A review of 20 years of technical literature on natural gas emissions in the United States and Canada found that methane (CH4) emissions appear significantly larger than official estimates, but they are not enough to negate the benefits of switching from coal power generation to natural gas-fired power.

The review was conducted by a group of 16 researchers whose affiliations include National Renewable Energy Laboratory, National Oceanic and Atmospheric Administration, Environmental Defense Fund and a half-dozen universities.

"People who go out and actually measure methane pretty consistently find more emissions than we expect," said Adam Brandt, an assistant professor of energy resources engineering at Stanford University and the lead author of the analysis, on the Stanford website. "Atmospheric tests covering the entire country indicate emissions around 50% more than EPA [Environmental Protection Agency] estimates."

Even so, replacing coal generation with gas-fired generation is an environmental improvement, the researchers concluded.

"We find measurements at all scales show that official inventories consistently underestimate actual CH4 emissions, with the NG [natural gas] and oil sectors as important contributors," the researchers said in a study published in the journal Science. Research suggests that a small number of "superemitters" could be responsible for a large fraction of methane leakage, they said.

However, the study also concluded that "recent regional atmospheric studies with very high emissions rates are unlikely to be representative of typical NG system leakage rates," and assessments using 100-year impact indicators "show system-wide leakage is unlikely to be large enough to negate climate benefits of coal-to-NG substitution."

The researchers said emissions estimates that have not aligned with the amount of CH4 observed in the atmosphere may be the result of sampling from devices that are not representative of current technologies and practices, and limited sample sizes.

"Data should improve with increased reporting requirements enacted by EPA," they said.

According to EPA, methane accounts for an estimated 9% of U.S. greenhouse gas emissions (see Daily GPI, Dec. 16, 2013). The Energy Department’s Energy Information Administration estimates that less than 1.5% of the total volume of produced natural gas is released as it travels from the wellhead to homes and businesses.

A study conducted last year by researchers at the Carnegie Institution for Science and others at Harvard University found that human activity created methane emissions about 1.5 times greater than the government had estimated (see Shale Daily, Nov. 27, 2013).