Current understanding of sources and quantities of geological and biological methane emissions into the atmosphere

Methane (CH₄) is an important greenhouse gas, but our understanding of the magnitude of its sources is fraught with considerable uncertainties. Quite neglected in early atmospheric methane budget studies, Earth’s degassing is today considered a major natural source of methane. Geological (natural fossil) methane emissions, including gas seepage in petroliferous sedimentary basins (macro-seeps, microseepage, marine seepage) and geothermal exhalations, were estimated at the global scale by several research groups, based on bottom-up and top-down procedures, and accounting for ~40-50 Tg CH₄ yr^-1 (latest review and discussions on conflicting estimates are in Etiope and Schwietzke, 2019, and Thornton et al. 2021). This value is equivalent to roughly one-third of the average emission attributed to wetlands, and it rivals with the high uncertainty of freshwater sources. Global gridded mapping was developed to provide the spatial distribution of the geological methane sources, as well as their isotopic (¹³C/¹²C) composition and potential intensity (Etiope et al. 2019), and it has been used to refine fossil fuel industry and microbial CH₄ emission budget. Geological emission breakdown at continental scale could be derived (e.g., Petrescu et al. 2023), with the emission values that must be considered only in terms of “order of magnitude”. The geo-CH₄ sources are now included in the methane budget of the Global Carbon Project (Saunois et al. 2020), where they are compared to other natural and anthropogenic sources, including an analysis of double-counting with some biological sources. Correct definitions, source attribution, uncertainties and limits define the roadmap strategy for refining emission estimates.