Wendy Koch

USA TODAY

Natural gas vehicles may be worse for the climate than diesel-fueled ones

The natural gas system leaks methane%2C a potent heat-trapping greenhouse gas

Leaks may explain why sudies find higher methane emissions than EPA estimates

Natural gas is widely hailed as cleaner than other fossil fuels, but new research says using it — instead of diesel — to power trucks and buses could actually exacerbate global warming over a 100-year period.

Diesel engines are relatively fuel-efficient while the natural gas infrastructure leaks more heat-trapping methane than federal or industry data suggest, says a study Thursday by 16 scientists from federal laboratories and seven universities including Stanford, Harvard and MIT. (

"There's lots of reasons to shift from diesel," says lead author Adam Brandt of Stanford, adding diesel buses are "stinky" and natural gas ones may help cut oil imports and improve local air quality. But from a climate perspective, "it's not likely to reduce greenhouse gas emissions."

Burning natural gas in vehicles emits less carbon dioxide than burning diesel, but the drilling and production of natural gas leaks methane, a potent greenhouse gas. Those leaks offset some of natural gas' CO2 benefit.

"Even running passenger cars on natural gas instead of gasoline is probably on the borderline in terms of climate," he says. His research, a review of 200-plus studies over 20 years , appears in Friday's edition of the journal Science.

An industry group disagrees. Richard Kolodziej, president of Natural Gas Vehicles for America, says a 2007 report by the California Energy Commission calculated that on a well-to-wheel basis — which includes extraction and distribution — that natural gas in vehicles emits 22% fewer greenhouse gases than diesel and 29% fewer than conventional gasoline.

Brandt says his analysis is based on calculations from a peer-reviewed 2012 study led by Ramon Alvarez of the Environmental Defense Fund, a private research group. Alvarez' team said switching buses and trucks from diesel to natural gas might not help the climate over a 100-year period if 1.7% or more of methane is leaked in producting and using natural gas. The Enviromental Protection Agency says 1.5% is leaked, but Brandt says EPA's estimate is about 50% too low.

Still, despite its leakiness, Brandt says natural gas is better for the climate long-term than coal as a way to generate electricity.

The production of natural gas is booming in the United States, and President Obama welcomed it in his 2014 State of the Union Address as "bridge fuel that can power our economy with less of the carbon pollution that causes climate change." He urged Congress to support the construction of natural gas fueling stations for cars and trucks.

Natural gas consists mostly of methane, a greenhouse gas that doesn't linger in the atmosphere nearly as long as carbon dioxide but traps about 30 times more heat while it does. So even small methane leaks, whether from pipelines under city streets or a power plant, add up.

The study says the U.S. Environmental Protection Agency underestimates methane emissions largely because of the way it tallies them. The agency takes a "bottom-up" approach in which it calculates emissions based on the amount released per cow or facility. It does not include emissions from abandoned oil and gas wells or natural sources such as wetlands

In contrast, "top-down" counts taken from airplanes or towers measure actual methane in the air. They suggest U.S. methane emissions are 25% to 75% higher than EPA estimates, says the study, which also notes the limits of these atmospheric counts.

"It's not clear where these emissions are coming from," Brandt told reporters. While scientists don't know exactly how much is due to hydraulic fracturing or fracking — the drilling method largely responsible for the natural gas boom — he said it's probably a small share of total emissions.

The study says methane leakage in the gas industry may also have been underestimated because emission rates for wells and processing plants were based on voluntary participation. One EPA study asked 30 gas companies to cooperate, but only six allowed the agency on site.

"It's impossible to take direct measurements of emissions from sources without site access," said co-author Garvin Heath, senior scientist with the National Renewable Energy Laboratory.

The authors call on the natural gas industry to clean up its leaks. Fortunately for gas companies, they say a few leaks probably account for much of the problem so repairs are doable. An earlier study of about 75,000 components at processing plants found just 50 faulty ones were behind nearly 60% of the leaked gas.

Eric Pooley, senior vice president of the Environmental Defense Fund, a research and advocacy group, says gas companies are moving in the right direction. He says they're being prodded by an upcoming EPA rule — effective January 2015 — that requires methane be captured when liquids are being removed after drilling.

"EPA has not yet had the opportunity to review the upcoming Science study on methane emissions," the agency said in a statement. The EPA says it's aware of studies that show emissions are higher than its estimates, adding this research will "help us refine our estimates going forward."