Global methane budget by inversion modelling utilising δ13C-CH4 observations in CarbonTracker Europe - CH4

Methane (CH₄) is an important greenhouse gas and its atmospheric burden has more than doubled since the 18^th Century. Currently the concentration of CH₄ in the atmosphere is well known, but the contribution from different emission sources are uncertain. CH₄ is emitted to the atmosphere from various sources including ocean, termites, fires, wetlands, coal industry, rice cultivation, landfills and waste water treatment, oil and gas industry, enteric fermentation and manure management, and residential. The contribution of different sources to the global CH₄ budget is important to study in order to understand the changes in the CH₄ growth-rate before and after 2006. Different CH₄ sources have process specific δ¹³C-CH₄ values, which can be used to identify emission sources.

In this study CarbonTracker Europe-CH₄ (CTE-CH₄) is modified to assimilate δ¹³C-CH₄ observations and to optimise methane emissions source-wise based on the knowledge of existing CH₄ emissions. The observations of CH₄ and δ¹³C-CH₄ are provided by the NOAA Global Monitoring Division, the Institute of Arctic and Alpine Research and Royal Holloway, University of London. CTE-CH₄ uses TM5 chemistry transport model as an observation operator, constrained by ECMWF ERA Interim meteorological fields, and uses off-line TM5 chemistry to account for the atmospheric CH₄ sink. Prior natural emission for wetlands, peatlands and mineral soils are taken from LPX-Bern DYPTOP ecosystem model and prior fire emissions from GFED v4. Prior emissions for coal industry, rice cultivation, landfills and waste water treatment, oil and gas industry, enteric fermentation and manure management, and residential are taken from EDGAR v4.2 FT2010 inventory and scaled to match the optimised emissions from CTE-CH₄, which is estimated by assimilating atmospheric CH₄ only. Also emissions for termites, and ocean are included in the model. Deriving detailed emission estimates of CH₄ will provide more comprehensive understanding of the methane source attribution. Preliminary results for western Europe, north eastern America, South America, and eastern Australia indicate an increase in emissions from rice cultivation and enteric fermentation and manure management.