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By Matt McGrath Environment correspondent
A scientific analysis of a natural gas leak near Los Angeles says that it was
the biggest in US history.
The Aliso Canyon blowout vented almost 100,000 tonnes of methane into the
atmosphere before it was plugged.
The impact on the climate is said to be the equivalent of the annual emissions
of half a million cars.
Researchers say it had a far bigger warming effect than the BP oil spill in the
Gulf of Mexico in 2010.
First detected on 23 October, the leak came from one of the 115 wells connected
to a massive underground natural gas storage facility, the fifth largest in the
US.
Seven unsuccessful attempts were made to shut down the billowing plumes of
methane and ethane by the owners, Southern California Gas Company (SoCalGas).
Concerns over the impacts of the spewing gas eventually led to more than 11,000
nearby residents being evacuated as California governor Jerry Brown declared a
state of emergency in the area.
The blowout pushed enough of the gas into the air every day to fill a balloon
the size of a football stadium.
At its peak, the flow doubled the rate of methane emissions from the entire Los
Angeles basin.
The leak was permanently sealed on 18 February. By then almost 100,000 tonnes
of methane had poured into the atmosphere.
Off the scale
Scientists have now completed an analysis based on 13 research flights that
captured air samples in and around the methane plume as well as from the
ground. The initial aircraft readings were so high that the researchers
rechecked their monitoring equipment for errors.
The amount of methane entering the atmosphere from the leaking well makes it
the largest of its kind recorded in the US. A bigger escape of gas occurred in
Texas in 2004 but as most of this methane burned off in a fire that followed an
explosion, the impact on the climate was muted.
The researchers say that the blowout will have a significant impact on
California's ability to meet its greenhouse gas emissions targets this year.
Methane is a short-lived atmospheric chemical but is highly potent as a warming
gas, with an effect 25 times higher than CO2 over a 100-year time period.
"In terms of the methane release, Aliso Canyon is by far largest," said lead
author Dr Stephen Conley, from the University of California, Davis.
"It had the largest climate impact; it beats the BP oil spill."
The analysis found above normal levels of several potentially harmful chemicals
that came from the natural gas leak. These included benzene, toluene and
xylenes which have been linked to health impacts from long term exposure.
Role of luck
The authors believe there are important lessons to be learned from the leak -
particularly the need to monitor oil and gas facilities more carefully.
They say that there has been little co-ordinated oversight of the biggest oil
and gas leaks in recent years. They point to Aliso Canyon, the BP spill and the
Total Elgin rig blowout in the North Sea, as examples where luck more than
intent ensured the impacts on the environment were monitored.
In the case of Aliso Canyon, the surveying aircraft was working on another
project searching for pipeline problems, when the scientists were asked to
overfly the leaking well.
"The state's response to Aliso Canyon was teed off by the first measurement we
took, at that point no one had any clue that this was 50,000kg per hour of
gas," said Dr Conley.
"That to me is a huge oversight, especially with the Paris Climate Agreement.
How can we commit to monitor and reduce our greenhouse gas emissions without
measuring our biggest emitters?
"These sort of leaks will continue to happen, let's try to be continuously
looking for them so we can seriously talk about reducing our emissions."
The research has been published in the journal, Science.