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  • Dick writes about Earth and planetary science for Science magazine.
 

Natural Gas for Coal a Good Climate Trade-Off Despite Leaks, Researchers Argue

13 February 2014 2:00 pm
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Emitters. Pads for natural gas wells in the Utah’s Uintah Basin.

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Emitters. Pads for natural gas wells in the Utah’s Uintah Basin.

A new analysis of more than 200 studies of how much natural gas escapes into the atmosphere to drive global warming finds that more gas is leaking than the U.S. Environmental Protection Agency (EPA) had been estimating. But the new review also finds that disturbingly high leak rates publicized lately—such as from Utah gas fields—cannot be typical. That suggests substituting gas for coal in power generation still offers a “bridge” to lower greenhouse gas emissions in the future, the authors argue.

Natural gas has a split personality. Burned to produce electricity, it yields less of the greenhouse gas carbon dioxide than does coal. But natural gas, which is almost all methane, is itself a powerful greenhouse gas that escapes into the atmosphere from leaks all along the production and distribution system, from the well where it is produced to the home furnace, gas stove, or power plant where it is burned.

Plenty of researchers have tried to quantify methane leakage. Since the 1990s, EPA has made an annual “bottom-up” estimate for the United States. The agency estimates how fast gas is leaking from each sort of device in the system—well, valve, compressor, and such—and multiplies that flux by the number of such devices in the system. But studies that measure methane in the atmosphere—where actual emissions are mixing together—tended to come up with higher leak rates.

So Novim, a nonprofit organization housed in the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, pulled together one of its expert panels to see what could be said about methane leakage from the published literature. The review study was funded through a grant from the Cynthia and George Mitchell Foundation; the late George Mitchell was the father of hydrofracturing, or fracking, for natural gas. The 16-member panel’s results are published in Science today in a peer-reviewed Policy Forum article accompanied by about 75 pages of supplementary materials.

The panel’s aggregation of studies shows that bottom-up leakage estimates like EPA’s are consistently smaller than those derived from atmospheric sampling of methane on a regional scale. Those regional estimates are, in turn, smaller than atmospheric estimates based on sampling on a national scale. And the panel, led by environmental scientist Adam Brandt of Stanford University in Palo Alto, California, finds the national atmospheric estimates to be the most reliable. Bottom-up estimates can mislead, the authors write, because their sampling is too spotty to catch the few “superemitters”—the broken valve or poorly maintained compressor—that appear to account for much of total methane emission. The panel concludes that actual methane emissions are 1.25 to 1.75 times the EPA estimate.

The one-in-a-thousand superemitter “does seem to be the problem,” says atmospheric chemist Paul Wennberg of the California Institute of Technology in Pasadena, who was not involved in the Science study. And “it’s economically and environmentally straightforward to address that.”

To sort out where, broadly speaking, the largest emissions of methane might be coming from, the panel set up a “thought experiment” in which plausible ranges of emissions were developed for natural gas production and distribution and compared with the panel’s national estimate. Fracking for shale gas—a target of environmentalists lately—seems to be a minor source. The largest source would be natural gas production and processing. In contrast, gas distribution—including leaks like those recently reported around Washington, D.C.—is a lesser source. And the recently reported high emissions from gas fields in Utah do not jibe with this broader view. “It just looks like that’s not common,” Brandt says. “It doesn’t make sense that that’s typical.”

Methane emissions may be higher than the “official” estimate, but the panel concludes that their assessment “still supports robust climate benefits from [natural gas] substitution for coal in the power sector.” The same cannot be said for substituting natural gas for petroleum in motor vehicles, however. Switching gas for gasoline in light-duty vehicles offers some benefit. But if gas is substituted for diesel fuel in buses and trucks, “the climate benefit will be small or negative,” Brandt says, though the proven health benefits of reduced soot emissions from diesel remain.

As the Policy Forum format allows, the panel closes with recommendations: “If natural gas is to be a ‘bridge’ to a more sustainable energy future, it is a bridge that must be traversed carefully.” Fixing leaks is already profitable, it notes, but scientists and engineers must develop practical methods for identifying and fixing superemitters. And bottom-up measuring methods still need to be improved, it warns.

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