Running out of oil: Is it only a matter of time?

Is the world running out of oil? The prospect seems unthinkable — mostly because the consequences, if true, would be unimaginable.

Permanent fuel shortages would tip the world into a generations-long economic depression. Millions would lose jobs. Farm tractors would be idled, triggering massive famines. Energy wars would flare. And carless suburbanites would trudge to their nearest big-box stores — not to buy Chinese-made clothing, but to scavenge glass and copper wire from abandoned buildings.

That may sound like the plot from a B-grade disaster flick. But with crude prices hitting record highs since 2004, global oil demand outstripping supplies like never before and major discoveries stagnant for 20 years, peak oil has migrated from the fringe to the center of the global energy debate.

"What peak-oil proponents focus on is discovered resources," said Scott Nauman, an executive with Exxon Mobil, which takes a skeptical stance on peak oil. "They overlook future discoveries and growth to unknown resources. There are very underexplored areas of the world ... where over 50 percent of the world's remaining resource is located."

But a diverse group of experts disagrees. Sources as sober as the U.S. Army Corps of Engineers and former Energy Secretary James Schlesinger have issued urgent wake-up calls about the economic, security and political repercussions awaiting the world as conventional crude supplies dwindle.

What "peakists" stress is that Earth's fuel gauge needn't droop near zero before shortages begin to roil the world economy. Output from petroleum reservoirs sags when they are only half-drained, geologists say. Several of the biggest fields appear to be sliding down that irreversible slope already.

Even skeptics don't deny the basic phenomenon. The controversy swirls around how much time we have left.

Optimists, including energy economists and oil giants such as Saudi Arabia, say the peak is at least decades away.

Pessimists, whose ranks are peppered with retired industry geologists, think the world will hit its peak within a few years, if it hasn't already.

Probably the most exhaustive study of peak oil was done in 2000 by the U.S. Geological Survey. A consensus of geologists concluded then — before China's roaring economy started soaking up the world's surplus oil — that the peak would come by 2037. Last year, the government moved its estimate to 2044.

Even that assessment is jolting. The fuel that powers our cars, our military, our technological way of life and our consumer culture likely will have to be replaced before today's preschoolers turn 40.