In a world where fossil fuel provides more than 80 percent of energy, what would it take to go completely green? Could the world switch over to power from only the wind, sun, waves, and heat from the Earth in only a few decades?

The question seems a fanciful one, when world leaders are stymied over proposals for far less dramatic cuts in the carbon dioxide emissions from global burning of coal, oil, and natural gas. But two U.S. researchers, a transportation expert and an atmospheric scientist, decided the time had come to apply blue-sky thinking to one of the world's greatest challenges.

"We wanted to show that wind, water, and solar power are available to meet demand, indefinitely," says study co-author