The logic is pretty straightforward. Carbon dioxide emissions are threatening the planet. In the U.S., coal plants are the second-largest source of those emissions, after transportation. Therefore, the Environmental Protection Agency should impose emissions limits on coal-fired plants.

The EPA is trying to do just that; it plans to issue rules by next June that would limit carbon dioxide emissions from existing power plants. As part of that process, it has just finished an 11-city “listening tour,” seeking public comment for how best to approach its task.

If the Washington leg of that tour was any indication, the undertaking also became a platform for the EPA’s opponents to challenge the idea that rules should be issued at all. Senate Minority Leader Mitch McConnell descended on the panel last week, complete with nine-member entourage, to ask the agency to end its “war on coal.”

It’s worth considering the various incarnations of that argument. If the EPA is to achieve its mission -- the greatest possible emissions reduction at the least possible cost -- it will have to lay them to rest:

-- Carbon emissions should be addressed through legislation, not regulations issued under the Clean Air Act.

There’s no disputing that a 40-year-old law that wasn’t written with carbon dioxide in mind is an imperfect vehicle for addressing greenhouse gases. But public policy, like politics, is the art of the possible, and Republicans have made it clear that they have no interest in passing climate-change legislation. For the moment, it’s regulation or nothing.

-- Regulations are unnecessary because market forces are already reducing the share of power produced by coal.

Again, no disputing that the share of U.S. power generated by coal has fallen, to 37 percent last year from 45 percent in 2000, thanks largely to the falling price of natural gas. That’s not a reason to curtail efforts to make coal plants less polluting. Besides, energy prices can change quickly, and they will almost certainly shift again.

-- The technology doesn’t exist yet to dramatically reduce emissions from existing coal plants.

The EPA’s critics are right to argue against imposing the same tough standards on existing plants that it has proposed for new plants. But that doesn’t mean states can’t find a way to rebalance their power systems in a way that reduces total carbon emissions. That could entail a combination of upgrading some coal plants, changing the mix of fuel for others, and replacing some with natural gas plants, renewable energy sources and greater efficiency measures.

-- The industry needs more time to address climate change.

The EPA can deal with this concern by phasing in these regulations over time. It’s not a reason to refrain from issuing regulations altogether.

-- Imposing carbon limits on U.S. coal plants will cost people their jobs.

There’s no getting around this, and the EPA would have done itself some good by holding listening sessions in Wyoming, Kentucky or West Virginia -- the three biggest coal-producing states. Coal mining employed about 87,000 people in the U.S. last year, supporting entire communities in some places. As the demand for coal dries up, so will those jobs.

But protecting those jobs must be balanced against putting the planet, and ourselves, at risk. Arguing over whether the EPA should cap emissions from existing plants is counterproductive. The focus should be on how best to make those caps work.

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