Coal is here to stay, but efforts to cut emissions are ambitious, expensive, and have largely stumbled

America runs on coal. It's cheap, plentiful (at least for another 100 years or so), and comfortingly domestic. Two hundred years ago, it powered the industrial revolution. Today, it spits out nearly half of the country's electricity.

Coal's problems, however, are getting to be so big and serious that they are not just overshadowing the industry but threatening to render it obsolete. About 80 percent of the electricity sector's carbon dioxide emissions come from burning coal. A price on CO2 pollution, which Congress might impose as early as this year, is expected to be so costly that the mere prospect of it is already shaking things up. Some states have banned new coal plants, and many companies are canceling their plans in other places.

The industry's greatest hope for survival, as far as CO2 emissions go, is a work-in-progress technological arsenal known as carbon capture and storage, or CCS. With all the makings—and risk—of a classic American gamble, it is in some ways the energy equivalent of missile defense. It's ambitious, expensive, intricate, and wildly controversial.

Cue the requisite political theatrics in Washington and, behind them, more serious questions about the promise of "clean coal" versus the reality. Last fall, at a campaign rally, then Sen. Barack Obama said, "Clean coal technology is something that can make America energy independent." The coal industry itself is spending millions of dollars on ads—placed in newspapers, pasted to public buses, and shellacked across subway station entryways—that boast of its commitment to "clean coal technology." Environmentalists have aggressively volleyed back. Clean coal, they say, doesn't exist.

It would be helpful if everyone were using the same definition. The term "clean coal," though alliteratively pleasing, is far from straightforward. Besides CO2, coal plants emit mercury, particulate matter, nitrogen oxides, and sulfur dioxides. What doesn't go into the air often ends up in the ground as fly ash, a sludgelike material that became big news last year when a retaining wall at an ash dump in Tennessee suddenly gave way, releasing thousands of pounds of waste into people's front yards. So, it's not just CO2 that's problematic. In fact, in the early 1990s, clean coal referred almost exclusively to efforts aimed at curbing nitrogen and sulfur pollution. Today, clean coal has morphed to mean coal from a plant that doesn't emit CO2. And it doesn't exist yet.