1. Laboratoire des Sciences du Climat et de l'Environnement, IPSL-LSCE, CEA-CNRS-UVSQ, F-91191, France
2. Université de Versailles Saint Quentin en Yvelines, F-78035, France
3. NOAA Earth System Research Laboratory, Global Monitoring Division, Boulder, Colorado 80305-3328, USA
4. Cooperative Institute for Research in Environmental Science, Campus Box 216, University of Colorado, Boulder, Colorado 80309, USA
5. LERMA, Observatoire de Paris, F-75014, France
6. Faculty of Earth and Life Sciences, Vrije Universiteit, Amsterdam, The Netherlands
7. Laboratoire de Biogéochimie Isotopique, LBI, F-78026, France
8. South African Weather Service, Stellenbosch 7599, South Africa
9. CSIRO, Marine and Atmospheric Research, Victoria 3195, Australia
10. NASA-GISS-Columbia University, New York, New York 10025, USA
11. Earth System Science Department, University of California, Irvine, California 92697, USA
12. Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309, USA

Correspondence to: P. Bousquet^1,2 Correspondence and requests for materials should be addressed to P.B. (Email: philippe.bousquet@cea.fr).

Abstract

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 uncertain^{2, 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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