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 new research.
The report, released on Thursday by researchers at seven major universities, reviewed some 200 studies from the past two decades and found that the Environmental Protection Agency (EPA) is missing leakage from various sources in North America, including faulty pipelines, wetlands, natural seepage from the earth's crust and other sources.
But a single culprit was not easily found, and not enough evidence existed 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 per cent 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 researchers focused on studies 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 per cent higher than EPA estimates, and our uncertainty range is 25 to 75 per cent," said lead author Adam Brandt, assistant professor of energy-resources engineering at Stanford University.
"This amounts to something like seven to 21 million tonnes of methane per year, with a central estimate of 14 million tonnes."
The article appears in the journal Science.