Methane is an important greenhouse gas, responsible for about 20% of the warming induced by long-lived greenhouse gases since pre-industrial times. By reacting with hydroxyl radicals, methane reduces the oxidizing capacity of the atmosphere and generates ozone in the troposphere. Although most sources and sinks of methane have been identified, their relative contributions to atmospheric methane levels are highly uncertain. As such, the factors responsible for the observed stabilization of atmospheric methane levels in the early 2000s, and the renewed rise after 2006, remain unclear. Here, we construct decadal budgets for methane sources and sinks between 1980 and 2010, using a combination of atmospheric measurements and results from chemical transport models, ecosystem models, climate chemistry models and inventories of anthropogenic emissions. The resultant budgets suggest that data-driven approaches and ecosystem models overestimate total natural emissions. We build three contrasting emission scenarios — which differ in fossil fuel and microbial emissions — to explain the decadal variability in atmospheric methane levels detected, here and in previous studies, since 1985. Although uncertainties in emission trends do not allow definitive conclusions to be drawn, we show that the observed stabilization of methane levels between 1999 and 2006 can potentially be explained by decreasing-to-stable fossil fuel emissions, combined with stable-to-increasing microbial emissions. We show that a rise in natural wetland emissions and fossil fuel emissions probably accounts for the renewed increase in global methane levels after 2006, although the relative contribution of these two sources remains uncertain.
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This paper is the result of an international collaboration of scientists organized by the Global Carbon Project, a joint project of the Earth System Science Partnership. This work was supported by: the UK NERC National Centre for Earth Observation; the European Commission's 7th Framework Programme (FP7/2007-2013) projects MACC (grant agreement no. 218793) and GEOCARBON (grant agreement no. 283080); contract DE-AC52-07NA27344 with different parts supported by the US DOE IMPACTS and SciDAC Climate Consortium projects; computing resources of NERSC, which is supported by the US DOE under contract DE-AC02-05CH11231; NOAA flask data for CH3CCl3 (made available by S. Montzka); the Australian Climate Change Science Program, and ERC grant 247349. Simulations from LSCE were performed using HPC resources from DSM-CCRT and CCRT/CINES/IDRIS under the allocation 2012-t2012012201 made by GENCI (Grand Equipement National de Calcul Intensif). We thank the EDGAR group at JRC (Italy) and US-EPA for providing estimates of anthropogenic emissions.
The authors declare no competing financial interests.
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Kirschke, S., Bousquet, P., Ciais, P. et al. Three decades of global methane sources and sinks. Nature Geosci 6, 813–823 (2013). https://doi.org/10.1038/ngeo1955
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