Improved Constraints on Global Methane Emissions and Sinks Using C-CH.

We study the drivers behind the global atmospheric methane (CH) 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 CH and C-CH, 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) CH emission trend from the Emissions Database for Global Atmospheric Research 4.3.2 inventory does not agree with observed C-CH. Increased FF CH emissions are unlikely to be the dominant driver for the post-2006 global CH 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 CH increase since it does not track the observed decrease in global mean C-CH. Different CH sinks have different fractionation factors for C-CH, thus we can investigate the uncertainty introduced by the reaction of CH with tropospheric chlorine (Cl), a CH sink whose abundance, spatial distribution, and temporal changes remain uncertain. Our results show that including or excluding tropospheric Cl as a 13 Tg/year CH 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.