -0.34 (-0.38)c +0.30 1.43
UIO1 6.5 -0.33 +0.29 1.41
UKMO 8.3 -0.31 (-0.34)c +0.27 1.37
ULAQ 13.8 -0.29 +0.25 1.33
TAR valued 9.6 -0.32   1.4
a Global mean tropospheric OH is weighted by the CH4 loss rate.
b Lifetime against tropospheric OH loss at 1,745 ppb.
c Evaluated at 4,300 ppb CH4 plus emissions for Y2100/draft-A2 scenario.
d TAR recommended OH lifetime for CH4, 9.6 yr, is scaled from a CH3 CCl3 OH lifetime of 5.7 yr (WMO, 1999; based on Prinn et al., 1995) using a temperature of 272K (Spivakovsky et al., 2000). Stratospheric (120 yr) and soil-loss (160 yr) lifetimes are added (inversely) to give mean atmospheric lifetime of 8.4 yr. Only the OH lifetime is diagnosed and is subject to chemical feedback factor, and thus the total atmospheric lifetime for a CH4 perturbation is 12 yr. In the SAR, the feedback factor referred only to the increase in the lifetime against tropospheric OH, and hence was larger. For Chemistry Transport Model (CTM) code see Table 4.10.

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