Methane Mystery: L.A. Emitting Twice as Much as Estimated

July 23, 2009 -- The greater Los Angeles area is emitting more than double the amount of methane than previously estimated, according to a new study.

A greenhouse gas 25 times more powerful than carbon dioxide (CO2), most man-made methane (CH4) emission comes from agriculture -- rice paddies, livestock, and biomass burning are all big contributors. As a result studies have largely ignored the methane coming from urban areas, and regulatory agencies have had to rely on guesswork to fill in the gaps.

Debra Wunch of the California Institute of Technology in Pasadena and a team of researchers measured greenhouse gas emissions in the south coast air basin, a bowl of smoggy air that hangs over Los Angeles, its suburbs, and some 15 million inhabitants.

According to the latest estimate by the California Air Resources Board, human activity in the basin should only emit about 260,000 tons of methane each year. But after taking readings from August 2007 through June 2008, the team found that the annual rate was much higher: some 600,000 tons.

Much of the area's electricity comes from burning natural gas, which is almost entirely made up of methane. Approximately 10.5 million tons of natural gas was shipped into the south coast air basin in 2006; if two to three percent of that is leaking into the atmosphere, it could account for a large part of the discrepancy.

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"If it's coming from a few potent leaks, one could hope to put a plug in it," Paul Wennberg also of California Institute of Technology, and a co-author on the study due to be published in the journal Geophysical Research Letters said. "This stuff is valuable. If you can get enough of it, you want to burn it."

The excess gas could also be trickling out of cars' exhaust pipes, landfills or sewage treatment plants.

Worldwide, the human activity and natural processes like ocean methane seeps and wetlands decomposition combine to emit approximately 582 million tons of methane each year. If cities around the world have similar methane emission patterns as Los Angeles, the team estimates urban areas could account for as much as 10-15 percent of that total.

"The hope is that the processes operating in California may be happening in other cities around the world," Amy Townsend-Small of the University of California, Irvine said. "If so, this model will be applicable in cities around the world."

Determining how much methane cities give off -- and the sources responsible -- are crucial steps toward controlling emissions in the future. Ultimately, getting a handle on methane emissions could go a long way toward limiting the effects of global warming.

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