Could the petroleum joyride -- cheap, abundant oil that has sent the global economy whizzing along with the pedal to the metal and the AC blasting for decades -- be coming to an end?

Some observers of the oil industry think so. They predict that this year, maybe next -- almost certainly by the end of the decade -- the world's oil production, having grown exuberantly for more than a century, will peak and begin to decline.

And then it really will be all downhill. The price of oil will increase drastically. Major oil-consuming countries will experience crippling inflation, unemployment and economic instability. Princeton University geologist Kenneth S. Deffeyes predicts "a permanent state of oil shortage."

According to these experts, it will take a decade or more before conservation measures and new technologies can bridge the gap between supply and demand, and even then the situation will be touch-and-go.

None of this will affect vacation plans this summer -- Americans can expect another season of beach weekends and road trips to Graceland relatively unimpeded by the cost of getting there. Though gas prices are up, they are expected to remain below $2.50 a gallon. Accounting for inflation, that's pretty comparable to what motorists paid for most of the 20th century. It only feels expensive because gasoline was unusually cheap between 1986 and 2003.

And there are many who doubt the doomsday scenario will ever come true. Most oil industry analysts think production will continue growing for at least another 30 years. By then, substitute energy sources will be available to ease the transition into a post-petroleum age.

"This is just silly," said Michael Lynch, president of Strategic Energy and Economic Research in Winchester, Mass. "It's not like industrial civilization is going to come crashing down."

Where you stand on "peak oil," as parties to the debate call it, depends on which forces you consider dominant in controlling the oil markets. People who consider economic forces most important believe that prices are high right now mostly because of increased demand from China and other rapidly growing economies. But eventually, high prices should encourage consumers to use less and producers to pump more.

But Deffeyes and many other geologists counter that when it comes to oil, Mother Nature trumps Adam Smith. The way they see it, Saudi Arabia, Russia, Norway and other major producers are already pumping as fast as they can. The only way to increase production capacity is to discover more oil. Yet with a few exceptions, there just isn't much left out there to be discovered.

"The economists all think that if you show up at the cashier's cage with enough currency, God will put more oil in the ground," Deffeyes said.

There will be warning signs before global oil production peaks, the bearers of bad news contend. Prices will rise dramatically and become increasingly volatile. With little or no excess production capacity, minor supply disruptions will send the oil markets into a tizzy. So will periodic admissions by oil companies and petroleum-rich nations that they have been overestimating their reserves.

Oil producers will grow flush with cash. And because the price of oil ultimately affects the cost of just about everything else in the economy, inflation will rear its ugly head.

Anybody who has been paying close attention to the news lately may feel a bit queasy at this stage. Could $5-a-gallon gas be right around the corner?

"The world has never seen anything like this before and so we just really don't know," said Robert L. Hirsch, an energy analyst at Science Applications International Corp., a Santa Monica, Calif., consulting firm.

Still, he added, "there's a number of really competent professionals that are very pessimistic." The pessimism stems from a legendary episode in the history of petroleum geology. Back in 1956, a geologist named M. King Hubbert predicted that U.S. oil production would peak in 1970.

His superiors at Shell Oil were aghast. They even tried to persuade Hubbert not to speak publicly about his work. His peers, accustomed to decades of making impressive oil discoveries, were skeptical.

But Hubbert was right. U.S. oil production did peak in 1970, and it has declined steadily ever since. Even impressive discoveries such as Alaska's Prudhoe Bay, with 13 billion barrels in recoverable reserves, haven't been able to reverse that trend.

A few years ago, geologists began applying Hubbert's methods to the entire world's oil production. Their analyses indicated that global oil production would peak some time during the first decade of the 21st century.

Deffeyes thinks the peak will be in late 2005 or early 2006. Houston investment banker Matthew Simmons puts it at 2007 to 2009. California Institute of Technology physicist David Goodstein, whose book "The End of Oil" was published last year, predicts it will arrive before 2010.

The exact date doesn't really matter, said Hirsch, because he believes it's already too late. In an analysis he did for the U.S. Department of Energy in February, Hirsch concluded that it will take more than a decade for the U.S. economy to adapt to declining oil production.