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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.

Thornton, B.F., Etiope, G., Schwietzke, S., Milkov, A.V., Klusman, R.W., Judd, A., Oehler, D.Z., 2021. Conflicting estimates of natural geologic methane emissions. Elementa: Science of Anthropocene 9(1). 

https://doi.org/10.1525/elementa.2021.00031

Global natural versus anthropogenic fossil CH4 emission estimates.Saunois et al. (2020) reported annual bottom-up fossil fuel emissions of approximately 180 Tg, 25% of which were natural emissions (Etiope and Schwietzke, 2019), out of a global annual total of 747 Tg. In contrast, Hmiel et al. (2020) suggested median annual geo-CH4 emissions of only 1.6 Tg. DOI: https://doi.org/10.1525/elementa.2021.00031.f1