In the United States, as in many countries, the single biggest source of electricity is also the dirtiest: Domestic coal-fired plants produced 1.45 billion metric tons of CO2 in 2012, according to Environmental Protection Agency data, equal to about 305 million cars. That’s more than 67 percent of the power sector’s total carbon emissions, even though coal accounts for just 37 percent of the energy mix. Put another way, coal-burning power plants in America alone exceed the total carbon emissions of Central and South America.

Limiting emissions could also have benefits beyond slowing climate change: A study released this week by public health researchers at Harvard and Syracuse universities found that controlling the amount of carbon released by power plants also cut down on emissions of toxic pollutants like sulfur dioxide and mercury, which contribute to asthma, heart disease, and other serious health impacts—not to mention that sulfur dioxide causes acid rain.

America’s carbon emissions have declined from their 2007 peak, and last fall the EPA issued rules for power plants that could prevent many (or any) new coal plants from being built. (For the time being, thanks to the fracking boom, cheap natural gas has made construction of new coal plants unlikely for economic reasons.) With the new rule, the EPA is looking to fill in the deepest part of America’s carbon footprint.

How big an effect will this have?

The rules require the power sector to cut emissions 30 percent from 2005 levels by 2030.

Does the EPA have legal authority to do this?

Yes. Based on the 2007 Supreme Court case Massachusetts v. EPA, the EPA has had the authority to treat greenhouse gases as dangerous pollutants, enabling it to use the Clean Air Act to place limits on them. More specifically, the 2011 Supreme Court case American Electric Power v. Connecticut found that the EPA has the authority to regulate carbon pollution from power plants.

That doesn’t mean legal challenges aren’t lining up already, from coal industry groups, the U.S. Chamber of Commerce, state Republican attorneys general, and others. “I’m ready to get it over with,” says Paul Bailey, vice president for federal affairs at the American Coalition for Clean Coal Electricity (ACCCE). “I want to see what this looks like, and whether it’s something we’ll have to push back hard on.”

The key legal question is whether the courts will validate the EPA’s approach of including “outside-the-fence” measures (see below) in its assessment of how much a state can be expected to cut emissions. Bailey says the Clean Air Act doesn’t provide for anything except regulations on the plants themselves, so the EPA can’t set a standard that requires states to do anything else. But environmentalists Doniger and Ceronsky argue that because the power system is, in fact, a system with many moving pieces, there’s no legal reason why regulators and utility companies should look just at power plants and ignore things like efficiency standards and the overall energy mix.