Matt Yglesias detailed what the EPA is capable of doing following the Supreme Court's ruling in Massachusetts v. EPA -- a ruling that allowed the federal regulation of carbon emissions.
The EPA dipped its toes in the water of actually regulating CO2 emissions with a "new source" rule last year. The terms of the rule are nuanced, but essentially make it unrealistic to build any new coal-fired power plants in the United States. That's been a source of anger to the coal industry, but realistically it doesn't make much difference. Ultra-cheap natural gas has made new coal uneconomical one way or another. What the new source rule does is offer a backstop—even if for some reason gas prices spike, new coal is still dead. But what about existing coal-fired power plants? That's a tough one. The EPA could promulgate a strict rule and start shutting them down, but it'd be a very hard lift politically that would risk having congress partially repeal the Clean Air Act and strip the agency of its regulatory authority.
The Natural Resources Defense Council is out with a new proposal to do existing source regulation in a more viable way, namely by regulating states' average power plant fleet emissions rather than regulating on a plant-by-plant basis. Grist's David Roberts has a great explanation of how this works that those interested in the details should check out. One salient point is that much like a carbon tax this creates a big short-term incentive for utilities to invest in energy efficiency, which is generally the lowest hanging fruit for de-carbonization. The other is that (unlike with a pure carbon pricing system) regulation would take states' existing emissions levels as the baseline meaning, in Roberts' words, "No state will be unfairly penalized for having a carbon-intensive fleet today."
Of course we're racing headlong towards what Yglesias cleverly called "the climate cliff" -- the 2 degree Celsius temperature increase that's the zero barrier between terrible and apocalyptic global warming -- and very little can be done to stop it. But slowing down the inevitable could help in terms of preparedness, I suppose.