Abstract
The observed rise in atmospheric methane (CH4) from 375 ppbv during the Last Glacial Maximum (LGM: 21,000 years ago) to 680 ppbv during the late preindustrial 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 preindustrial 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 δ 13CH4.
Highlights
Atmospheric methane (CH4) is an important greenhouse gas
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
Summary
Atmospheric methane (CH4) is an important greenhouse gas. Its concentration has risen sharply over the past two centuries, reaching 1,799 ppbv by Common Era (CE) 2010 (Kirschke et al, 2013). We performed late preindustrial and LGM atmosphere-only simulations with HadGEM2-ES similar to those reported previously (Hopcroft & Valdes, 2015; H17) but with new intercellular leaf CO2 diagnostics (implemented within the dynamic vegetation scheme of Cox (2001), see supporting information, to calculate the isotopic discrimination by vegetation for the two time periods (following Kaplan, Prentice, & Buchmann, 2002; Lloyd & Farquhar, 1994). The calculated changes in the isotopic signature of wetland δ13C averaged in three latitude bands (−0.1, 0.1, and 0.23‰ in the northern, tropical, and southern boxes, respectively) and for biomass burning (2.6, −0.1, and 4.5‰) were included along with the small change in fractionation factors due to the temperature dependence of methanogenesis and uptake in the box model. This does not capture a substantial fraction of the observed change and implies that changes to both the source mixture and the individual source signatures are required
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