Natural gas and oil production is the second-biggest source of U.S. greenhouse gases, the government said, emboldening environmentalists who say tighter measures are needed to curb the emissions from hydraulic fracturing.

In its second-annual accounting of emissions that cause global warming from stationary sources, the U.S. Environmental Protection Agency for the first time included oil and natural- gas production. Emissions from drilling, including fracking, and leaks from transmission pipes totaled 225 million metric tons of carbon-dioxide equivalents during 2011, second only to power plants, which emitted about 10 times that amount.

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