Atmospheric methane levels off: Temporary pause or a new steady-state?

3.1. CH₄ Observations

[4] Globally averaged surface CH₄ mole fractions (nmol mol⁻¹, abbreviated ppb) are plotted in Figure 1a. Annually-averaged CH₄ (circles) increased from 1625 ppb during 1984 to 1751 ppb during 1999; CH₄ has since remained essentially constant through 2002. The solid line in Figure 1a is the deseasonalized trend determined from a fit to the global averages at ∼weekly resolution; its derivative with respect to time represents the instantaneous growth rate of methane, which is plotted in Figure 1b. Also plotted in Figure 1b are annual increases (circles with error bars) determined directly from the trend curve. Though atmospheric CH₄ has increased during most of our measurement record, its growth rate decreased [Steele et al., 1992; Dlugokencky et al., 1998] from ∼14 ppb yr⁻¹ (corresponding to an imbalance between emissions and losses of 40 Tg yr⁻¹) in 1984, to near zero recently. During 2000 and 2001, global CH₄ decreased, in agreement with Simpson et al. [2002]; these decreases were compensated by a small increase in 2002 to maintain constant global annual mean CH₄ from 1999–2002. It is too soon to tell if the small increase in 2002 indicates a return to the small growth rates, ∼2 to 7 ppb yr⁻¹, observed in the mid-1990s, or if atmospheric CH₄ is at steady state. Abrupt changes in the CH₄ growth rate during 1991, 1992, and 1998 in Figure 1b have been used to test our understanding of the chemical sink [Dlugokencky et al., 1996], wetland emission rates [Walter et al., 2001; Dlugokencky et al., 2001], and emissions from biomass burning [Langenfelds et al., 2002].

[5] We previously showed that if the atmospheric lifetime of CH₄ were constant, the decreasing CH₄ growth rate could be explained as a system approaching steady state, with constant annual emissions [Dlugokencky et al., 1998]. If this scenario were true, CH₄ would have reached steady state in the mid- to late-2010s at ∼1780 ppb. An important assumption in this analysis was that the global [OH] had been approximately constant [Prinn et al., 1995]. Recent measurements of methyl chloroform (MC) in the free troposphere and boundary layer over central Europe [Krol et al., 2003] suggest that previous reports of trends in [OH] [Prinn et al., 2001; Krol et al., 1998] are in doubt, and that our earlier assumption of constant CH₄ lifetime was reasonable. Figure 1c (circles) shows total global CH₄ emissions calculated from E = d[CH₄]/dt + [CH₄]/τ [Dlugokencky et al., 1998], where E is emissions in Tg CH₄, [CH₄] is the global CH₄ burden determined from the observations, and τ is the CH₄ lifetime (assumed constant at 8.9 yr). Figure 1c suggests that there is no significant trend in emissions during the past 2 decades, but there is significant interannual variability in emission rate (or [OH], which is assumed constant in our analysis).

[6] We next explore changes in the CH₄ latitudinal distribution. Differences in annual mean CH₄ mole fractions between polar northern (PNH = 53° to 90°N) and southern (PSH = 53° to 90°S) latitudes (IPD = interpolar difference) are plotted in Figure 1d as circles (uncertainties are ±1σ). From 1984 to 1991, the difference increased at an average rate of (0.5 ± 0.4) ppb yr⁻¹, but after a decrease of ∼10 ppb between 1991 and 1992, the difference has continued to decrease (−1.0 ± 0.3 ppb yr⁻¹), not recovering to pre-1992 levels.

[7] The same temporal pattern as described above for the IPD is seen when individual high northern latitude sites are compared with South Pole. For example, during 1984–1991, the difference between annually averaged CH₄ mole fractions at Barrow, Alaska and South Pole was 151 ± 4 ppb (1σ); during 1992–2001, the difference between these sites decreased to 140 ± 6 ppb. To improve our understanding of where CH₄ sources and sinks were changing to produce the observed change in IPD, we averaged differences in annual mean CH₄ mole fractions between each surface site and South Pole for 1997, 1999, and 2000 (1998 was excluded because there were large anomalous emissions from northern wetlands in that year [Dlugokencky et al., 2001]) and subtracted the average differences calculated for 1984, 1985, and 1986. These differences represent the observed change in CH₄ latitude distribution from the mid-1980s to the late-1990s, and they are plotted as circles in Figure 2. The decrease in gradient is largest at very-high northern latitudes, decreasing towards mid- and tropical-latitudes; this indicates that the change in source/sink imbalance that caused the change in gradient (and IPD) occurred in high northern latitudes. Since little CH₄ is destroyed in polar latitudes, the change in IPD must be caused by a change in emissions, or by a change in atmospheric transport.

3.2. Evidence for Permanent Changes in CH₄ Emissions

[8] The change in IPD provides observational evidence that CH₄ emission rates from polar northern latitudes decreased in the early-1990s. Further observational evidence for this change comes from in situ quasi-continuous measurements of CH₄ and CO₂ at Alert, Canada [Poß et al., 2003]. During winter, when there is often strong flow from Eurasia to Alert, CH₄ and CO₂ are highly correlated in well-defined pollution events. The ratio of CH₄/CO₂ during these events decreased, while anthropogenic CO₂ emissions in the former Soviet Union (fSU) decreased by 30% [Olivier and Berdowski, 2001; Olivier, 2002]. Poß et al. [2003] combine these observed ratios with CO₂ emission estimates and a trajectory model and calculate that CH₄ emissions decreased from high northern latitudes by 30 Tg from 1988 to 1999.

[9] Estimates of anthropogenic methane emissions in EDGAR3 [Olivier and Berdowski, 2001; Olivier, 2002], tabulated for 1980–1995 on 5-year intervals, also support decreased emissions. EDGAR3 emissions from the fSU decreased by 10.6 Tg CH₄ between 1990 and 1995. Major contributors to this decrease come from the fossil fuel sector (gas, coal, and oil) and animal emissions (enteric fermentation in ruminants and waste management). About 8 Tg of the decreased CH₄ emissions from the fSU in EDGAR3 resulted from decreased fossil fuel production: extraction of oil and coal peaked in the late-1980s, dropping to less than 2/3rds of their peak values by the mid- to late-1990s [B.P. Statistical Review of World Energy, 2002]. Also, natural gas extraction peaked in 1990, and then fell by 17% between 1991 and 1997. Economic incentives existed for decreased emissions from the natural gas sector after the Soviet economy collapsed in 1992. A comparison of the emission estimates from the natural gas industry in the fSU by Reshetnikov et al. [2000] and Dedikov et al. [1999] suggests that emissions decreased by more than 20 Tg CH₄ yr⁻¹ (worth ∼$4 billion) from 1991 to 1997. But, we note that there is no evidence in Gazprom (Russia's partly state-owned gas company) data to indicate that such large decreases in emissions occurred during the early-1990s (A.I. Reshetnikov, private communication).

3.3. Model Results

[10] We used a 3-D transport model (Tracer Model, Version 3: TM3 [Houweling et al., 2000]) to see if the changes in emissions as reported in EDGAR3 are consistent with the observed decrease in IPD. A simulation was run at 4° latitude × 5° longitude resolution using assimilated, reanalyzed meteorological fields from the National Centers for Environmental Prediction for 1983–2000. Anthropogenic emission rates were from EDGAR3. Emissions distributions by source sector are available at 5-year intervals for 1980–1995. For other years in our simulation, global totals for lumped sectors were scaled linearly to prescribe emissions for each sector. Emission rates from natural sources and CH₄ sinks, both constant throughout the simulation, were as in Houweling et al. [2000], except that the ratio of emissions for bogs/swamps was as in Bergamaschi et al. [2001].

[11] Triangles in Figure 1d are model-simulated IPD for 1986–2000 calculated using only those model grid boxes that contain CMDL air sampling sites (the first 3 years of the model run were excluded to remove the influence of the initial conditions). Our simulation overestimates and lacks the abrupt decrease in observed IPD, but the change in IPD from the late-1980s to the late-1990s is of similar magnitude (∼15 ppb) to the observed change. A test simulation (not shown) was run with emissions held at 1983 values, but meteorology was allowed to vary. This resulted in interannually varying, but approximately constant IPD, indicating that changes in simulated IPD were due to changes in EDGAR3 emissions, and were not due to changes in transport. Model results (triangles) are also plotted in Figure 2; the general pattern in ΔCH₄ is similar, but agreement in the tropics is poor.

[12] Natural variability in CH₄ emissions and sinks have cumulative effects on the CH₄ burden. In Figure 1b, triangles are CH₄ annual increases calculated by mass balance with constant CH₄ lifetime and emissions, except for a decrease in emissions of 5 Tg CH₄ yr⁻¹ during 1992 and 1993 (total decrease of 10 Tg CH₄). The simulated annual increase is consistent with the observations, but interannual variability in observed CH₄ growth rate, particularly since 1990, makes it impossible to confirm that a decrease in emissions of 10 Tg CH₄ yr⁻¹ from high northern latitudes during the early-1990s had an impact on observed global CH₄ growth rate. The IPD is not sensitive to changing emission rates in mid-latitudes and the tropics. In EDGAR3, decreased emissions from the fSU are compensated by increases in emissions at mid- and tropical-latitudes, so global anthropogenic emissions in the inventory were the same for 1990 and 1995 (consistent with Figure 1c). Assuming all other emission and sink rates remained constant, a decrease of 10 Tg CH₄ yr⁻¹ would lower the steady-state globally averaged CH₄ mole fraction by ∼30 ppb.