Constraining the Sources and Sinks of Atmospheric Methane Using Stable Isotope Observations and Chemistry Climate Modeling

Abstract:

Methane acts as both a greenhouse gas and a driver of atmospheric chemistry. There is a lack of consensus for the explanation behind the atmospheric methane trend in recent years (1980-2010). High uncertainties are associated with the magnitudes of individual methane source and sink processes. Methane isotopes have the potential to distinguish between the different methane fluxes, as each flux is characterized by an isotopic signature. Methane emissions from each source category are expressed explicitly in a chemistry climate model SOCOL, including wetlands, rice paddies, biomass burning, industry, etc. The model includes 48 methane tracers based on source type and geographical origin in order to track methane after it has been emitted. SOCOL simulations for the years 1980-2010 are performed in “nudged mode”, so that model dynamics reflect observed meteorology. Available database estimates of the various surface emission fluxes are inputted into SOCOL. The model diagnostic methane tracers are compared to methane isotope observations from measurement networks. Inconsistencies between the model results and observations point to deficiencies in the available emission estimates or model sink processes. Because of their dependence on the OH sink, deuterated methane observations and methyl chloroform tracers are used to investigate the variability of OH mixing ratios in the model and the real world. The analysis examines the validity of the methane source and sink category estimates over the last 30 years.