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The actual levels of U.S. emissions of methane could exceed estimates from the U.S. Environmental Protection Agency (EPA) by 50 percent, according to a new study published in the Proceedings of the National Academy of Sciences.

Methane is one of the most powerful causes of the greenhouse effect and is a major contributor to climate change. U.S. emissions of the gas are most concentrated in three states — Texas, Kansas and Oklahoma. The study revealed that much of the excess gas comes from livestock, from refining operations and from leaks from oil and gas drilling wells.

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For their research, scientists estimated emissions in 2007 and 2008 based on almost 13,000 measurements taken on ground level, in telecommunication towers and from the air to compile a comprehensive database of methane emissions for analysis. Results showed that the United States dramatically underestimates methane releases from certain industries, including agriculture and oil and gas. Data for 2012 is yet to be analyzed but it is likely to add the impact from expanding hydraulic fracturing operations.

An analysis of the data revealed that in 2008 U.S. emissions of methane amounted to 49 million tons. This compares to the 32 million tons that the EPA estimated were released into the air in the same year. The figure is also much higher than the European Commission estimate of 29 million tons, the study noted.