E.P.A. Sets Greenhouse Emission Limits on New Power Plants In the United States, the electric power sector produces 40 percent of the nation's heat-trapping carbon dioxide emissions, the bulk of them generated by coal-fired plants. Still, the agency emphasized that the proposed rules would apply only to future construction, not to existing plants or others for which permits have already been granted. The declining price of natural gas has made it the fuel of choice for companies planning new plants, and the latest gas-fired generation on the drawing boards is expected to easily meet the new standards without adding new controls. The challenge will be far greater and possibly prohibitive for new coal plants, whose emissions are dirtier but will have to meet the same standard. The new rule sets a limit of 1,000 pounds of carbon dioxide emissions per megawatt-hour of electricity produced. That would make it impossible for new coal plants to be approved without the addition of equipment that traps carbon dioxide before it leaves the smokestack and then stores it underground. But, in an apparent concession to companies still planning to build coal plants, the E.P.A. said it would allow new plants to begin operating with higher levels of emissions as long as the average annual emissions over a period of 30 years met the standard. In a statement, the E.P.A. administrator, Lisa P. Jackson, called the proposed rules "a common-sense step to reduce pollution in the air, protect the planet for our children and move us into a new era of American energy." Environmental groups generally applauded the standards, although some expressed disappointment with the agency's refusal, for the moment, to regulate existing coal plants. Senator James M. Inhofe, Republican of Oklahoma, and other conservatives threatened to block the rules in Congress, and coal industry representatives denounced them as an attack on a widely used fuel that makes electricity more affordable. Frances Beinecke, president of the Natural Resources Defense Council, offered measured praise for the new rules while urging the agency to go further. "This Environmental Protection Agency action means any new coal plants built in America must use modern, state-of-the-art carbon pollution controls," she said in a statement "The logical next step is to improve the aging fleet of existing coal-fired power plants, which remain the major source of industrial carbon pollution in our country." In a conference call with reporters, Ms. Jackson said that if such action was to be taken in the future, the agency would thoroughly consult with the industry and all others affected. The proposed regulations arose from a 2009 finding by the E.P.A. that carbon dioxide is a pollutant threatening human health and therefore must be regulated under the Clean Air Act. The agency acted in accordance with a Supreme Court directive that the agency decide whether carbon dioxide was a pollutant or not. At Peabody Energy, the largest coal mining company in the United States, Vic Svec, a spokesman, questioned the legality of the standard, arguing that the E.P.A. was supposed to set standards based on existing technology and that carbon capture technology was not ready. A standard of 1,000 pounds per megawatt-hour for coal plants would "require something that doesn't exist as a commercial technology," he said. Steve Miller, chief executive of the lobbying group the American Coalition for Clean Coal Electricity, said, "This latest rule will make it impossible to build any new coal-fueled power plants, and could cause the premature closure of many more coal-fueled power plants operating today. "So far, other E.P.A. regulations are responsible for the announced closure of more than 140 electricity generating units in 19 states," he said in a statement. The regulation the E.P.A. proposed today could raise the number of closures even higher and put more workers out of jobs. Michael Brune, executive director of the Sierra Club, said in an interview that ending coal-fired generation as it now exists was the point. "It's a rule that follows the marketplace," he said, adding: "Right now, next to no coal plants are being built. This basically means that new coal plants are going extinct. As for coal plants built in the 1970s or earlier, he said, "we can put them in our rear-view mirror." An earlier version of this article misstated the new Environmental Protection Agency limit for carbon dioxide emissions from new energy plants. It is 1,000 pounds of carbon dioxide emissions per megawatt-hour of electricity produced, not 1,000 tons.