Developing cleaner power sources for transportation is perhaps the trickiest piece of the energy puzzle. The difficulty stems from two discouraging facts. First, the number of vehicles worldwide, now 750 million, is expected to triple by 2050, thanks largely to the expanding buying power of customers in China, India and other rapidly developing countries. And second, 97 percent of transportation fuel currently comes from crude oil.

In the near term, improving fuel economy is the best way to slow the rise in oil use and greenhouse gas emissions from cars and trucks. But even if automakers triple the efficiency of their fleets and governments support mass transit and smart-growth strategies that lessen the public's reliance on cars, the explosive growth in the number of vehicles around the world will severely limit any reductions in oil consumption and carbon dioxide emissions. To make deeper cuts, the transportation sector needs to switch to low-carbon, nonpetroleum fuels. Liquid fuels derived from woody plants or synthesized from tar sands or coal may play important roles. Over the long term, however, the most feasible ways to power vehicles with high efficiency and zero emissions are through connections to the electric grid or the use of hydrogen as a transportation fuel.