Studies performed since 2000 have demonstrated that geologic emissions of methane are animportant global greenhouse-gas source (Etiope, 2004; Kvenvolden and Rogers, 2005; Etiopeal, 2008). It is recognised that significant amounts of methane, produced within the Earth crust,released naturally into the atmosphere through faults and fractured rocks. Major emissions arerelated to hydrocarbon production in sedimentary basins (microbial and thermogenic methane),through continuous exhalation and eruptions from more than 1 200 onshore and offshore mudvolcanoes, more than 10 000 onshore and shallow marine seeps and through diffuse soilmicroseepage. Specifically, six source categories must be considered: mud volcanoes, gas seeps(independent of mud volcanism), microseepage (diffuse exhalation from soil in petroleum basins),submarine seepage, geothermal (non-volcanic) manifestations and volcanoes. Global emissionestimates range from 42 to 64 Tg y-1 (mean of 53 Tg y-1), almost 10 % of the total CH4 emission,representing the second most important natural methane source after wetlands. Geo-CH4 sourceswould also represent the missing source of fossil methane recognised in the recent re-evaluationthe fossil methane budget in the atmosphere (about 30 %; Lassey et al,, 2007; Etiope et al, 2008),which implies a total fossil methane emission much higher than that due to fossil fuel industry.The global geo-CH4 emission estimates are of the same level as or higher than other sources orsinks considered in the Intergovernmental Panel on Climate Change (IPCC) tables, such asbiomass burning, termites and soil uptake. Recent studies indicate that Earth’s degassing alsoaccounts for at least 17 % and 10 % of total ethane and propane emissions (Etiope and Ciccioli,2009).
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