Global Methane Emissions Through an Isotopic Lens

Changes in atmospheric methane (CH₄) are mainly driven by natural, anthropogenic and pyrogenic emissions and oxidation by OH.

There is no consensus about the underlying explanations about hemispheric-scale changes in atmospheric methane (CH₄). This is partly due to sparse data that do not exclusively identify individual changes in surface emissions and surface and atmospheric losses of CH₄. This challenge represents a major scientific weakness in our understanding of this potent greenhouse gas, with implications for meeting global climate policy obligations.  A confounding challenge is that the regional importance of individual emission sources change with time due to, for example, innovations in agricultural practices, climate-sensitive wetlands, and political decisions associated with climate friendlier transitional fuels.  

Here we use bulk isotope ratios δ¹³C and δD of CH₄ that have been previously shown to provide effective constraints on source apportionment: different CH₄ sources have characteristic isotope ratios. One of the key challenges associated with using these data is that region-specific isotope ratios change with time due to varying source prevalance, in addition to source signatures having inherent uncertainties. We use the GEOS-Chem global 3-D chemical transport model to describe the spatial and temporal isotopic behaviour of atmospheric CH₄. We develop a Maximum A-Posteriori inverse method to simultaneously infer time dependent CH₄ emissions and isotope ratios from in situ data. 

We will report the magnitude, distribution and source attribution of CH₄ emissions from 2004 to 2017, inferred from in situ measurements of total atmospheric CH₄ mole fraction and the corresponding measurements of δ¹³C and δD. We will compare our results with previous studies.