US emissions of methane -- a potent greenhouse gas -- could be significantly higher than indicated in estimates by the US Environmental Protection Agency, according to a new study published Monday.

The study found the EPA numbers could underestimate by as much as 50 percent the true amount of the gas being produced by the United States.

The most striking discrepancy, the researchers said, was in the oil-producing south-central United States, where their results were nearly three times higher than EPA estimates.

"It will be important to resolve that discrepancy in order to fully understand the impact of these industries on methane emissions," said lead author Scot Miller, a doctoral student at Harvard University.

Methane is produced by livestock, landfills, coal mining, and natural gas production and distribution, among other natural and man-made activities, the authors explained, adding that humans are thought to contribute around 60 percent of the total.

The researchers explained their figures differ from the government ones because of a difference in methodology.

The EPA, they explained, uses a "bottom-up" approach that multiplies amounts typically released, for example, by each cow, per unit of coal, or per unit of natural gas sold.

But in this new study, researchers took the opposite "top-down" approach, calculating how much methane is actually present in the atmosphere and then tracing it to its sources using meteorological and statistical analysis.

"When we measure methane gas at the atmospheric level, we're seeing the cumulative effect of emissions that are happening at the surface across a very large region," said Steven Wofsy, a Harvard professor and co-author of the PNAS study.

"That includes the sources that were part of the bottom-up inventories, but maybe also things they didn't think to measure," he explained.

For the analysis, the researchers used observational data from 2007-2008, when the US sharply increased its natural gas production, and compared it with the EPA figures from the same period.

They intend to repeat the analysis using present-day data.

"Now that we know the total does not equal the sum of the parts, that means that either some of those parts are not what we thought they were, or there are some parts that are simply missing from the inventories," said co-author Anna Michalak of the Carnegie Institution for Science.

"It really offers an opportunity for governments to reexamine the inventories in light of what we now know."

Methane is the second-most important greenhouse gas after carbon dioxide, emphasized the researchers, who also hailed from the University of Michigan, NOAA's Earth System Research Laboratory, Lawrence Berkeley National Laboratory, Atmospheric and Environmental Research, the European Commission Joint Research Centre in Italy, and the University of Colorado Boulder.

It traps 70 times more heat than CO2 in the atmosphere, but it only lasts 10 years in the atmosphere, compared to 100 years for carbon dioxide.

The study was published in the Proceedings of the National Academy of Sciences.