Aliso Canyon Study

Aliso Canyon
largest U.S. methane leak ever

methane pollution

Graphic: A. Reiser, NOAA

A team of researchers led by NOAA and Scientific Aviation published the first study to quantify total methane emissions from the Aliso Canyon blowout. During the 112-day event, the well leaked almost as much methane pollution as the entire Los Angeles Basin. The accident's impact on climate will be equivalent to the annual greenhouse gas emissions from over half a million passenger cars.

Where: northwest of Los Angeles, California

When: October 2015 - February 2016

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.

Aliso Canyon Study Data

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

methane comparisons

Important Conclusions

  • At its peak, the methane emission rate from the Aliso Canyon blowout was one-quarter the size of the emission rate of the entire State of California, and effectively doubled the emission rate of the entire Los Angeles Basin.
  • A total of 97,100 tonnes of methane were released over the course of the 112-day event. This is about 24% of the annual methane emissions for the entire Los Angeles Basin, and is the largest methane leak in U.S. history.


  • Methane is a potent greenhouse gas with a global warming potential (GWP) about 28 times higher than that of carbon dioxide (100-year time horizon). Emissions from the Aliso Canyon gas leak will add to California's GHG emissions budgets and impact the state's efforts to meet greenhouse-gas emission targets for the year.
  • This work demonstrates how rapid-response airborne chemical sampling has societal value during episodic releases on the scale of the Aliso Canyon blowout. Such information can help to document human exposure, formulate optimal well control intervention strategies, quantify the effectiveness of deliberate control measures during the crisis, and assess the climate and air quality impacts of large unintended releases.
  • Using the measured amounts of other (non-methane) components in the natural gas, the researchers also could estimate the amounts of benzene, butanes, pentanes, sulfur-containing odorants, and other constituents released during the leak. This gave air quality and health officials a basis for estimating the levels of these other compounds in the impacted area.