Conflicting estimates of natural geologic methane emissions

Global bottom-up and top-down estimates of natural, geologic methane (CH4) emissions (average
approximately 45 Tg yr–1) have recently been questioned by near-zero (approximately 1.6 Tg yr–1) estimates
based on measurements of 14CH4 trapped in ice cores, which imply that current fossil fuel industries’ CH4
emissions are underestimated by 25%–40%. As we show here, such a global near-zero geologic CH4 emission
estimate is incompatible with multiple independent, bottom-up emission estimates from individual natural
geologic seepage areas, each of which is of the order of 0.1–3 Tg yr–1. Further research is urgently needed to
resolve the conundrum before rejecting either method or associated emission estimates in global CH4
accounting.