Mystery methane belched out by megacities

by Bob Holmes, NewScientist, July 17, 2009

The Los Angeles metropolitan area belches far more methane into its air than scientists had previously realised. If other megacities are equally profligate, urban methane emissions may represent a surprisingly important source of this potent greenhouse gas.

Atmospheric researchers have long had good estimates of global methane emissions, but less is known about exactly where these emissions come from, particularly in urban areas.

To fill this void, a research team led by Paul Wennberg, an atmospheric chemist at Caltech in Pasadena, estimated methane emissions for the Los Angeles region, then subtracted all known sources of methane, such as livestock, landfills and sewage. They ended up with an enormous amount of methane – about 0.14–0.34 megatonnes per year, or up to half of the total emissions that could not be accounted for by known sources.

'Put a stopper in'

"There's a sizeable amount of emissions that seem not to be catalogued right now," says Wennberg. "You'd like to find out what's the source, so you can put a stopper in it."

If other urban areas have a similar share of uncatalogued emissions, then these unknown urban sources would account for 7–15% of global methane emissions from human activity.

This extra methane may result from higher than expected leakage from landfills, sewage treatment plants or natural gas pipelines, or it could be natural seepage from oil wells or other geologic sources, the researchers say.

Carbon clues

A separate study now underway by Amy Townsend-Small, a biogeochemist at the University of California at Irvine, may help pinpoint the source.

She has been measuring the carbon isotope composition of methane released from different sources – for example, natural gas and other geologically old methane should be devoid of carbon-14, a relatively short-lived radioactive isotope.

Townsend-Small plans to use these measurements to calculate what proportion of emissions come from each source. "It seems likely that we'll be able to use isotopes to distinguish between urban sources," she says.

Journal reference: Geophysical Research Letters (DOI: 10.1029/2009GL039825)

Link to NewScientist article: http://www.newscientist.com/article/dn17479-mystery-methane-belched-out-by-megacities.html