Photo by NRK P3.

Way back in his 2011 State of the Union speech, President Obama introduced a new idea, a “Clean Energy Standard” to boost America’s use of low-carbon electricity. As a piece of legislation it went nowhere, of course, because … Congress. But as a conceptual sketch of the future of energy, it had an interesting feature: It included everything except coal.

Why? Because any metric of “clean” rules out coal. You can loosen the definition to include natural gas (half as much CO2 as coal, a small fraction of the conventional pollutants) and nuclear. But there is no comparative standard of cleanliness that will accommodate coal. It is the benchmark of filthiness.