Major U.S. cities may be leaking far more methane into the atmosphere than government estimates suggest.

New measurements found that up to twice as much gas is being released from six cities on the East Coast — Washington, D.C.; Philadelphia; Boston; New York; Providence, R.I.; and Baltimore — than estimates recorded by EPA.

Their combined methane emissions are higher than those at some of the nation's biggest natural gas production centers, including the Four Corners region and the Bakken Shale in the Dakotas.

"For context, it's quite a bit of methane for a small number of cities to be emitting," study co-author Eric Kort, a climate scientist at the University of Michigan, said in an interview with E&E News. "Does it drive the global budget? No. But is it an important component that potentially drives mitigation opportunities and is potentially important to consider when we consider the footprint of natural gas? Certainly."

While carbon dioxide remains the most significant greenhouse gas affecting the planet's climate, methane emissions are a growing concern among climate scientists. Methane persists for a shorter period of time in the atmosphere than CO2, but its climate-warming influence is much more potent while it lasts.

The new study isn't the first to suggest that the amount of methane released during the production of natural gas may be higher than previously thought. A striking analysis published in Science last year found that methane from the oil and gas supply chain could be up to 60% higher than estimates by EPA.

But what about cities? Emissions from urban centers have been an "under-examined part of the methane budget," the authors of the new study write.

Several studies have begun investigating the issue at a local level, looking at specific urban centers such as Boston or Washington. Yesterday's study builds on those efforts. The findingswere published last week in the journal Geophysical Research Letters.

The six cities are notable not only for their size — they account for 12% of the U.S. population — but also for their aging infrastructure, which the authors note may make them more prone to leaks in the natural gas distribution network.

The researchers, including scientists from the University of Michigan, Harvard University and NOAA, collected their data using aircraft equipped with sensors that measure gases in the atmosphere, including carbon dioxide, carbon monoxide, ethane and methane.

The measurements suggest that the cities' combined methane emissions come to about 8.9 teragrams — or 890,000 metric tons — each year. That's more than twice the estimate currently reported for those cities in EPA's most recent methane inventory.

Methane can be produced from a variety of sources. The researchers narrow down those possibilities by pointing to the presence of other gases typically associated with the distribution of natural gas, namely ethane. That would seem to indicate that the majority of the emissions is coming from the natural gas supply chain.

Exactly where in the distribution process the leaks are occurring remains a question, Kort notes.

"We can pinpoint pretty well ... that this much is coming from emissions that are coming out of the natural gas distribution system somewhere in this domain," he said. "But what we can't say very well is did that come out of pipelines that are sending things into different neighborhoods, or pipes into people's homes, or does it come out in people's homes?"

The researchers also can't say for sure whether these findings are likely to apply to other cities in other regions of the United States.

"It's probably likely that cities that have a similar profile — that is, similar age distribution profile, similar compositions, similar climates — probably behave similarly," Kort said. "But more measurements would certainly be needed. And newer cities might be expected to be less leaky because they have newer pipeline systems."

What it all means for U.S. methane emissions as a whole remains unclear. At least one paper, published in May in Geophysical Research Letters, has suggested that the nation's total methane emissions have remained fairly flat over the past decade, even if they're rising at oil and gas facilities (Climatewire, May 16).

Still, identifying new sources of methane leaks from any sector can provide opportunities to reduce the climate-warming emissions even further.

"This type of measurement and this type of knowledge really helps us understand U.S. emissions of methane, how we might attribute it, what the impacts are of using natural gas as a fuel source," Kort said. "That's where this really gives us information and provides context for mitigation policy."

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at