The proposal would help reshape where Americans get electricity, away from coal and toward cleaner sources of energy. It's a key step in President Barack Obama's plans to address climate change.
The proposed rule eventually would force the government to limit emissions from existing power plants, which account for a third of all US greenhouse gas emissions. Obama has given the Environmental Protection Agency until next summer to propose those regulations.
That's because to meet the standard, new coal-fired power plants would need to install expensive technology to capture carbon dioxide and bury it underground. No coal-fired power plant has done that yet, in large part because of the cost.
Coal, which is already struggling to compete with cheap natural gas, accounts for 40 percent of U.S. Electricity, a share that was already shrinking. And natural gas would need no additional pollution controls to comply.
The EPA already has issued rules aimed at curbing global warming pollution from automobiles and the largest industrial sources.