Study: EPA likely underestimating methane emissions by at least 50%

Methane emissions are worse than the conventional wisdom would have you believe, according to a new study by researchers at Stanford University.

Methane, which is the primary component of natural gas, is an especially powerful greenhouse gas, packing more than two dozen times as much global warming potential than carbon dioxide.

Traditionally, environmental regulators and energy industry groups have estimated methane emissions by multiplying the amount of methane emitted by a specific source – e.g., belching cattle or methane leaks at natural gas processing plants – by the number of that source type in a geographic region.

For example, imagine that a cow emits 1/10 of a metric ton of methane every year. If the United States has 10 cows, the total methane emissions attributable to cattle is one metric ton annually. By adding the total methane emissions from cattle with the totals from every other source of methane emissions, we can derive the total methane emissions for the United States.

That is how the U.S. Environmental Protection Agency has traditionally calculated methane emissions since the 1990s. If the methane emissions rates (e.g., how much methane does a cow emit in a year?) are wrong, the total estimated methane emissions are also wrong.

Several studies have tested the accuracy of these traditional methane emissions estimates by using airplanes and towers to measure actual methane in the air.

The new study, “Methane Leakage from North American Natural Gas Systems,” evaluated more than 200 of these atmospheric studies and concluded that the EPA’s methane emissions estimates are too low.

The key take-away: the EPA is likely underestimating U.S. methane emissions from natural gas by at least 50% or more.

“People who go out and actually measure methane pretty consistently find more emissions than we expect,” said Adam Brandt, an assistant professor of energy resources engineering at Stanford University and the study’s lead author. “Atmospheric tests covering the entire country indicate emissions around 50% more than EPA estimates. And that’s a moderate estimate.”

This means that methane leaks from the natural gas system are likely to worse than previously thought.  Nevertheless, generating electricity by burning gas rather than coal still reduces the total greenhouse effect over 100 years, the new analysis shows. Burning coal releases enormous amounts of carbon dioxide.  Mining coal also releases a lot of methane.

More importantly, the majority of methane emitted from the natural gas system can be traced to a relatively small number of large leaks, which means the problem would likely not be terribly difficult to fix.

While natural gas is cleaner than coal when used for electric power, it is dirtier than diesel when used for transportation.

The new study concluded that powering trucks and buses with natural gas instead of diesel fuel probably accelerates global warming, because diesel engines are relatively clean. Natural gas will only be cleaner than diesel if the gas system is less leaky than the EPA’s current estimate, which the study suggests is unlikely.

“Fueling trucks and buses with natural gas may help local air quality and reduce oil imports, but it is not likely to reduce greenhouse gas emissions,” said Brandt. Even running passenger cars on natural gas instead of gasoline is probably on the borderline in terms of climate.”

The study, which will be published in the February 14, 2014 issue of the journal Science, was supported by the Cynthia and George Mitchell Foundation.

< Go Back