The Obama administration will press ahead on Friday with tough requirements for new coal-fired power plants, moving to impose for the first time strict limits on the pollution blamed for global warming.
The proposal would help reshape where Americans get electricity, away from a coal-dependent past into a future fired by cleaner sources of energy.
It's also a key step in President Barack Obama's global warming plans, because it would help end what he called "the limitless dumping of carbon pollution" from power plants.
Although the proposed rule won't immediatedly affect plants already operating, it eventually would force the government to limit emissions from the existing power plant fleet, which accounts for a third of all US greenhouse gas emissions.
Obama has given the Environmental Protection Agency until next northern summer to propose those regulations.
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The official announcement is expected on Friday. The public will have a chance to comment on the rule before it becomes final.
Despite some tweaks, the rule packs the same punch as one announced last year, which was widely criticised by industry and Republicans as effectively banning any new coal projects in the US.
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, already struggling to compete with cheap natural gas, accounts for 40 per cent of US electricity, a share that was already shrinking. And natural gas would need no additional pollution controls to comply.
The regulations have been in the works since 2011 and stem from a 1970 law passed by Congress to control air pollution. In 2007, the Supreme Court ruled that that law, the Clean Air Act, could be applied to heat-trapping pollution.
The EPA already has issued rules aimed at curbing global warming pollution from cars and the largest industrial sources.
