{mosads}The biggest drop in 2012 came from declining use of coal, a fuel facing fierce competition from low natural gas prices, according to the EIA, which is the Energy Department’s independent statistical arm.

“Lower natural gas prices resulted in reduced levels of coal generation, and increased natural gas generation — a less carbon-intensive fuel for power generation, which shifted power generation from the most carbon-intensive fossil fuel (coal) to the least carbon-intensive fossil fuel (natural gas),” said the agency.

But there’s considerable concern that leakage of the potent greenhouse gas methane from natural-gas wells and transport are sapping the fuel’s advantage from a climate standpoint.

The World Resources Institute, a prominent environmental think tank, explores the topic in a major new paper found here.

Beyond the coal-to-gas trend, the EIA said that other factors also helped lower U.S. carbon emissions in 2012, including “decreased demand for transportation fuels and mild winter temperatures that reduced demand for heating.”