Conflicting estimates of natural geologic methane emissions

Abstract

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.