"It takes 15 years to roll over the vehicle stock," Kopp said.
Another approach is by switching to ethanol and other plant-based biofuels (photo gallery: Powering the Future).
In the U.S. ethanol is made by fermenting corn in industrial plants. (Brazil makes ethanol from sugar cane.) (See "Ethanol More Energy Efficient Than Thought, Study Says.")
Ethanol is already a billion-dollar industry in the U.S., says Surya Prakash, a chemistry professor at the University of Southern California in Los Angeles.
The U.S. ethanol industry produces about 4 billion gallons (15 billion liters) a year. But in terms of energy output, those 4 billion gallons equal only 2.5 billion gallons (9.5 billion liters) of gasoline, Prakash says.
That's because ethanol generates less power per gallon than gas.
"It's a drop in the bucket," Prakash says. "It can hardly cover three or four days' usage" of gasoline in the U.S.
Other biofuels may hold greater promise.
These include products made from the switchgrass and wood chips mentioned in President Bush's address.
Prakash estimates that the different forms of biofuel can together probably replace 10 to 15 percent of total U.S. gasoline usageenough to meet the president's goal of reducing oil imports from the Mideast by 75 percent.
Daniel Kammen, also in the ERG at U.C., Berkeley, is more optimistic.
Currently, he says, enough waste biomass is being generated by lumbering, by farming, and as urban waste to meet 10 percent of U.S. transportation needs.
With a major commitment, Kammen thinks it might be possible to replace all of the nation's oil with biofuels.
Even with major improvements in fuel economy, however, that would require putting an additional 250 million acres (100 million hectaresan area two and a half times as large as California) into agricultural production.
"It's doable," Kammen says, "but it would take some rough choices."
In the State of the Union address Bush also said, "We will increase our research in pollution-free cars that run on hydrogen." (See "Hydrogen-Fueled Race Car Showcases Future Technologies.")
Most people don't realize that hydrogen takes energy to produce, USC's Prakash says.
"You can't make a hole in the ground and pump hydrogen," Prakash said. "You have to make it."
Furthermore, he said, "storage and distribution are a nightmare."
The problem is that hydrogen is an extremely permeable gas. If placed in current gas tanks and pipelines, it would seep through the walls and be lost.
Prakash estimates that a hydrogen economy would require two trillion to three trillion U.S. dollars in new pipelines. (See "New Process Could Help Make Hydrogen Fuel Affordable.")
"All of these things look nice to the layman," Prakash said. "But there are lots of pitfalls."
Other energy sources receiving attention include coal liquefaction (the chemical conversion of coal into synthetic liquid fuel) and tar sands, which can be processed into synthetic oil. However, these have the drawback that, like oil, they contribute to global warming.
Many experts prefer nuclear power, though critics point to safety and waste-storage issues.
In the long run we may see entirely new energy technologies.
U.C.'s Kammen thinks it's possible to add extra batteries to today's gasoline-electric hybrid cars to create plug-in hybrids that can be recharged at home, the office, and the mall.
Such cars might be able to go 200 to 300 miles (320 to 480 kilometers) on a gallon (3.8 liters) of gas, he says.
Already, an organization called CalCars is at work on this project.
"It's a cool technology," U.C.'s Farrell says.
Kammen is also a fan of wind power and rooftop solar generators.
"There are parts of northern Germany that already get a quarter of their electricity from wind. In a good month they get 50 percent," Kammen said.
Another prospect is methanol, which can be made from natural gas. And, unlike hydrogen, it's easy to store and transport.
In the future green electrical power could be used to make methanol from recycled carbon dioxide, Prakash says.
Such a technology would not only provide fuel but also ease global warming, since carbon dioxide in the atmosphere is known to trap heat.
To a lot of folks, that would be the ultimate win-win solution.
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