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 At its peak, 60 tons per hour of natural gas was spewing from a
ruptured underground pipeline at the Aliso Canyon storage field,
effectively doubling the methane emissions of the entire Los Angeles
metropolitan area, the researchers said.
Their study, published in the journal Science, represents the first
comprehensive effort to quantify a gas leak that made scores of
people ill and prompted the temporary relocation of more than 6,600
households from the northern Los Angeles community of Porter Ranch
at the edge of the gas field.
The damaged injection well discharged a total of 97,100 tons - or 5
billion cubic feet (142 million cubic meters) - of methane into the
environment from the time it was first detected on Oct. 23 until it
was plugged earlier this month, the study said.
Methane, the chief component of natural gas and a far more potent
greenhouse agent than carbon dioxide, persists in the atmosphere for
10 years. The total release from Aliso Canyon is equivalent to the
annual energy-sector methane emissions of a medium-sized European
Union country, it said.
 "Our finding means that the Aliso Canyon leak was the largest
accidental release of methane in the history of the U.S.," Tom
Ryerson, a scientist for the National Oceanic and Atmospheric
Administration , and co-lead of the study, said in a NOAA statement
about the research.
A 2004 accident in Texas actually expelled more natural gas but it
was mostly consumed in an explosion and fire, so that methane never
reached the atmosphere, the study said.
"Aliso Canyon will have by far the largest climate impact" and will
"substantially impact the state of California greenhouse gas
emission targets for the year," the study said.
In terms of heat-trapping greenhouse potential, the volume of leaked
methane was equivalent to putting 572,000 passenger cars on the road
for a year, according to the scientists.
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Aliso Canyon, owned by Southern California Gas Co, a division of San
Diego-based Sempra Energy, is the fourth largest gas storage field
of its kind in the United States.
The utility issued a statement late Thursday minimizing the extent
of the greenhouse gas impact, citing earlier state Air Resources
Board data showing that methane released at Aliso Canyon was
equivalent to 2.4 million tons of carbon dioxide.
That amount, the utility said, represents less than 1 percent of
California's entire year-round greenhouse gas emissions, and about
one-eighth of the all methane produced annually by dairy cows in the
state.
Still, environmental groups have seized on the Aliso Canyon disaster
in calling attention to hazards posed by the state's aging fossil
fuel infrastructure. The crippled well there was converted from oil
production to gas storage in 1973.
The blowout has also triggered a wave of legal action. SoCal Gas has
pleaded not guilty to criminal charges stemming from the leak, which
prosecutors said the utility failed to report in a timely manner.
Local, state and regional authorities, as well as dozens of
residents, have also sued the company.
The study was based on data collected from airborne and ground
measurements taken during the course of the leak and analyzed by
teams from NOAA, the California Energy Commission, two University of
California campuses and the University of Colorado at Boulder.
(Reporting by Steve Gorman; Editing by David Gregorio, Alistair Bell
and Peter Cooney)
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