Improved Constraints on Global Methane Emissions and Sinks Using δ ¹³C-CH₄

We study the drivers behind the global atmospheric methane (CH₄) increase observed after 2006. Candidate emission and sink scenarios are constructed based on proposed hypotheses in the literature. These scenarios are simulated in the TM5 tracer transport model for 1984-2016 to produce three-dimensional fields of CH₄ and δ ¹³C-CH₄, which are compared with observations to test the competing hypotheses in the literature in one common model framework. We find that the fossil fuel (FF) CH₄ emission trend from the Emissions Database for Global Atmospheric Research 4.3.2 inventory does not agree with observed δ ¹³C-CH₄. Increased FF CH₄ emissions are unlikely to be the dominant driver for the post-2006 global CH₄ increase despite the possibility for a small FF emission increase. We also find that a significant decrease in the abundance of hydroxyl radicals (OH) cannot explain the post-2006 global CH₄ increase since it does not track the observed decrease in global mean δ ¹³C-CH₄. Different CH₄ sinks have different fractionation factors for δ ¹³C-CH₄, thus we can investigate the uncertainty introduced by the reaction of CH₄ with tropospheric chlorine (Cl), a CH₄ sink whose abundance, spatial distribution, and temporal changes remain uncertain. Our results show that including or excluding tropospheric Cl as a 13 Tg/year CH₄ sink in our model changes the magnitude of estimated fossil emissions by ∼20%. We also found that by using different wetland emissions based on a static versus a dynamic wetland area map, the partitioning between FF and microbial sources differs by 20 Tg/year, ∼12% of estimated fossil emissions.