Mystery of Methane Levels in 90’s Seems Solved

[jpak]

http://www.nytimes.com/2006/09/28/science/28methane.html?_r=2&adxnnl=1&oref=login&ref=us&adxnnlx=1159531895-omU+5FcyneMakd09IkzNAg&oref=slogin

For years, scientists have been trying to figure out why the atmospheric concentration of methane, a heat-trapping gas, stopped increasing in the early 1990’s after tripling during the preceding 200 years.

Some scientists and environmental groups said the change could be a sign of success in efforts to stem emissions of methane, a gas that contributes to global warming. They included stanching pipeline leaks and capturing methane from landfills.

Now a new study by an international research team supports the suspicions of some experts that the leveling off was probably temporary and caused by a downturn in emissions from industry and most likely related to the collapse of the Soviet Union and its economy.

After 1999, emissions from industry and other human activities began rising again, particularly in China, according to the study, which will be published today in the journal Nature. But that increase in methane from manmade sources appears to have been masked by a reduction in methane from sources in nature. Tropical droughts reduced methane released by bacteria in muddy wetlands, the study said.

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and Nature abstract...

Contribution of anthropogenic and natural sources to atmospheric methane variability

P. Bousquet, P. Ciais, J. B. Miller, E. J. Dlugokencky, D. A. Hauglustaine, C. Prigent, G. R. Van der Werf, P. Peylin, E.-G. Brunke, C. Carouge, R. L. Langenfelds, J. Lathière, F. Papa, M. Ramonet, M. Schmidt, L. P. Steele, S. C. Tyler and 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 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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