Which has done more in the last five years to cut America's greenhouse-gas emissions and slow the pace of global warming?
1) Development of renewable energy such as wind and solar.
2) Increasing the fuel efficiency of vehicles, especially through hybrid technology.
3) Fracking.
You see where this is headed.
The equation of national energy use can be tough to parse. As energy consultant Geoffrey Styles says, it has many "moving parts."
But you can make a reasonable argument that fracking, or hydraulic fracturing -- the recently developed process of springing gas and oil from previously impenetrable rock formations -- has done more than any other policy or technology in the last few years (except, perhaps, for our disastrous recession) to slow the production of greenhouse gases.
How? By producing abundant, cheap natural gas. That gas has replaced a lot of coal in generating electricity, while releasing only half the carbon dioxide.