Contribution of anthropogenic and natural sources to atmospheric methane variability

Methane: masked intruder Changes in atmospheric methane levels over the past twenty years are well documented, but the causes of these changes remain uncertain. Bousquet et al . use inverse (top-down) modelling to quantify variations in methane emissions from different sources between 1983 and 2004. They find that a decrease in the growth rate of atmospheric methane during the 1990s was caused by a decrease in anthropogenic emissions, but that anthropogenic emissions have increased again since 1999. To date, this trend has been masked by a coincident decrease in wetland emissions, but it is possible that it will cause total methane levels to rise again in the near future. Inverse modelling is used to quantify variations in methane emissions from different sources from 1983–2004 reveals that a decrease in the growth rate of atmospheric methane during the 1990s was caused by a decrease in anthropogenic emissions, but that anthropogenic emissions have increased again since 1999. Methane is an important greenhouse gas, and its atmospheric concentration has nearly tripled since pre-industrial times 1 . 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 decreased 2 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 next 2 , 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 aga