2016 News & Events

California's Aliso Canyon blowout led to largest U.S. methane leak ever

25 February 2016

natural gas well blowout and site
Methane and other gases leaking from the Aliso Canyon natural gas well blowout northwest of Los Angleles, California (left). Site aerial of the Aliso Canyon well blowout, as seen from the research aircraft used in the study (right). Infrared Image (left): Environmental Defense Fund; Aerial Photo (right): S. Conley, Scientific Aviation / UC Davis

The first published study since the natural gas well was plugged shows the leak was equivalent to one-quarter of Los Angeles' annual methane emissions.

On February 11, workers in California ended one of the largest natural gas leaks in history, the Aliso Canyon leak that had been releasing methane and other gases into the atmosphere since October 23rd. The leak caused the evacuation of more than 5,000 households in Porter Ranch, a suburb northwest of Los Angeles.

Scientists in California and Colorado quickly joined forces to assess the atmospheric impacts of the leak, commencing airborne and ground measurements about two weeks after the blowout to quantify how much methane, a powerful greenhouse gas, was escaping. Scientists from the Chemical Sciences Division of NOAA's Earth System Research Laboratory and from Scientific Aviation (a research company in Boulder) co-led the Aliso Canyon Study.

The enormity of the methane leak surprised even the scientists. At its peak, the methane emission rate doubled that of the entire Los Angeles Basin. The findings are published in the February 25 issue of Science.

figure 2 adapted
Measurements of methane and ethane leak rate throughout the Aliso Canyon well blowout (top panel). Efforts of Southern California Gas slowed the leak on about 1 December and halted the leak on 11 February. Researchers used their measurements to calculate the total methane released (bottom panel). Figure: Science (adapted from figure 2 of the published paper)

The scientists continued to make measurements as workers took steps to reduce the leak throughout recent months, until the leak was finally plugged. Their 13 research flights gave them an unprecedented "end-to-end" opportunity to document the total amount of methane that was released over the 112-day leak. The amount was over 100,000 tons of methane – one quarter of the methane that is typically emitted by the entire Los Angeles Basin over the course of a year.

"Our finding means that the Aliso Canyon leak was the largest accidental release of methane in the history of the U.S.," said Tom Ryerson, a scientist at the NOAA ESRL Chemical Sciences Division and co-lead of the study.

The Aliso Canyon gas leak will substantially impact California's ability to meet greenhouse gas emission targets for the year, according to the study's authors.

"The impact of the Aliso Canyon leak on climate will be equivalent to the annual greenhouse gas emissions from more than a half-million passenger cars," said study coauthor Jeff Peischl, a scientist with the Cooperative Institute for Research in Environmental Sciences at University of Colorado Boulder who works in the NOAA ESRL Chemical Sciences Division.

The 13 flights also were real-time snapshots of the effectiveness of mitigation measures taken by the Southern California Gas Company in December and January. After about December 1, the emission rate gradually fell from its peak of 50 tons of methane per hour to about 20 tons per hour, where it steadied until the successful intervention on February 11. The researchers relayed their findings to the California Air Resources Board throughout the leak.

figure 1
Flight tracks (white lines) of the Scientific Aviation research aircraft that sampled methane and ethane in the air around and downwind of the Aliso Canyon natural gas well blowout (marked as the SS-25 well). Whole air samples (WAS) were gathered on the ground near the well site and later analyzed to determine the composition of the leaking natural gas and to compare to the airborne measurements. Estimates of the release of benzene, odorants added to the natural gas, and other constituents were made based on the comparison. Figure: Science (figure 1 of the published paper)

Lead Author Stephen Conley, a scientist with Scientific Aviation and University of California Davis, gathered the airborne data using his single-engine Mooney TLS aircraft, carrying instruments that made real-time measurements of methane and ethane. On the ground near the well site, coauthor Donald Blake of University of California Irvine collected samples of the leaking natural gas to determine its composition and compare to Conley's airborne measurements. Using these samples, the researchers also could estimate the trace amounts of benzene, toluene, sulfur-containing odorants, and other constituents released during the leak. The estimates gave air quality and health officials a basis for gauging the levels of these other compounds in the impacted area.

The mega-leak has drawn attention to the broader problem of fugitive emissions from natural gas extraction and processing and from the pipelines and storage infrastructure across the country. The leaky well is one of 115 wells connected to this particular subsurface storage reservoir, which has been used for natural gas storage since 1973 and is the fourth largest facility of its kind in the U.S.

The new study highlights the value of responding rapidly with airborne chemical sampling during crises involving the development of energy resources. Such information can help to document human exposure, formulate optimal well control intervention strategies, quantify the effectiveness of deliberate control measures during well breaches, and assess the climate and air quality impacts of large unintended releases. The NOAA and CIRES researchers in the Aliso Canyon study also provided critical airborne measurements during the 2010 Deepwater Horizon Oil Spill in the Gulf of Mexico. Their work in the Gulf established that air quality was safe for relief workers and the public downwind, and also pioneered a new method of using airborne measurements to quantify the subsurface oil leak rate during offshore oil spills.

In the case of the Aliso Canyon leak, the researchers came together to make unprecedented measurements of the event from nearly its start until the leak was plugged.

"If we don't measure these things quickly, we won't have any idea of what kind of response might be called for," said Conley. "We're happy that we could provide state officials with the scientific information they needed."

The work was funded by the California Energy Commission, the Southern California Gas Company, the California Agricultural Experiment Station, and NOAA.

Stephen Conley (Scientific Aviation, Boulder, CO and University of California, Davis), Guido Franco (California Energy Commission), Ian Faloona (University of California, Davis), Donald R. Blake (University of California, Irvine), Jeff Peischl (University of Colorado CIRES and NOAA), and Thomas B. Ryerson (NOAA), Methane emissions from the 2015 Aliso Canyon blowout in Los Angeles, CA, Science, doi:10.1126/science.aaf2348, 2016.


Single-point failures of the natural gas infrastructure can hamper deliberate methane emission control strategies designed to mitigate climate change. The 23 October 2015 blowout of a well connected to the Aliso Canyon underground storage facility in California resulted in a massive release of natural gas. Analysis of methane (CH4) and ethane (C2H6) data from dozens of plume transects from 13 research aircraft flights between 7 Nov 2015 and 13 Feb 2016 shows atmospheric leak rates of up to 60 metric tonnes of CH4 and 4.5 metric tonnes of C2H6 per hour. At its peak this blowout effectively doubled the CH4 emission rate of the entire Los Angeles Basin, and in total released 97,100 metric tonnes of methane to the atmosphere.


In a study co-led by NOAA, scientists found that at its peak, the unprecedented gas leak in Aliso Canyon near Los Angeles effectively doubled the methane emission rate of the entire LA Basin, and that in total it released more methane than any other leak in U.S. history. In the 25 February issue of Science, researchers from the Chemical Sciences Division of NOAA's Earth System Research Laboratory, its Cooperative Institute CIRES in Boulder, and partners published data and analyses quantifying the methane leak. They made aircraft and ground-based measurements of methane, a potent greenhouse gas, from shortly after the leak's October 23 start until its end on February 11. Their results showed that at its peak, this blowout was releasing methane at one quarter of the total methane emission rate for the entire state, and was doubling the methane emission rate of the Los Angeles Basin. A total of over 100,000 tons (97,100 metric tonnes) of methane were released over the course of the 112-day event. Ground samples near the site of the breached well determined the composition of the leaking natural gas and enabled the authors to calculate the release of other gases such as butanes and higher hydrocarbons, benzene, toluene, and xylenes. Researchers from Scientific Aviation and NOAA ESRL CSD led the study, with coauthors from CIRES, University of California, and the California Energy Commission.

methane comparisons

Important Conclusions