By Valerie Volcovici
WASHINGTON (Reuters) - Environmental regulators may have underestimated by 50 percent the amount of the greenhouse gas methane emitted in the United States, according to a study published on Monday by the Proceedings of the National Academy of Sciences.
The study, conducted by scientists at several institutions and led by researchers at Harvard University, found the discrepancy was greatest in south central United States, where total emissions are nearly five times greater than measurements by the U.S. Environmental Protection Agency and others.
The researchers said their findings also "cast doubt on the EPA's recent decision to downscale its estimate of national natural gas emissions by 25 to 30 percent."
The EPA is in the middle of setting federal limits on greenhouse gas emissions, and accurate measurements of methane - the second most prevalent greenhouse gas - are essential, the report's authors said. States will be asked to devise their own plans to carry out those rules.
The report said methane emissions are likely to be 1.5 times higher than EPA's estimate, and 1.7 times higher than that of the international Emissions Database for Global Atmospheric Research. Methane is produced in a variety of ways from gas escaping during oil and gas production to manure from livestock.
The scientists' approach differed from the EPA's because they measured what is in the atmosphere using meteorological data and statistical analysis. The government agency's method is based on estimated emissions per cow, for example, or per unit of coal or gas sold.
"Effective national and state greenhouse gas reduction strategies may be difficult to develop without appropriate estimates of methane emissions from these source sectors," the report said.
The EPA said Monday it is reviewing the PNAS study and appreciates the fact that new data is available to the public.
"EPA is committed to using the best available data for our inventory and continually seeks opportunities to update and improve our estimates," the agency said in a statement.
"Research studies like these will add to our knowledge base of greenhouse gas emissions and will help us refine our estimates going forward."
(Reporting by Valerie Volcovici; editing by Ros Krasny and Jackie Frank)