Balancing the global methane budget: constraints imposed by isotopes and anthropogenic emission inventories

Figures & data

Table 1. Representative construction of the global contemporary methane sink and isotope fractionation.^a

Sink	Strength, Tg yr^−1	εsink, ‰
Tropospheric OH	511 ± 103^a	−4.65 ± 0.75^b
Soil	30 ± 15^a	−20 ± 2^b
Stratosphere	40 ± 8^a	−3 ± 3^c
Atomic chlorine	25 ± 12^d	−60 ± 1^b
Total	606 ± 105^e	−7.6 ± 1.4^e
^aSink strengths and 95% confidence intervals for all sinks except chlorine are taken from the IPCC Fourth Assessment Report (Denman et al. 2007). All sink strengths scale with mixing ratio, but are used in the present work only for their role in estimating fractionation εtotal and its uncertainty.
^bThe kinetic isotope shifts, εsink, are taken from the literature, with confidence interval reflecting the spread in measured values: εOH from Cantrell et al. (1990) and Saueressig et al. (2001); εsoil from Tyler et al. (1994) and Snover and Quay (2000); εchlorine from Crowley et al. (1999) and Tyler et al. (2000).
^cThe stratospheric sink is a net cross-tropopause flux with corresponding δ^13C estimated from the δ^13C contrast in the two gross fluxes, themselves poorly determined: see Lassey et al. (2007a) for detail.
^dThe chlorine sink strength is taken from Allan et al. (2007) who cite 25 Tg yr^−1 as a mid-point between bounds assessed at 13 and 37 Tg yr^−1 in the contemporary troposphere.
^eThe confidence intervals for the total sink and corresponding kinetic isotope shift are calculated with the assumption that all individual confidence intervals are uncorrelated.

Table 2. Natural global source scenarios, Nat1 and Nat2.

Source category	Nat1^a, Tg yr^−1	Nat2^b, Tg yr^−1	δ^13C^c, ‰
Wetlands	163 (83–240)	129 (83–240)	−60
Termites	20 (10–30)	20 (10–30)	−57
Wild animals	15	15	−62
Geologic (incl. oceans)	19 (9–29)	53 (42–64)	−45
Boreal wildfires	5	5	−25
Total natural	222	222
Fossil fraction^d	8.6%	23.9%
δ^13C in total	−57.8‰	−55.5‰
^aIndividual source strengths and confidence intervals are taken from Houweling et al. (2000), who also estimate the total source on the basis of the assessed pre-industrial sink (“top–down” estimate).
^bNat2 is adjusted from Nat1 by substantially increasing the geologic source and its confidence limits after Etiope et al. (2008) at the expense of some wetland emission, maintaining the same total source.
^cThe δ^13C assignments are from Houweling et al. (2000) except that the estimate by Etiope et al. (2009) is preferred for the geologic source.
^dThe “fossil fraction” is the proportion of geologic CH4, which is distinguished by having negligible ^14CH4.

Figure 1. EPA06 dataset of anthropogenic CH₄ emissions by eight country groupings at 5-yearly increments from 1990 projected to 2020, along with the world total shrunk by a factor of 5.

Table 3. Anthropogenic global source EPA06 for 2000.^a

Source component	Strength, Tg yr^−1	δ^13C, ‰
Biogenic sources
Ruminants	86	−62
Rice agriculture	30	−64
Landfills	35	−55
Waste water	25	−55
Animal waste	11	−55
Total biogenic	187	−60.1
Pyrogenic sources
C3 biomass	18	−25
C4 biomass	12	−12
Total pyrogenic	30	−19.8
Fossil sources
Coal mining	18	−35
Oil, gas, industry	52	−40
Total fossil	70	−38.7
Total source	287	−50.7
Fossil fraction	24.3%
^aSources: USEPA (2006) for source strengths, Houweling et al. (2000) for δ^13C values.

Figure 2. The upper two panels show the 1950–2005 sub-intervals of the model inputs, and the lower panels the corresponding modelled output, with atmospheric data in the left-hand panels. Figure 2a is the globally-adjusted MacFarling Meure (2006) mixing ratio dataset, C(t), with illustrative annual global-mean data from the NOAA/ESRL network (E.J. Dlugokencky, personal communication), and Southern Hemisphere datasets from Baring Head, New Zealand (Lassey et al. 2010) and from archived samples collected at Cape Grim, Australia (Francey et al. 1999). Figure 2b shows the back-extrapolated EPA06 and Edgar4 constructions of the anthropogenic source history, each of which is augmented by a natural source of 222 Tg yr⁻¹ to give S(t). Figure 2d shows the corresponding turnover times, λ(t)⁻¹, along with modelled estimates of turnover time based on OH chemistry alone (Karlsdóttir and Isaksen 2000; Dentener et al. 2003). Figure 2c shows the modelled atmospheric δ¹³C evolution for four combinations of natural and anthropogenic source histories, each for a value of ε (cited in square brackets in the legend) chosen to provide an adequate fit to δ¹³C data from the Southern Hemisphere (Craig et al. 1988, Francey et al. 1999, Ferretti et al. 2005).

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