CH4 sources estimated from atmospheric observations of CH4 and its C-13/C-12 isotopic ratios: 2. Inverse modeling of CH4 fluxes from geographical regions

We present a time-dependent inverse modeling approach to estimate the magnitude of CH4 emissions and the average isotopic signature of the combined source processes from geographical regions based on the observed spatiotemporal distribution of CH4 and C-13/C-12 isotopic ratios in CH4. The inverse estimates of the isotopic signature of the sources are used to partition the regional source estimates into three groups of source processes based on their isotopic signatures. Compared with bottom-up estimates, the inverse estimates call for larger CH4 fluxes in the tropics (266 +/- 25 Tg CH4/yr) and southern extratropics (98 +/- 15 Tg CH4/yr) and reduced fluxes in the northern extratropics (252 +/- 18 Tg CH4/yr). The observations of C-13/C-12 isotopic ratios in CH4 indicate that the large a posteriori CH4 source in the tropics and Southern Hemisphere is attributable to a combination both bacterial sources and biomass burning and support relatively low estimates of fossil CH4 emissions