Guide to alternative fuels

Experts warn that it won't be easy to get off of petroleum or reduce how much the world uses. The amount of energy per liter derived from petroleum is far better than most of the alternatives, a worldwide infrastructure based on it already exists, and people tend to be lazy.

If oil drops below $55 a barrel, most of these alternatives stop becoming attractive, says Dan Arvizu, director of the National Renewable Energy Laboratory. And the Organization of the Petroleum Exporting Countries is watching what is going on in alternative fuels and can gauge oil prices accordingly.

"We do have a problem with how serious we are about our energy challenges," he said.

Global warming and improving technology, however, are making the alternatives more realistic all the time. Here's a guide to the main alternatives.