The natural gas system in the United States is leaking far more methane, a harmful greenhouse gas, into the air than official estimates say, according to research released Thursday.

The report by researchers at seven major universities reviewed some 200 studies from the past two decades and found that the Environmental Protection Agency is missing leakage from various sources in North America, including faulty pipelines, wetlands, natural seepage from the Earth's crust and other sources.

However, a single culprit was not easily found, and not enough evidence exists to point the finger solely at the booming hydraulic fracturing industry, experts said.

Methane is a potent source of pollution, some 30 times more harmful than carbon dioxide, and the latest study adds to a suite of recent reports about excess leakage that have raised worldwide concerns about how natural gas, a fuel touted for its benefits over coal, may be speeding up the process of global warming.

The EPA greenhouse gas annual inventory estimates about 1.5 percent of methane from natural gas leaks out of the system before it is burned.

The EPA focuses on human-caused emissions and gets its figures by multiplying the amount of methane thought to be emitted by a particular source -- whether from cattle farms or natural gas processing -- by the number of that source type in a given area.

The studies reviewed by the research team focused on those with original measurements that compared their results to official methods.

"Our best guess as to the degree of underestimation suggests that methane emissions are something like 50 percent higher than EPA estimates, and our uncertainty range is 25 to 75 percent," said lead author Adam Brandt, assistant professor of energy resources engineering at Stanford University.

"This amounts to something like seven to 21 million tons of methane per year, with a central estimate of 14 million tons."

- Fracking not to blame -

While the fracking boom has raised concerns about the impact on the environment, Brandt told reporters the current findings do not suggest that leaks from operations to extract gas from beneath deep shale beds are a major source of methane in the air.

The evidence suggests that fracking is leaking more than official counts indicate, but that it may be contributing just around one million of the total 14 million extra tons of methane leakage, he added.

"High emissions were observed before the recent hydraulic fracturing boom," said Brandt.

"It doesn't appear to be the main contributor. The math just doesn't work out."

Given the high uncertainty associated with the precise sources of the leaks -- whether from natural gas operations, coal, farming, landfills, or other sources -- scientists were unable to say how the problem should be fixed.

However, the study authors said leakage must be reduced in the future so that natural gas can be the "clean" fuel it is touted as being.

"Over the long term, one has to realize that the gas of course is still a greenhouse gas emitting fuel source," said Doug Arent of the National Renewable Energy Laboratory and the Joint Institute for Strategic Energy Analysis.

In the meantime, even the concern about excess leaks is not enough to outweigh the benefits of moving toward natural gas as a fuel source and away from coal over the next century.

"Shifting from coal to natural gas is going to still be significantly beneficial," said Brandt.

The article appears in the journal Science.

The research was funded by a grant from the Cynthia and George Mitchell Foundation.