Emissions of the greenhouse gas methane in the United States, due to human activity, were much greater than previously estimated according to a scientific report just published.
According to a new analysis by 15 climate scientists published in The Proceedings of the National Academy of Sciences the emissions were roughly 1.5 times greater in the US in the middle of the last decade than prevailing estimates.
The analysis also said that methane discharges in Texas and Oklahoma, where oil and gas production was concentrated at the time, were 2.7 times greater than conventional estimates.
Emissions from oil and gas activity alone could be five times greater than the prevailing estimate, the report said.
The study relies on nearly 12,700 measurements of atmospheric methane in 2007 and 2008.
Its conclusions are sharply at odds with the two most comprehensive estimates of methane emissions, by the Environmental Protection Agency and an alliance of the Netherlands and the European Commission.
The EPA had stated that all emissions of methane, from both man-made and natural sources, had been slowly but steadily declining since the mid-1990s.
In April, the agency reduced its estimate of methane discharges from 1990 through 2010 by eight to 12 per cent, largely citing sharp decreases in discharges from gas production and transmission, landfills and coalmines.
The new analysis calls that reduction into question, saying that two sources of methane emissions in particular, from oil and gas production and from cattle and other livestock, appear to have been markedly larger than the EPA estimated during 2007 and 2008.
One of the study’s principal authors, Dr Scot Miller of Harvard University’s department of earth and planetary sciences, said its higher estimates underscore methane’s significant contribution to rising temperatures.
“These are pretty substantial numbers we’re dealing with, and an important part of greenhouse gas emissions,” he said.
“Our study shows that there could be large greenhouse gas emissions in places in the country where we may not necessarily have accounted for them.”
Methane made up only about nine per cent of greenhouse gas emissions in 2011, the EPA said; carbon dioxide is easily the most prevalent gas.
However, methane is much more potent, even though it rapidly breaks down in the atmosphere, its contribution to global warming is 21 times greater than carbon dioxide’s over a 100-year period.
The E.P.A. and Europe’s Emissions Database for Global Atmospheric Research largely agree on how much methane is discharged annually in the United States.
At the most basic level, both arrive at estimates by assigning an average discharge to each category of methane emission, such as landfills, and multiplying the average by the number of sources in each category.
The latest analysis differs from those estimates because it relies on actual measurement of methane concentrations.
The data did not directly identify the sources of methane discharges.
The study concluded that livestock produced roughly twice as much methane during the reporting period as the European database estimated.
Most striking, the analysis reported that oil and gas operations in a north-south swath of Kansas, Oklahoma and Texas may have produced five times more methane, and, combining all sources of discharge, the three states may have been responsible for a quarter of all man-made methane discharges in the United States.
Mr. Miller cautioned that both estimates were subject to large margins of uncertainty; the methane from oil and gas activity could be as small as 2.3 times the European estimates, or as great as 7.5 times.
The reason, he said, is that the potential for inaccuracy rises as the area being surveyed or the category of emissions grows smaller.
That said, the study’s overall conclusion that methane emissions were 1.5 times EPA’s latest estimates is statistically accurate to within about five per cent, Dr Miller said.
