IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis

7.4.1 Methane Biogeochemistry and Budgets of Methane

Atmospheric CH4 originates from both non-biogenic and biogenic sources. Non-biogenic CH4 includes emissions from fossil fuel mining and burning (natural gas, petroleum and coal), biomass burning, waste treatment and geological sources (fossil CH4 from natural gas seepage in sedimentary basins and geothermal/volcanic CH4). However, emissions from biogenic sources account for more than 70% of the global total. These sources include wetlands, rice agriculture, livestock, landfills, forests, oceans and termites. Emissions of CH4 from most of these sources involve ecosystem processes that result from complex sequences of events beginning with primary fermentation of organic macromolecules to acetic acid (CH3COOH), other carboxylic acids, alcohols, CO2 and hydrogen (H2), followed by secondary fermentation of the alcohols and carboxylic acids to acetate, H2 and CO2, which are finally converted to CH4 by the so-called methanogenic Archaea: CH3COOH → CH4 + CO2 and CO2 + 4H2 → CH4 + 2H2O (Conrad, 1996). Alternatively, CH4 sources can be divided into anthropogenic and natural. The anthropogenic sources include rice agriculture, livestock, landfills and waste treatment, some biomass burning, and fossil fuel combustion. Natural CH4 is emitted from sources such as wetlands, oceans, forests, fire, termites and geological sources (Table 7.6).

The net rate of CH4 emissions is generally estimated from three approaches: (1) extrapolation from direct flux measurements and observations, (2) process-based modelling (bottom-up approach) and (3) inverse modelling that relies on spatially distributed, temporally continuous observations of concentration, and in some cases isotopic composition in the atmosphere (top-down approach). The top-down method also includes aircraft and satellite observations (Xiao et al., 2004; Frankenberg et al., 2005, 2006). When the bottom-up approach is used to extrapolate the emissions to larger scales, uncertainty results from the inherent large temporal and spatial variations of fluxes and the limited range of observational conditions. The top-down approach helps to overcome the weaknesses in bottom-up methods. However, obstacles to extensive application of the top-down approach include inadequate observations, and insufficient capabilities of the models to account for error amplification in the inversion process and to simulate complex topography and meteorology (Dentener et al., 2003a; Mikaloff Fletcher et al., 2004a, 2004b; Chen and Prinn, 2005, 2006). Measurements of isotopes of CH4 (13C, 14C, and 2H) provide additional constraints on CH4 budgets and specific sources, but such data are even more limited (Bergamaschi et al., 2000; Lassey et al., 2000; Mikaloff Fletcher et al., 2004a, 2004b).

Since the TAR, availability of new data from various measurement networks and from national reporting documents has enabled re-estimates of CH4 source magnitudes and insights into individual source strengths. Total global pre-industrial emissions of CH4 are estimated to be 200 to 250 Tg(CH4) yr–1 (Chappellaz et al., 1993; Etheridge et al., 1998; Houweling et al., 2000; Ferretti et al., 2005; Valdes et al., 2005). Of this, natural CH4 sources emitted between 190 and 220 Tg(CH4) yr–1, and anthropogenic sources (rice agriculture, livestock, biomass burning and waste) accounted for the rest (Houweling et al., 2000; Ruddiman and Thomson, 2001). In contrast, anthropogenic emissions dominate present-day CH4 budgets, accounting for more than 60% of the total global budget (Table 7.6).

The single largest CH4 source is natural wetlands. Recent estimates combine bottom-up and top-down fluxes, and global observations of atmospheric CH4 concentrations in a three-dimensional Atmospheric Transport and Chemical Model (ATCM) simulation (Chen and Prinn, 2005, 2006). In these estimates, southern and tropical regions account for more than 70% of total global wetland emissions. Other top-down studies that include both direct observations and 13C/12C ratios of CH4 suggest greater emissions in tropical regions compared with previously estimates (Mikaloff Fletcher et al., 2004a, 2004b; Xiao et al., 2004; Frankenberg et al., 2006). However, several bottom-up studies indicate fewer emissions from tropical rice agriculture (Li et al., 2002; Yan et al., 2003; Khalil and Shearer 2006). Frankenberg et al. (2005, 2006) and Keppler et al. (2006) suggest that tropical trees emit CH4 via an unidentified process. The first estimate of this source was 10 to 30% (62–236 Tg(CH4) yr–1) of the global total, but Kirschbaum et al. (2006) revise this estimate downwards to 10 to 60 Tg(CH4) yr–1. Representative 13C/12C ratios (δ13C values) of CH4 emitted from individual sources are included in Table 7.6. Due to isotope fractionation associated with CH4 production and consumption processes, CH4 emitted from each source exhibits a measurably different δ13C value. Therefore, it is possible, using mixing models, to constrain further the sources of atmospheric CH4.

Geological sources of CH4 are not included in Table 7.6. However, several studies suggest that significant amounts of CH4, produced within the Earth’s crust (mainly by bacterial and thermogenic processes), are released into the atmosphere through faults and fractured rocks, mud volcanoes on land and the seafloor, submarine gas seepage, microseepage over dry lands and geothermal seeps (Etiope and Klusman, 2002; Etiope, 2004; Kvenvolden and Rogers, 2005). Emissions from these sources are estimated to be as large as 40 to 60 Tg(CH4) yr–1.

Table 7.6 Sources, sinks and atmospheric budgets of CH4 (Tg(CH4) yr–1).a

References Indicative 13C, ‰b Hein et al., 1997c Houweling et al., 2000c Olivier et al., 2005 Wuebbles and Hayhoe, 2002 Scheehle et al., 2002 J. Wang et al., 2004c Mikaloff Fletcher et al., 2004ac Chen and Prinn, 2006c TAR AR4 
Base year   1983–1989   2000   1990 1994 1999 1996–2001 1998 2000–2004 
Natural sources     222   145   200 260 168     
Wetlands –58 231 163   100   176 231 145     
Termites –70   20   20   20 29 23     
Ocean –60   15               
Hydrates –60                 
Geological sources –40     14             
Wild animals –60   15                 
Wildfires –25                 
Anthropogenic sources   361   320 358 264 307 350 428     
Energy           74 77         
Coal mining –37 32   34 46     30 48d     
Gas, oil, industry –44 68   64 60     52 36e     
Landfills & waste –55 43   66 61 69 49 35       
Ruminants –60 92   80 81 76 83 91 189f     
Rice agriculture –63 83   39 60 31 57 54 112     
Biomass burning  –25 43     50 14 41 88  43e     
C3 vegetation –25     27               
C4 vegetation –12                   
Total sources   592     503   507 610 596 598 582 
Imbalance   +33               +22 +1 
Soils –18 26     30   34 30   30 30g 
Tropospheric OH –3.9 488     445   428 507   506  511g 
Stratospheric loss   45     40   30 40   40  40g 
Total sink   559     515   492 577   576 581g 


a Table shows the best estimate values.

b Indicative δ13C values for sources are taken mainly from Mikaloff Fletcher et al. (2004a). Entries for sinks are the fractionation, (k13/k12–1) where kn is the removal rate of nCH4; the fractionation for OH is taken from Saueressig et al. (2001) and that for the soil sink from Snover and Quay (2000) as the most recent determinations.

c Estimates from global inverse modelling (top-down method).

d Includes natural gas emissions.

e Biofuel emissions are included under Industry.

f Includes emissions from landfills and wastes.

g Numbers are increased by 1% from the TAR according to recalibration described in Chapter 2.

The major CH4 sinks are oxidation by OH in the troposphere, biological CH4 oxidation in drier soil, and loss to the stratosphere (Table 7.6). Oxidation by chlorine (Cl) atoms in the marine atmospheric boundary layer is suggested as an additional sink for CH4, possibly constituting an additional loss of about 19 Tg(CH4) yr–1 (Gupta et al., 1997; Tyler et al., 2000; Platt et al., 2004; Allan et al., 2005). However, the decline in the growth rate of atmospheric CH4 concentration since the TAR shows no clear correlation with change in sink strengths over the same period (Prinn et al., 2001, 2005; Allan et al., 2005). This trend has continued since 1993, and the reduction in the CH4 growth rate has been suggested to be a consequence of source stabilisation and the approach of the global CH4 budget towards steady state (Dlugokencky et al., 1998, 2003). Thus, total emissions are likely not increasing but partitioning among the different sources may have changed (see Section 2.3). Consequently, in the Fourth Assessment Report (AR4) the sink strength is treated as in the TAR (576 Tg(CH4) yr–1). However, the AR4 estimate has been increased by 1% (to 581 Tg(CH4) yr–1) to take into account the recalibration of the CH4 scale explained in Chapter 2. The main difference between TAR and AR4 estimates is the source-sink imbalance inferred from the annual increment in concentration. The TAR used 8 ppb yr–1 for a period centred on 1998 when there was clearly an anomalously high growth rate. The present assessment uses 0.2 ppb yr–1, the average over 2000 to 2005 (see Section 2.3 and Figure 2.4). Thus, using the CH4 growth rate for a single anomalous year, as in the TAR, gives an anomalously high top-down value relative to the longer-term average source. For a conversion factor of 2.78 Tg(CH4) per ppb and an atmospheric concentration of 1,774 ppb, the atmospheric burden of CH4 in 2005 was 4,932 Tg, with an annual average increase (2000–2005) of about 0.6 Tg yr–1. Total average annual emissions during the period considered here are approximately 582 Tg(CH4) yr–1.

Uncertainty in this estimate may arise from several sources. Uncertainty in the atmospheric concentration measurement, given in Chapter 2 as 1,774 ± 1.8 ppb in 2005, is small (about 0.1%). Uncertainty ranges for individual sink estimates are ±103 Tg(CH4) (20%), ±15 Tg(CH4) (50%), ±8 Tg(CH4) (20%) for OH, soil and stratospheric loss, respectively (as reported in the Second Assessment Report). The use of a different lifetime for CH4 (8.7 ± 1.3 years) leads to an uncertainty in overall sink strength of ±15%. Thus, the top-down method used in AR4 is constrained mainly by uncertainty in sink estimates and the choice of lifetime used in the mass balance calculation.