Methane is an important greenhouse gas, and its atmospheric concentration has nearly tripled since pre-industrial times1. The growth rate of atmospheric methane is determined by the balance between surface emissions and photochemical destruction by the hydroxyl radical, the major atmospheric oxidant. Remarkably, this growth rate has decreased2 markedly since the early 1990s, and the level of methane has remained relatively constant since 1999, leading to a downward revision of its projected influence on global temperatures. Large fluctuations in the growth rate of atmospheric methane are also observed from one year to the next2, but their causes remain uncertain2,3,4,5,6,7,8,9,10,11,12,13. Here we quantify the processes that controlled variations in methane emissions between 1984 and 2003 using an inversion model of atmospheric transport and chemistry. Our results indicate that wetland emissions dominated the inter-annual variability of methane sources, whereas fire emissions played a smaller role, except during the 1997–1998 El Niño event. These top-down estimates of changes in wetland and fire emissions are in good agreement with independent estimates based on remote sensing information and biogeochemical models. On longer timescales, our results show that the decrease in atmospheric methane growth during the 1990s was caused by a decline in anthropogenic emissions. Since 1999, however, they indicate that anthropogenic emissions of methane have risen again. The effect of this increase on the growth rate of atmospheric methane has been masked by a coincident decrease in wetland emissions, but atmospheric methane levels may increase in the near future if wetland emissions return to their mean 1990s levels.
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We thank P. Rayner, F. Chevallier and F.-M. Breon for comments on the manuscript, and P. Quay for published δ13C-CH4 measurements for the period 1989–1995. Atmospheric CH4 measurements from Réseau Aatmosphérique de Mesure des Composés à Effet de Serre (RAMCES) at Laboratoire des Sciences du Climat et de l'Environnement (LSCE) were partly funded by Institut National des Sciences de l'Univers (INSU). All calculations were realized with Commisariat à l'Energie Atomique (CEA), Centre National de la Recherche Scientifique (CNRS), Institut Pierre Simon Laplace (IPSL) and LSCE computers and support. The development of the Global Fire Emissions Dataset (GFED) used here was supported by a grant from the National Aeronautics and Space Administration (NASA). Author Contributions The main contributions of each author are: P.B.: inversions, data analysis and coordination. P.C.: inverse method and manuscript improvements. J.B.M.: CH4 and δ13C-CH4 data analysis and inversion analysis. E.J.D.: CH4 measurements and manuscript improvements. D.A.H.: chemistry-transport model and manuscript improvements. C.P and F.P.: satellite retrievals of flooded areas. G.R.V.d.W.: CH4 emissions from fires. P.P. and C.C.: inversion method. R.L.L.: CH4 measurements and manuscript improvements. E.G.B., M.R., M.S., L.P.S. and S.C.T.: CH4 measurements. J.L.: plant source analysis. J.W.: δ13C-CH4 measurements.
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Scientific Reports (2018)