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Abstract:
Atmospheric methane (CH4) increased from similar to 900 ppb (parts per billion, or nanomoles per mole of dry air) in 1900 to similar to 1800 ppb in 2010 at a rate unprecedented in any observational records. However, the contributions of the various methane sources and sinks to the CH4 increase are poorly understood. Here we use initial emissions from bottom-up inventories for anthropogenic sources, emissions from wetlands and rice paddies simulated by a terrestrial biogeochemical model, and an atmospheric general circulation model (AGCM)-based chemistry-transport model (i.e. ACTM) to simulate atmospheric CH4 concentrations for 1910-2010. The ACTM simulations are compared with the CH4 concentration records reconstructed from Antarctic and Arctic ice cores and firn air samples, and from direct measurements since the 1980s at multiple sites around the globe. The differences between ACTM simulations and observed CH4 concentrations are minimized to optimize the global total emissions using a mass balance calculation. During 1910-2010, the global total CH4 emission doubled from similar to 290 to similar to 580 Tg yr(-1). Compared to optimized emission, the bottom-up emission data set underestimates the rate of change of global total CH4 emissions by similar to 30% during the high growth period of 1940-1990, while it overestimates by similar to 380% during the low growth period of 1990-2010. Further, using the CH4 stable carbon isotopic data (delta C-13), we attribute the emission increase during 1940-1990 primarily to enhancement of biomass burning. The total lifetime of CH4 shortened from 9.4 yr during 1910-1919 to 9 yr during 2000-2009 by the combined effect of the increasing abundance of atomic chlorine radicals (Cl) and increases in average air temperature. We show that changes of CH4 loss rate due to increased tropospheric air temperature and CH4 loss due to Cl in the stratosphere are important sources of uncertainty to more accurately estimate the global CH4 budget from delta C-13 observations.
