Global emission estimates

Global emission estimates for natural geological CH4 sources are listed in Table 4.1; the latest estimates are also summarized in Figure 4.4.

Figure 4.4 Estimates of methane emissions from geological sources

Source: Based on data from Etiope et al (2008)

Figure 4.4 Estimates of methane emissions from geological sources

Source: Based on data from Etiope et al (2008)

For global submarine emission estimates, a dual approach was utilized based on the seep flux and on the amount of geological CH4 produced and available to seep (Kvenvolden et al, 2001). The two approaches produced comparable results, 30 and 10Tg yr-1, respectively, with the average, ~20Tg yr-1, still a consensus value (Judd, 2004) awaiting refinement.

Unlike seabed seeps, onshore global emission estimates were derived on the basis of hundreds of flux measurements performed since 2001; some estimates (mud volcanoes, microseepage and geothermal sources) follow upscaling methods recommended by the EMEP/CORINAIR Guidelines, which are based on the concepts of an 'emission factor' and 'area' or 'point' sources (EEA, 2004; Etiope et al, 2007a).

Table 4.1 Global emissions of methane from geological sources

Emission (Tg yr-1) Reference

Table 4.1 Global emissions of methane from geological sources

Emission (Tg yr-1) Reference

Marine seepage

18-48

Hornafius et al (1999)

10-30 (20)

Kvenvolden etal (2001)

Mud volcanoes

5-10

Etiope and Klusman (2002)

10.3-12.6

Dimitrov (2002)

6

Milkov etal (2003)

6-9

Etiope and Milkov (2004)

Other macroseeps

3-4

Etiope et al (2008)

Microseepage

>7

Klusman et al (1998)

10-25

Etiope and Klusman (2010)

Geothermal/volcanic areas

1.7-9.4

Lacroix (1993)

2.5-6.3a

Etiope and Klusman (2002)

<1b

Etiope et al (2008)

30-70c

Etiope and Klusman (2002)

TOTAL

13-36d

Judd (2004)

35-45e

Etiope and Milkov (2004)

45ce

Kvenvolden and Rogers (2005)

40-60c

Etiope (2004); Etiope and Klusman (2010)

42-64c

Etiope et al (2008) - best estimate

30-80c

Etiope et al (2008) - extended range

Note:a Volcanoes not considered;b only volcanoes;c gas hydrates not considered; d microseepage not considered; e former microseepage estimate.

Note:a Volcanoes not considered;b only volcanoes;c gas hydrates not considered; d microseepage not considered; e former microseepage estimate.

The CH4 emission estimates from mud volcanoes differ slightly, although they were derived from different data sets and approaches. The latest estimate, by Etiope and Milkov (2004), was based on direct measurements of flux, and is probably the only one that includes both focused venting and diffuse microseepage around craters and vents. The estimate was also based on a classification of mud volcano sizes in terms of area, following a compilation of data from 120 mud volcanoes. Global emissions from other seeps were based on a database of fluxes measured directly or visually estimated from 66 gas seeps in 12 countries, with the assumption that their flux and size distributions were representative of the global macroseep population, at least 12,500 seeps (Etiope et al, 2008).

The most recent global microseepage emission value was derived on the basis of an accurate estimate of the global area of oil and gas fields, and on TPS, the average flux from each of the three microseepage levels recognized in the global data set (563 measurements), assuming that the percentage of occurrence of the three levels (3 per cent, 12 per cent and 34 per cent) was valid at the global scale (Etiope and Klusman, 2010). Since measurements were made in all seasons, seasonal variations were incorporated into the data set.

Upscaling the measurement to all gas/oil field areas would give a total microseepage of the order of 11-13Tg yr-1. Extrapolating to the global potential microseepage area (TPS: ~8 million km2) would result in an emission in the order of 25Tg yr-1. These estimates are coherent with the lower limit of 7Tg yr-1 initially suggested by Klusman et al (1998) and Etiope and Klusman (2002). However, more measurements in various areas and for different seasons are needed to refine the three-level classification and the actual area of seepage. Finally, global geothermal CH4 flux estimates of 0.9-3.2Tg yr-1 were preliminarily proposed by Lacroix (1993). A wide data set was then reported by Etiope and Klusman (2002), who conservatively derived a global geothermal flux between 2.5 and 6.3Tg yr-1. Lacroix (1993) also suggested a global volcanic CH4 flux of 0.8-6.2Tg yr-1. More recently, volcanic emissions have been considered as not exceeding 1Tg yr-1 (Etiope et al, 2008).

Thus, global geo-CH4 emission estimates, stemming from mud volcanoes plus other seeps, plus microseeps, plus submarine emissions, plus geothermal and volcanic emissions range from 42 to 64Tg yr-1 (mean of 53Tg yr-1), almost 10 per cent of total CH4 emissions, representing the second most important natural CH4 source after wetlands. Geo-CH4 sources would then also represent the missing source of fossil CH4 as recognized in the recent re-evaluation of the fossil CH4 budget for the atmosphere (~30 per cent) (see Lassey et al, 2007; Etiope et al, 2008), which implies a total fossil CH4 emission much higher than that due to the fossil fuel industry. Global geo-CH4 emission estimates are on the same level or higher than other sources (such as biomass burning, termites, wild animals, oceans and wildfires) considered by the IPCC (Denman et al, 2007) (Figure 4.5).

Recent studies indicate that earth's degassing also accounts for at least 17 per cent and 10 per cent of global emissions of ethane and propane, respectively (Etiope and Ciccioli, 2009), hydrocarbons that contribute to photochemical pollution and ozone production in the atmosphere.

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