How much methane escapes? (Image: Eddie Seal/Bloomberg via Getty Images)

Emissions of methane, a powerful greenhouse gas, are 1.5 to 1.7 times higher in the US than current estimates. The findings are the latest in a series of contentious studies attempting to determine the climate impact of the nation’s recent boom in fossil fuel production.

The switch from burning dirty coal to cleaner natural gas should have cut greenhouse gas emissions in the US. But the latest evidence suggests this effect may be outweighed by the amount of gas leaking into the atmosphere during fossil fuel production.

To work out how much methane the US was emitting, Scot Miller of Harvard University and colleagues took a “top-down” approach – measuring how much methane was in the atmosphere and then deducing its source. They did this by using data from research flights that monitor the chemical make-up of the air, as well as taking daily readings of methane concentrations from the tops of 10 telecommunication towers across the country.


“If you are measuring methane at the top of these towers you are really seeing an amassed signal from across a broad region,” says Miller. “You can really get a good sense of emissions nationwide.”

Gas, gas everywhere

The team made nearly 13,000 measurements between 2007 and 2008. They then plugged the data into atmospheric models that factor in things like wind speed and direction, to figure out where the emissions came from.

The approach differs from frequently used “bottom-up” estimates that look at individual sources such as cows, landfills and waste water treatment plants, and then extrapolate total emissions nationwide.

Emissions in the south central US, which includes Texas, Oklahoma and Kansas, were 2.7 times higher than recorded in existing emissions inventories. The higher methane levels correlated with elevated concentrations of propane, a gas that is only emitted during fossil fuel extraction and refining. Methane emissions from the region’s fossil fuel production alone were nearly five times higher than prior estimates.

Emissions cuts?

The top-down approach the authors used is better than existing bottom-up estimates, says Britton Stephens of the National Center for Atmospheric Research in Boulder, Colorado, who was not involved in the study. “The atmosphere is this great integrator that records the sum of all emissions,” he says. “The great thing about it is it doesn’t lie, it doesn’t make mistakes.”

According to the US Environmental Protection Agency, the US’s greenhouse gas emissions have fallen over the last few years. There are many reasons, but the switch from coal to natural gas is thought to have contributed, since natural gas releases less carbon dioxide than coal for every unit of energy. But Stephens says the methane leaks may cancel out this benefit.

“The bad news is the reductions in greenhouse gas emissions that have been attributed to the conversion of coal to natural gas are either much less than hoped for or nonexistent,” says Stephens.

“The good news is that, through more attention to monitoring and fixing leaks, this is a relatively easy problem to fix,” he adds.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1314392110