The Environmental Protection Agency has proposed the first limits on greenhouse-gas emissions from U.S. power plants, the largest source of carbon dioxide, a pollutant linked to climate change.

The rules would permit emissions from new power plants at 1,000 pounds of carbon dioxide per megawatt hour, about the level for a modern natural-gas plant, the EPA said Tuesday in an e-mailed statement. The limit would effectively preclude construction of new coal-fired plants.

"We're taking a common-sense step to reduce pollution in our air, protect the planet for our children, and move us into a new era of American energy," Lisa Jackson, EPA administrator, said in a statement.

The standard is the first of its kind issued by the EPA for carbon-dioxide emissions. With the failure of Congress to cut carbon emissions, environmental groups see agency actions as the best chance to combat man-made climate change.

"This is a milestone in the fight to rein in climate change," Joe Mendelson, climate policy director for the National Wildlife Federation, said in a statement. "The EPA is taking a big step toward protecting the world our children will inherit."

The rule would apply only to new plants, and the administration said it would exempt plants that are proposed to be built within the next year.

The initial impact would be minimal as utilities are closing, not building, coal plants because natural gas prices are at 10-year lows. The share of coal in electricity generation dropped below 40 percent by the end of 2011, the first time it had been that low since 1978, according to the U.S. Energy Information Agency in Washington.

The EPA said the rule would not add costs for power providers because no coal plants are likely to be built in the next two decades unless the facilities have carbon-capture technology. Industry groups say that technology is not ready for commercial use.

It "is neither economically viable nor commercially available," Scott Segal, a lobbyist representing power providers such as Southern Co., wrote in a letter to the White House on March 12 in an attempt to head off the regulation.

The average U.S. coal plant emits 2,249 pounds of carbon dioxide for each megawatt hour of power produced, compared with 1,135 pounds for a natural gas plant, according to the EPA.

"If old King Coal isn't dead already, he's certainly teetering toward life support," said Frank O'Donnell, president of Clean Air Watch in Washington.