Reappraisal of the fossil methane budget and related emission from geologic sources

Converging evidence from new top-down and bottom-up estimates of fossil "radiocarbon-free" methane emissions indicates that natural geologic sources account for a substantial component of the atmospheric methane budget. Comparing emission estimates based on atmospheric (CH4)-C-14 ("radiomethane") with geologic emissions from seepage, including terrestrial macroseeps, microseepage, marine seeps, and geothermal/volcanic emissions from the Earth's crust, shows that such "geo-CH4" sources can be conservatively estimated at 53 +/- 11 Tg yr(-1) globally. This makes geo-CH4 second in importance to wetlands as a natural methane source. Such a new appraisal can easily be accommodated within the uncertainty of the global methane budget as recently compiled, and recognizes the importance of geophysical out-gassing of methane generated within the lithosphere. We propose a new coherent contemporary budget in which 30 +/- 5% (based on atmospheric radiomethane measurements) of the global source of 582 +/- 87 Tg yr(-1) has fossil origin, both natural and anthropogenic.