It will take decades before anything other than liquid fuels powers a significant portion of the nearly one billion cars on the road. Still, cars will rely more and more on electricity. Hybrids, which accounted for only about 2 percent of U.S. sales of light-duty vehicles in 2007, could account for 40 percent by 2030. Then there are plug-in hybrids, which are just starting to be sold and which could account for 2 percent of sales by 2030. Unlike conventional hybrids, which derive all their power from gasoline-­powered internal-­combustion engines, plug-in hybrids have batteries that can be charged from the electrical grid, ideally using nighttime excess generating capacity. They can go the distance of an average commute on this energy alone, using an electric motor; an onboard gasoline engine kicks in for longer trips. Since some of the energy for propelling the car comes from power plants, overall greenhouse-gas emissions depend on the fuel those power plants use. Assuming typical driving patterns, a plug-in hybrid with a 20-mile electric range will generate about 325 grams of carbon dioxide emissions per mile if the electricity comes from a coal-powered plant (a conventional vehicle emits about 450 grams per mile). If the electricity comes from wind power, the hybrid will generate 150 grams per mile.

The high cost of batteries will initially slow the adoption of hybrids and all-electric vehicles (see "Scaling Up Is Hard to Do"). Lithium-ion batteries that can provide a 40-mile range currently cost more than $16,000, according to an estimate by Carnegie Mellon University. But technological improvements and mass production could reduce this price by 75 percent or more. Meanwhile, researchers are exploring different chemistries, such as lithium-air batteries. These technologies could store 10 times as much energy as conventional lithium-ion batteries, extending range and potentially lowering costs.