Those emissions could drop even more in the future, as low natural gas prices, expanded renewable electricity generation and an abnormally warm winter last year curbed coal-fired generation.

On top of that, the EPA has proposed the first-ever emissions standards for new coal-fired power plants. Environmental groups hope the agency will soon roll out similar rules for existing plants.

"Greenhouse gases are the primary driver of climate change, which can lead to hotter, longer heat waves that threaten the health of the sick, poor or elderly; increases in ground-level ozone pollution linked to asthma and other respiratory illnesses; as well as other threats to the health and welfare of Americans," EPA said in a Tuesday statement.

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The EPA data comes from the Greenhouse Gas Reporting Program, which measured emissions from 41 different sources.

EPA released its first report from the program last year, when it considered 2010 emissions from 29 sources. Emissions from those sources fell 3 percent in 2011.

Petroleum and natural gas systems were the second greatest emitters, clocking in at 225 million tons of carbon dioxide equivalent emissions. Refineries ranked third, at 182 million tons.

According to the 2011 data released Tuesday, some of the nation’s most coal-dependent states were among the top emitters. Those states included Illinois, Pennsylvania, Ohio and Kentucky.

Lawmakers from those states, many of them Republicans, worry that EPA’s emissions regulations for coal-fired power plants might raise power prices for their constituents. They also contend the agency’s rules might shutter too much coal generation to meet power demand.

House Republicans held a series of votes last year to end EPA's ability to regulate greenhouse gas emissions and block air toxics rules, among other efforts. None gained traction in the Senate.

Rep. Ed Whitfield Wayne (Ed) Edward WhitfieldBottom Line Why Republicans took aim at an ethics watchdog What Azerbaijan wants from Israel? MORE (R-Ky.), the chairman of the House Energy and Commerce Subcommittee on Energy and Power, has said his subcommittee would continue to focus on the proposed power plant rules and their effects this Congress.