Contribution of anthropogenic and natural sources to atmospheric methane variability
P. Bousquet1,2, P. Ciais1, J. B. Miller3,4, E. J. Dlugokencky3, D. A. Hauglustaine1, C. Prigent5, G. R. Van der Werf6, ` P. Peylin7, E.-G. Brunke8, C. Carouge1, R. L. Langenfelds9, J. Lathiere1, F. Papa5,10, M. Ramonet1, M. Schmidt1, 9 11 12 L. P.Steele , S. C. Tyler & J. White
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 theearly 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 uncertain2–13. Here we quantify the processes that controlled variations in methane emissions between 1984and 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 emis˜ sions played a smaller role, except during the 1997–1998 El Nino event. These top-down estimates of changes in wetland and ﬁre emissions are in good agreement with independent estimates based on remotesensing 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 coincidentdecrease in wetland emissions, but atmospheric methane levels may increase in the near future if wetland emissions return to their mean 1990s levels. The global growth rate of atmospheric methane (CH4) decreased from nearly þ12 ^ 2 p.p.b. yr21 in the 1980s to þ4 ^ 4 p.p.b. yr21 in the last decade (all values are means ^ s.d.), but with large yearto-year variations2 (Fig. 1a). A peak in growth rateoccurred in 1991 in the tropics, followed by a large and abrupt drop in 1992, which began in the northern regions. The past few years have been marked by two positive growth-rate anomalies in 1997–1998 and in 2002– 2003, which seem more pronounced north of 30 8N than in the tropics. To understand better why the growth rate of CH4 has remained persistently smaller after the early 1990s, we have analysedthe regional trends in CH4 differences between sampling sites in the National Oceanic and Atmospheric Association (NOAA) global cooperative air sampling network2 and the South Pole site, taken as a reference (Fig. 1b and Supplementary Information). This analysis
suggests that either northern CH4 emissions have declined persistently since 1992 or that the destruction of CH4 by the hydroxylradical (OH) has increased north of 30 8N. Several conﬂicting hypotheses have been proposed to explain interannual and long-term variations in atmospheric CH4, focusing on wetland CH4 emissions10,13,14, anthropogenic CH4 emissions5, wild ﬁres6–9,15, OH photochemistry3,4,11 and interannual wind changes12. Various models have been used, but the contribution of each process has not been disentangledin a coherent framework, except for short periods6,16. Our understanding of the current methane budget therefore remains plagued by very large uncertainties. Atmospheric CH4 measurements can be linked quantitatively to regional sources and sinks by inverse modelling. For the period 1984–2003, the CH4 concentration responses to the action of OH sinks and regional surface sources were simulated...