Global and Regional CH₄ Emissions for 1995–2013 Derived From Atmospheric CH₄, δ¹³C‐CH₄, and δD‐CH₄ Observations and a Chemical Transport Model

Abstract

Abstract To better understand the current global CH 4 budget, biogenic, fossil fuel, and biomass burning CH 4 fluxes for the period 1995–2013 were inversely estimated from the observed mole fraction data of atmospheric CH 4 using a three‐dimensional chemical transport model. Then, forward simulations of carbon and hydrogen isotope ratios of atmospheric CH 4 (δ 13 C‐CH 4 and δD‐CH 4 ) were conducted using the inversion fluxes to evaluate the source proportion of the global total CH 4 emission. Model‐simulated spatiotemporal variations of atmospheric CH 4 reproduce the observational results well; however, the simulated δ 13 C‐CH 4 and δD‐CH 4 values significantly underestimate their observed values as a whole. This implies that the proportion of biogenic CH 4 sources in the global CH 4 emission, deduced by inverse modeling, is overestimated, although the proportion is fairly comparable with the medians of recent multiple CH 4 inverse modeling. To reduce the disagreement between the observed and calculated isotope ratios, the CH 4 fluxes of individual source categories were adjusted using our atmospheric δ 13 C‐CH 4 and δD‐CH 4 data observed at Arctic and Antarctic surface stations. The resultant global average biogenic, fossil fuel, and biomass burning CH 4 fluxes over 2003–2012 are 346 ± 11, 162 ± 2, and 50 ± 2 TgCH 4 year −1 , respectively. It is also strongly suggested that the leveling‐off of atmospheric CH 4 in the early 2000s and the renewed growth after 2006/2007 are, respectively, explainable by the decrease in biogenic and biomass burning CH 4 emissions for 2000–2006 and the increase in biogenic CH 4 emissions after that period. These emission changes mainly originate in the tropics.

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