For the past century most of the advances in transportation and industry have relied on energy from petroleum and other fossil fuels, but some experts warn that the peak of oil production worldwide may be reached soon and that the economic and political consequences could be enormous.

Is the world running out of oil? In the debate over that question between energy experts, environmentalists and government policy makers it often boils down to the old question about whether the glass is half full or half empty. The question is tricky because it involves not just the amount of oil potentially available beneath the earth's surface, but the difficulties, dangers and the expense of getting it out.

Paul Roberts is author of a new book: The End of Oil: On the Edge of a Perilous New World. He says that most of the easily obtainable oil, the cheap oil, has already been discovered and that it will become more and more difficult to meet world demand.

"That cheap oil is not gone completely by any means, but it is getting harder to find," he said. "You are seeing signs of it all over the place with companies like Shell Oil having to restate their reserves. Basically, they are not discovering oil as fast as they need to."

Mr. Roberts says that a peak in worldwide oil production could come within the next few decades. Making matters worse, demand for energy is increasing, driven by emerging nations like China and India. Paul Roberts says this will make the world ever more dependent on politically unstable nations that have plenty of oil.

"All of these countries have pretty severe political problems and any one of them could, at any time, erupt in some sort of civil chaos that could make production and exporting tough," he added. "We have seen that repeatedly. We saw it in Venezuela a year-and-a-half ago and we are seeing it right now in Nigeria. All it takes is for one of these countries to go off-line in any significant way and we would be in real trouble. If we think $40 a barrel is expensive now, try $80 to $100."

Mr. Roberts says the only way to avoid possible catastrophe is to begin to work toward a new energy future that would reduce the demand for oil. While many industry analysts agree that oil is a finite resource that could be subject to political dislocations, not everyone thinks the world is close to running out.

Dallas-based H. Sterling Burnett, a senior fellow at the private National Center for Policy Analysis, says there is still plenty of oil.

"Unless politics intervenes, the age of oil is just beginning," he said. "Currently, we have in reserves 3 trillion barrels of oil. If we did not discover one more drop, if we did not come up with new technology to extract more from the reserves we know of, we would still have enough oil for the next 56 years at the current rate of consumption with a 1.4 percent annual growth rate."

Mr. Burnett goes further, saying that new technologies could make it possible to extract oil from sites that were previously considered too challenging or expensive.

"The estimate is that we have about 14 trillion barrels of oil in shale oil and tar sands," he added. "Now, that is enough to fuel us for the next 500 years."

Of course, the only way for it to be cost-effective to recover such oil would be if the cost of crude went even higher. That would also make many alternative energy technologies more attractive to investors and consumers.

Mr. Burnett says the market will sort these things out, but author Paul Roberts suggests the market may need some help. He recommends government backing of alternative energy research. He says the government role should be to fund research and then let the market determine which technology offers the best alternative to oil.

"What has made oil such a great and powerful part of our energy economy is what we call its power density. You pack a lot of energy in a relatively small volume of fluid," he explained. "Finding something that does that cleanly, that replaces oil and gives the same bang for the energy buck, in a way that does not affect climate and does not require us to go over to the Middle East, that is a technological challenge that has not been met yet."

Mr. Roberts and Mr. Burnett agree that conservation could help keep down demand for oil and that the energy future may consist of a number of new technologies, perhaps working in tandem with a system in which petroleum is still used, but at much reduced levels. Already there are promising experimental programs utilizing not only the wind and the sun, but such things as used vegetable oil from restaurants and animal manure from large farms to produce fuel. Such projects offer the possibility of producing energy for the future that would also greatly reduce pollution and the carbon gases that many scientists believe contribute to global warming.