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 14 CH 4 (“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‐CH 4 ” sources can be conservatively estimated at 53 ± 11 Tg yr −1 globally. This makes geo‐CH 4 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.