Bayesian Analysis of the Glacial‐Interglacial Methane Increase Constrained by Stable Isotopes and Earth System Modeling

Abstract

The observed rise in atmospheric methane (CH 4 ) 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 CH 4 sources, but process‐based estimates fail to reproduce the required amplitude. CH 4 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 CH 4 cycles, including process‐based estimates of the isotopic discrimination of vegetation, in a box model of atmospheric CH 4 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 CH 4 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 13 CH 4 .

Polar amplification of Pliocene climate by elevated trace gas radiative forcing

“…CH 4 emissions are a function of microbial activity, i.e., temperature, available substrate, and wetland area (63). Total preindustrial emissions are scaled to 140 TgCH 4 /y for each model, consistent with the observed methane isotope and concentration measurements (64).…”

“…We combine the CH 4 source estimates with the lifetime calculation in a global mean budget calculation B = S × τ , where B is the atmospheric CH 4 burden in Tg, S is the global mean CH 4 source, and τ is the lifetime in years (64). The budget equation is combined with the parametric lifetime model and the soil uptake, which is multiplied by the resultant concentration divided by preindustrial value.…”

Strong sensitivity of the isotopic composition of methane to the plausible range of tropospheric chlorine

“…Our ability to reproduce the observed latitudinal distribution of δ 13 C depends not only on the assumed value of global mean Cl, but also its geographic distribution. The detailed halogen chemistry model (TOMCAT) of Hossaini et al (2016) places the maximum Cl values in the continental NH, in contrast to the large MBL Cl sink used in Allan et al (2007) to explain SH observations. We find that the strong NH Cl maximum, along with the resulting reduction in OH fractionation required to maintain consistency with observations, acts to flatten the interhemispheric gradient of δ 13 C, while the MBL Cl sink increases the hemispheric differences in NH winter and also strengthens the seasonal cycle.…”

“…Both SimStd and SimGC are thus below the 1 % loss via Cl suggested by Gromov et al (2018). We conduct a third sensitivity simulation, SimTom, which uses Cl from the TOMCAT model simulations that include chlorine sources from chlorocarbons (including very short lived substances), HCl from industry and biomass burning, and very short lived substances (Hossaini et al, 2016). This simulation leads to Cl accounting for 2.5 % of tropospheric CH 4 loss in our simulation.…”

“…Cl concentrations are highly variable and not well constrained by direct observations. Modeling work by Hossaini et al (2016) and Sherwen et al (2016) suggests that chlorine provides 2 %-2.5 % of tropospheric methane oxidation. This agrees well with estimates based on the isotopic fractionation, which also sug-gest Cl provides several percent of the total sink (Allan et al, 2007;Platt et al, 2004).…”