EOMF-24. Improved Constraints on Global Methane Emissions and Sinks Using Stable Carbon Isotopes of Methane
The atmospheric burden of methane (CH4) has been increasing since 2007 after a relatively stable period from 1999 to 2006, but the reasons behind are still debated. Around the same time that the increase started, the ratio of stable carbon isotopes of CH4 (13C/12C), denoted by δ13C-CH4, started to decrease after two centuries of increase. We use these observations to study the drivers behind the global atmospheric CH4 increase after 2006. Candidate emission and sink scenarios are constructed based on proposed hypotheses in the literature, and long-term mass balance constraint from δ13C-CH4. These scenarios are simulated in the TM5 tracer transport model for 1984–2016 to produce three-dimensional fields of CH4 and δ13C-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 δ13C-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), the primary CH4 sink, cannot explain the post-2006 global CH4 increase since it does not track the observed decrease in global mean δ13C-CH4. Emission increases from microbial sources (from anthropogenic sources and natural wetlands) are most likely the dominant contributors.