When factories belch smoke, everybody pays. Shouldn’t polluters be the ones to feel the sting instead? That’s the big idea behind carbon pricing: Add a levy so that emissions of greenhouse gases have a cost in line with their environmental damage. Using market forces should be the most efficient way to get companies to change their ways and to fight climate change. More countries are warming to the concept, but policy makers can’t agree on the best way to do it. Europe, parts of the U.S. and China use exchanges where companies buy and sell permits to pollute. There's vigorous debate about whether those markets work better than a simple carbon tax.