Why LGM methane needs a bigger wetland collapse
Ice-core CH4 and isotopes imply about a 50% source drop at the LGM; process models underpredict both concentration and isotopic shifts.
Source
Bayesian Analysis of the Glacial-Interglacial Methane Increase Constrained by Stable Isotopes and Earth System Modeling
Study at a glance
- Design
- Computational / modelling — MCMC inversion of LGM vs preindustrial CH4 and isotopes in a three-box model
- N
- Ice-core constraints and Earth-system flux priors; not an observational sample N
- Population
- Atmospheric methane budget from LGM to late preindustrial (ice-core constrained)
- Outcome
- Required source reductions and isotope shifts poorly matched by process models
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Inspect source passages
Why LGM methane needs a bigger wetland collapse
These passages occur verbatim in the stored paper text. They support the points listed below; they do not verify every statement in the explanation.
Current estimates suggest a 50% reduction in sources is required for LGM CH4.
Current process‐based estimates suggest that a 50% reduction in sources is required to explain the CH 4 concentration during the LGM.
Which Earth-system methane study supplies the priors?
Here we use process‐based Earth System model simulations of the late preindustrial and LGM methane cycles (Hopcroft et al., , H17 hereafter), to better understand the change in CH 4 .
What they did
Authors nested Hopcroft et al. Earth-system methane budgets in a three-box isotope model, added leaf-δ13C and chlorine sinks, then used MCMC to invert LGM versus preindustrial ice-core CH4, δ13C, and δD.
What they found
CH4 rose from 375 to 680 ppbv while δ13CH4 fell from −43 to −48‰. Standard LGM fluxes give 447 ppbv and almost no isotope shift. Leaf discrimination adds only ~0.6‰. Posterior wetlands must fall more than models simulate; geologic and fire sources also matter.
The limits
What it doesn't show
High-latitude peat and CH3Cl sources remain poorly represented; a 54 Tg/yr modern geologic source cannot fit the LGM reduction; Cl from extra LGM sea salt is not in the model.
Key terms
- LGM
- Last Glacial Maximum, the last peak ice-age climate.
- δ13CH4
- Carbon-13 composition of atmospheric methane.
- Three-box model
- Simple atmosphere used to mix methane sources and sinks isotopically.
- Wetland methane
- Biogenic CH4 from inundated soils, the largest natural source.
- MCMC
- Markov chain Monte Carlo sampling of posterior source strengths.
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LGM to preindustrial CH4:
Common questions
LGM vs preindustrial CH4?
About 375 to 680 ppbv.
Needed source change?
About a 50% reduction at the LGM.
Do bottom-up models match isotopes?
No—they underpredict concentration and isotope shifts.
Leaf δ13C role?
Limited (~0.6‰ of the atmospheric signal).
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