The Los Angeles metropolitan area belches far more methane into its air than scientists had previously realized. 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.
The Los Angeles metropolitan area belches far more methane into its air than scientists had previously realized. 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.
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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 to 0.34 megatonnes per year, or up to half of the total emissions that could not be accounted for by known sources.
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.
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