Optimal estimation of the present-day global methane budget

Historical observations of the C-13/C-12 ratio of atmospheric CH4 are used to constrain the present-day methane budget using optimal estimation. Three methane emission scenarios with basis in the recent literature are evaluated against historical (CH4)-C-13 observations, considering all uncertainties together. We estimate that present-day methane emissions are composed of 64%-76% biogenic, 19%-30% fossil, and 4%-6% pyrogenic sources. It is found that, barring any changes in the isotopic signatures of sources and sink processes, satisfying the C-13/C-12 record requires estimates of present-day anthropogenic fuel-related emissions that are on the high end of the assumed uncertainties, even when a significant geological source is included. Extending present-day results to the time of the Last Glacial Maximum (LGM), emissions from wetlands are implied to be 40%-62% of the present-day value, the higher number being valid only for a scenario with strong (similar to 30 Tg/a) geological emissions and roughly 20% greater biomass burning emissions at LGM relative to the present-day.