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

Converging evidence from new top-down and bottomup 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 14CH4 (‘‘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 yr1 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 yr1 has fossil origin, both natural and anthropogenic.