As world leaders convene at the United Nations this week for a climate change summit, the overarching question is how, exactly, to reduce global carbon dioxide emissions. (See "With UN Climate Summit Opening, Marchers Rallied Around the World.")

The United States, most experts believe, must play a key role in any meaningful global agreement on the issue. But the U.S. has dragged its feet on promoting a global deal for years, as Republicans—and some Democrats—argued that it could cripple the nation's economy. (Related: "Ahead of UN Climate Summit, Environmental Report Sees Economic Opportunities.")

One idea for reducing global emissions that's sure to surface at this week's summit: a carbon tax. The proposal has no chance of passing in the