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 will likely 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.
"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," lead author Adam Brandt, of Stanford said in releasing the findings. "Even running passenger cars on natural gas instead of gasoline is probably on the borderline in terms of climate."
Brandt says they used greenhouse-gas calculations from a 2012 study, which looked at past practices to assess the impact of vehicle fuels. He says future improvements in natural gas usage could alter its impact on climate.
Despite its leakiness, natural gas is still better for the climate long-term than coal as a way to generate electricity, according to the review of 200-plus earlier studies that appears in Friday's edition of the journal Science.
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." The burning of natural gas emits less carbon than oil or coal, but scientists say methane emissions are offsetting some its benefits.
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 of methane typically released per cow or per unit of coal or natural gas sold. 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 the EPA estimates, says the study, which also note 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 agency says it's aware of studies that show emissions are higher than EPA estimates, adding this research will "help us refine our estimates going forward."