Optimal estimation of the present‐day global methane budget

Historical observations of the 13 C/ 12 C ratio of atmospheric CH 4 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 13 CH 4 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 13 C/ 12 C 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 (∼30 Tg/a) geological emissions and roughly 20% greater biomass burning emissions at LGM relative to the present‐day.