Emissions of the potent heat-trapping gas, methane, the main component of natural gas, are likely 50 percent higher than U.S. government has estimated in its official greenhouse gas inventory, says a new study that is the most comprehensive effort yet to assess the problem.

But the team of scientists, after reviewing more than 200 earlier studies spanning 20 years, concluded that methane leakage was not great enough to negate the climate benefits of switching from coal to natural gas as a fuel for electricity. (See related “Quiz: What You Don’t Know About Natural Gas.”) The study cautioned, however, that switching to natural gas as a vehicle fuel was not likely to help cut carbon dioxide emissions, given probable leakage rates. (See related, “For Natural Gas-Fueled Cars, A Long Road Looms Ahead,” The New Truck Stop: Filling Up With Natural Gas For the Long Haul,” “Trading Oil for Natural Gas in the Truck Lane.”)

The study, published Thursday in the journal Science, is the first effort to systematically assess and compare the findings of the existing published literature on North American methane emissions based on both “bottom-up” measurements of leaks from wells, pipelines, and other infrastructure, and “top-down” ambient air readings gathered from monitors in high towers and aircraft. We reported on one such top-down study in November: Natural Gas Reality Check: U.S. Methane Emissions May Exceed Estimates By 50.

No matter how the data is gathered, the team said, the literature consistently shows methane emissions higher than the estimates by the U.S. Environmental Protection Agency (EPA.)  EPA’s inventory is likely to lead to undersampling, the authors said, in part because of changes in the industry, the authors said. The EPA’s data sampling relies on facilities that are not representative of current industry methods and technologies. And hydraulic fracturing and horizontal drilling for natural gas and oil, or fracking, is now happening at a scale far greater than when the EPA developed its measurement methods in the 1990s. (See related, “Methane: Good Gas, Bad Gas.”)

In a prepared statement sent by email, the EPA said it had not yet had the opportunity to review the study in Science.

“EPA is committed to using the best available data for our Inventory and continually seeks opportunities to update and improve our estimates,” the agency statement said. “EPA is aware of methane studies that result in estimates of national methane emissions that differ from EPA’s estimates, and is interested in feedback on how information from such studies can be used to improve U.S. GHG Inventory estimates.”

The agency said studies like this one, along with data the agency is receiving through its relatively new greenhouse gas reporting system, will help EPA refine and update estimates in the future.

Graphic courtesy John Bellamy/Stanford University

Graphic courtesy John Bellamy/Stanford University

The authors said that fracking likely accounts only for a small portion of the excess methane emissions, about 1 teragram of the 14 teragrams (1 million metric tons of the 14 million metric tons) that current inventory methods are failing to capture. Natural gas production and processing, leaks from distribution systems, and abandoned oil and gas wells are all likely to be larger sources of fugitive emissions. (See related, “Natural Gas Stirs Hope and Fear in Pennsylvania,” and interactive, “Breaking Fuel From Rock.”)

“We conclude that the emissions that could be associated with hydraulic fracturing are significant, and important, but it cannot explain the larger gap that is occurring,” said Francis O’Sullivan, director of research and analysis for the MIT Energy Initiative, and one of the paper’s co-authors. He also noted that the EPA is enacting regulations to cut fugitive emissions from fracking. (See related blog post: “A Move to Capture ‘Fugitive’ Natural Gas Emissions.”)

When viewed on a 100-year basis, over which methane has 30 times the heat-trapping potential of carbon dioxide, use of natural gas over coal is still preferable and will cut greenhouse gas emissions, said Adam Brandt, an assistant professor of energy resources engineering at Stanford University, who was the study’s lead author.

But Brandt admitted in a pre-publication news briefing that natural gas’s climate benefits seem far less when viewed only in the short term. (That’s because methane’s heat-trapping potential is far higher when calculated over a shorter time frame–it is short-lived in the atmosphere compared to long-lasting carbon dioxide.)

But, Brandt added, “That does not then give us license to burn coal. [When we do so,] we are shifting a much larger burden to our grandchildren, effectively shifting the problem into the future.”(See related, “U.S. Energy-Related Carbon Emissions Fall to an 18-Year Low” and “Natural Gas Nation: EIA Sees Future Shaped By Fracking.”)

O’Sullivan added that natural gas is not a long-term solution to the climate crisis, because over the long term, the world needs to reduce greenhouse gas far more significantly. “But it can have significant benefits in the near term,” he said. “So how do we enact policy that ensures we use it wisely, and it is a useful tool, especially in the near term for coal substitution, knowing that this cannot be the solution for 100 years.”

In addition to Stanford and MIT, researchers from Harvard, four other universities, the U.S. National Oceanic and Atmospheric Administration, the National Renewable Energy Laboratory, Lawrence Berkeley National Laboratory, and the nonprofit organization, the Environmental Defense Fund participated in the study. It was funded through a grant by the George Mitchell Foundation, named after the late oil industry hydraulic fracturing pioneer, whose work in Texas led to the shale fracking revolution.(See related, “Forcing Gas Out of Rock With Water.”) An acknowledgement on the study said that Mitchell “believed this technology should be pursued in ecologically sound ways.”