Bayesian analysis of the glacial-interglacial methane increase constrained by stable isotopes and Earth System modelling

The observed rise in atmospheric methane (CH₄) from 375 ppbv during the last glacial maximum (LGM: 21,000 yr ago) to 680 ppbv during the late pre-industrial era is not well understood. Atmospheric chemistry considerations implicate an increase in CH₄ sources, but process-based estimates fail to reproduce the required amplitude. CH₄ stable isotopes provide complementary information that can help constrain the underlying causes of the increase. We combine Earth System model simulations of the late pre-industrial and LGM CH₄ cycles, including process-based estimates of the isotopic discrimination of vegetation, in a box-model of atmospheric CH₄ and its isotopes. Using a Bayesian approach, we show how model-based constraints and ice core observations may be combined in a consistent probabilistic framework. The resultant posterior distributions point to a strong reduction in wetland and other biogenic CH₄ emissions during the LGM, with a modest increase in the geological source, or potentially natural or anthropogenic fires, accounting for the observed enrichment of δ¹³CH₄.