With the excitement over renewable energy, it might be reasonable to assume that fossil fuels such as coal, oil, and natural gas will go the way of the steam engine in the next 20 years.

Not so fast, says Daniel Yergin, author and one of the most influential voices in the world of energy.

"There is always the possibility that something big will happen very quickly, but probably not," Mr. Yergin said.

"On a worldwide basis, about 80 percent of energy today is oil, gas, and coal. You say, what's it going to be in 2030? Most studies say somewhere about 75 percent of the bigger pot."

Mr. Yergin, the chairman of IHS Cambridge Energy Research Associates in Washington, D.C., spends a lot of time thinking about energy. He won the Pulitzer Prize in 1992 for a history of the oil industry. He is promoting his latest book, "The Quest," a comprehensive examination of energy.

So after taking five years to write an 800-page book that explores everything from wind turbines to electric vehicles, is Mr. Yergin convinced we're stuck with greenhouse-gas-producing fuels?

"I'm convinced there will be major changes," he said. "But given how massive the energy system is, how complex it is, things just don't happen overnight."

Existing energy systems contain an enormous amount of embedded capital. New technologies have long lead times. Automobile fleets take a decade to turn over. And world energy demand is expected to grow 35 to 40 percent by 2030.

Wind turbines, after decades of development, are only now cost-competitive, he said. Photovoltaic cells, first used in spacecraft in 1958, still require subsidies.