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CH4 sources estimated from atmospheric observations of CH4 and its 13C/12C isotopic ratios: 1. Inverse modeling of source processes

A time‐dependent inverse modeling approach that estimates the global magnitude of atmospheric methane sources from the observed spatiotemporal distribution of atmospheric CH4, 13C/12C isotopic ratios, and a priori estimates of the source strengths is presented. Relative to the a priori source estimates, the inverse model calls for increased CH4 flux from sources with strong spatial footprints in the tropics and Southern Hemisphere and decreases in sources in the Northern Hemisphere. The CH4 and 13C/12C isotopic ratio observations suggest an unusually high CH4 flux from swamps (∼200 ± 44 Tg CH4/yr) and biomass burning (88 ± 18 Tg CH4/yr) with relatively low estimates of emissions from bogs (∼20 ± 14 Tg CH4/yr), and landfills (35 ± 14 Tg CH4/yr). The model results support the hypothesis that the 1998 CH4 growth rate anomaly was caused in part by a large increase in CH4 production from wetlands, and indicate that wetland sources were about 40 Tg CH4/yr higher in 1998 than 1999.