when will the world start to run out of oil?

The moment they were concerned with is known as "peak oil", the point at which production stops increasing and goes into inexorable decline. Some commentators believe that moment may be as little as two years away, others that peak production is so far off that it is pointless to try to put a time on it.

All are agreed, though, that when peak oil does happen, its impact on the world economy . . . and the lifestyles so many of us take for granted . . .will be profound. Chris Skrebowski, editor of the Energy Institute's Petroleum Review, believes peak oil will occur in 2008, at which point the world will move into "a land without maps where we are all likely to be poorer".

Ninety percent of world transport is oil-dependent; petrochemicals are produced from oil; 99% of food supply requires oil either to grow or to reach the market; and 95% of lubricants are oil-based.

And oil is not easily replaceable. There are no realistic alternatives to oil for fuelling aircraft and ships, producing petrochemicals or powering cars without massive investments in technology such as hydrogen.

The fact that world oil consumption has doubled from 42m barrels in 1970 to 84m today poses a stark challenge.

At present rates of depletion, 5m barrels a day of new production needs to be brought onstream for the next 10 years to keep world output rising.

The peak oil debate tends to divide into two camps. On the one hand there are geologists who argue, based on past production and discovery rates and field exhaustion, that it is almost upon us. On the other, there are economists, political scientists and oil majors who believe that oil producers will always find a way to meet demand, whether through clever ways of finding and extracting oil or greater fiscal incentives to discover and produce more.

Both strands of opinion were in evidence at last week's conference in London, organised by the Dutch investment bank Insinger de Beaufort.

Skrebowski says that the world's big five oil majors all produced less in 2005 than in 2004, and that North Sea oil production is declining so rapidly that it will halve by 2013.

The evidence is irrefutable, according to the University of Reading's Roger Bentley, secretary of the Association for the Study of Peak Oil & Gas, who says 64 of the world's 100 or so oil-producing countries are already past peak production.

The downward trend is clear, he says . . . although there may be a "mini-glut" as output is stepped up from Russia, the Caspian and Iraq, and new sources come onstream, such as deepwater oil and oilsands.

Bentley believes that nonOpec production will reach a peak within the next 30 months and global output will start to decline between 2010 and 2020 at the latest, depending on non-conventional sources such as oilsands.

"Alongside global warming, this is one of the two extraordinary challenges facing mankind, " he says. "The numbers may slip a little, but the fundamental underlying direction does not change."

Jeremy Leggett, an oil industry geologist turned environmental campaigner turned chief executive of a solar energy firm, says that peak oil will occur some time this decade.

Not only will that produce "horrible economic pain" as oil prices rise to choke off demand, Leggett argues, but it will precipitate environmental disaster as oil consuming countries switch to coal and hasten global warming.

"The shortfall between current expectations of oil supply and availability will be such that neither gas, nor renewables, nor liquids from gas and coal, nor nuclear, nor any combination thereof will be able to plug the gap in time to head off economic trauma, " he warns.

Despite the ingenuity of the oil industry in extracting oil from ever more hostile environments, Leggett says, it is a quarter of a century since the world discovered more oil in one year than it produced. In 2000 there were 16 discoveries of giant fields containing 500m barrels or more: in 2003 there were none.

Not all those who attended the conference are sold on the idea of peak oil. Energy consultant Mike Lynch, an adviser to the US government, says that the study of peak oil is not a science and there is a lot of naivete, ignorance and manipulation of the data among proponents.

"It is true that oil is finite, " Lynch says, " but, since 1989, people have repeatedly predicted the peak too soon and have had to keep on increasing their estimate of reserves."

Lynch is one of the few pundits who forecasts that oil prices will begin to ease. Yet, as even he jokes: "I have predicted nine of the last two price decreases."