Potent greenhouse-gas methane has been rising
Methane levels in the atmosphere rose in 2007 after 10 years. Scientists are trying to find out why.
After nearly a decade of holding relatively steady, levels of methane in the atmosphere appear to be rising, and scientists are trying to find out why.Skip to next paragraph
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The uptick is tiny, especially compared with the growth in carbon-dioxide emissions from industrial activity and land-use changes. But the shift has still raised eyebrows.
Pound for pound, methane is 25 times more effective as a greenhouse gas than carbon dioxide. So powerful is its effect that some experts have proposed that the world could meet climate targets more readily if it made big strides in reducing methane and other greenhouse gases that are less abundant but more potent.
If the increase seen last year continues unabated, researchers worry that over the long term, it could help bring on significant additional warming. As Earth's climate warms, it could trigger a steady release of methane as permafrost in the Northern Hemisphere's high latitudes thaws and the organic material it kept on ice begins todecompose. The more methane the permafrost loses, the more the climate warms.
The increase "comes on top of a period where methane barely increased at all," says Ed Dlugokencky, a researcher at the National Oceanic and Atmospheric Administration's Earth Systems Research Laboratory in Boulder, Colo. "I think that's why it looks so remarkable."
During the early 1980s, he says, methane levels in the atmosphere rose at an average annual rate of 14 molecules for every billion molecules of other gases, although the growth rate was beginning to slow over that period. From 1999 to 2006, methane's abundance in the atmosphere was relatively constant.
Last year, however, the concentration rose by 10 parts per billion to an overall level of 1.8 parts per million (p.p.m).
When its direct and indirect effects on the atmosphere are considered, its current warming effect is about half that of CO2, even though it is far less abundant. Both gases are substantially more abundant now than at any time during the last 650,000 years, according to research summarized by the Intergovernmental Panel on Climate Change last year.
Last year's methane rise may stem from wetlands' emissions at high latitudes in the northern hemisphere, Dr. Dlugokencky speculates. He notes that 2007 was a particularly warm year there. "We're pretty sure it didn't come from biomass burning" because the data doesn't show a big increase in carbon monoxide, another byproduct of burning biomass.
Methane rises from natural sources such as wetlands – as submerged plant material decays – wildfires, permafrost, and ocean sources. But estimates put human-related emissions – from farming to landfills – at some 60 percent of the methane in the air.