It's one of the biggest, and greenest, selling points for natural-gas drilling in the Marcellus Shale: the idea that the gas, as the Interstate Natural Gas Association of America puts it, "can play a significant role in helping to reduce [greenhouse-gas] emissions when used in place of other fossil fuels."
But Cornell University professor Robert W. Howarth is challenging that argument. In a preliminary report released Nov. 15, he wrote, "[N]atural gas is not better than coal and may in fact be worse than coal in terms of its greenhouse-gas footprint [over] the next several decades."
Some of his premises, however, are unconventional.
Most discussions of greenhouse gas involve CO2, the most common climate-change pollutant. And when burned, natural gas does release 25 percent less CO2 than diesel, and just half as much as coal. But Howarth, a professor of ecology and environmental biology, focuses on another greenhouse gas: methane. It's the principal component of natural gas, and it can trap even more heat than an equal amount of CO2.
Extracting, processing and transporting natural gas involves "fugitive emissions" of methane. These include intentional venting of gas at wellheads and unintentional leaks along the nation's 306,000 miles of gas pipeline. While definitive numbers are elusive, fugitive emissions from natural gas comprise an estimated 20 percent of all human-caused U.S. methane emissions. (Other sources include landfills, livestock and coal-mining.)
Howarth wanted to compare the greenhouse-gas footprint of natural gas from hydraulic-fracturing wells (commonly used in Marcellus Shale) to that of diesel and coal. For each fuel, he and his team added three numbers: the CO2 emissions from burning the fuel; the "indirect" emissions from energy burned to recover and process it; and fugitive methane.
Their paper has been submitted for peer review at a science journal. But last month, Howarth released a tentative finding: Even his low-ball estimate of fugitive emissions makes natural gas worse than diesel and nearly as bad as coal. Howarth also estimated that fugitive emissions could account for up to 45 percent of the greenhouse impact from natural gas.
If correct, such conclusions would undermine arguments for natural gas as a "bridge fuel" between our fossil-driven economy and a renewable-energy future.
But some question Howarth's findings -- and not just industry groups like Energy In Depth, which in April assailed an earlier version of Howarth's release as "fact-free" and "slanted scholarship," while noting that he's been an outspoken opponent of shale-gas drilling in New York state.
One criticism involves how Howarth rated methane's potency as a greenhouse gas. Over the 100-year time period scientists use to measure environmental impact, methane traps 21 times as much heat as carbon dioxide. Howarth used a much larger factor of 105, reflecting methane's 20-year heat-trapping potential as calculated in a 2009 paper in the journal Science.
Methane's atmospheric lifespan is actually shorter than 20 years (while CO2 hangs around for centuries). Howarth, however, says that 20-year span more accurately reflects the damage methane does, because its global-warming impact would be most acute even as we're using it as a bridge fuel, and just when reducing emissions is most crucial. But even some who agree that gas-drilling is risky question Howarth's methods.
Using a 20-year number is "a bit unconventional," says Mike Griffin, head of Carnegie Mellon University's Green Design Center.
Another issue is the extent of leakage. Regulators have long assumed that 1.5 percent of natural gas is either deliberately vented or leaks accidentally. Howarth assumes a much higher leakage rate of 3.5 percent.
New evidence suggests all those estimates may be too low. An October report from the federal Government Accountability Office, for instance, states that more than 4 percent of methane is lost to intentional venting and flaring of gas alone. And in November, the Environmental Protection Agency estimated that 2006 methane emissions from the petroleum and natural-gas industry were actually 57 percent higher than previously thought -- and that venting from some operations at "unconventional" wells (like those used in shale-gas drilling) was thousands of times higher than earlier estimates.
Howarth believes future research will bolster his claims. "This is really an area that deserves a lot more attention than it's getting," he says.