U.S. carbon-dioxide emissions have fallen dramatically in recent years, in large part because the country is making more electricity with natural gas instead of coal.

Energy-related emissions of carbon dioxide, the greenhouse gas that is widely believed to contribute to global warming, have fallen 12% between 2005 and 2012 and are at their lowest level since 1994, according to a recent estimate by the Energy Information Administration, the statistical arm of the U.S. Energy Department.

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