London — The perfect storm that has swept oil prices to $135 a barrel may subside over the coming months as rising crude supply from unexpected corners of the world finally comes on stream, just as the global economic downturn begins to bite.

The forces behind the meteoric price rise this spring are slowly receding. Nigeria has boosted output by 200,000 barrels a day (BPD) this month, making up most of the shortfall caused by rebel attacks on pipelines in April.

The Geneva consultancy PetroLogistics says Iraq has added 300,000 BPD to a total of 2.57 million as security is beefed up in the northern Kirkuk region.

“There is a strong rebound in supply,” the group’s president, Conrad Gerber, said.

Saudi Arabia is adding 300,000 BPD to the market in response to a personal plea from President Bush, and to placate angry Democrats on Capitol Hill — even though officials in the capital of Saudi Arabia, Riyadh, insists that there are abundant supplies for sale.

Like the rest of OPEC, the Saudis blame “speculators” for running amok, pushing paper contracts into the stratosphere.

The ever-diminishing reserves of oil in the earth’s crust will doubtless drive crude prices to much higher levels over time — provided no new technology such as nuclear fusion abruptly changes the picture — but that will not stop cyclical ups and downs along the way.

The world’s finely balanced market for crude has been creeping into surplus for several weeks. OPEC’s monthly report says that demand this quarter will average 85.75 million BPD. Supply was 86.8 million BPD in April. The fresh output from Nigeria, Iraq, and Saudi Arabia may push it significantly further into surplus.

The signs are already surfacing in global inventories. OPEC says that stocks held by the OECD club of rich countries are above their five-year average, with “comfortable” cover for 53 days of use. American stocks have edged up for the last four months, though they fell last week.

While it is widely reported that output from the non-OPEC trio of Norway, Britain, and Mexico has relentlessly fallen, it is less well known that a clutch of other countries are gradually filling the breach.

The U.S. Energy Information Agency says non-OPEC supply will edge up by 600,000 BPD over coming months as Brazil, Azerbaijan, and the Sudan raise production. By next year, America itself will be producing enough extra oil to shave its import needs.

None of this has been enough to curb the buying frenzy this spring. Goldman Sachs has warned that prices could reach $200 in a final spike, and even the bears at Lehman Brothers say there may be enough momentum to keep the boom going until Christmas.

It is unclear whether hedge funds and investors piling into futures contracts have now become the driving force in a speculative bubble. The Bank of England said Thursday that they were not a factor.

Lehman’s latest report — Is it a Bubble? — says commodity index funds have exploded from $70 billion to $235 billion since early 2006. This includes $90 billion of fresh money. Energy takes the lion’s share. Every $100 million flow of investment money into oil lifts crude prices by 1.6%, it said.

“We see many of the ingredients for a classic asset bubble,” Lehman’s oil expert, Edward Morse, said.

This week has seen a dramatic surge in oil contracts dated as far forward as 2016. Futures have moved higher than the spot price, a rare event known as “contango.” This can cut both ways: either as a sign of an impending supply crunch years hence; or that the futures market has become unhinged from reality.

What we know is that the International Monetary Fund has cut its forecast for world growth for 2008 three times since last autumn to 3.7%, and the United Nations is predicting just 1.8% — technically, a global recession. The major oil forecasters have halved their estimates for crude demand growth to 1.2 million BPD.

The bulls say that the housing crash in America and spreading contagion in Britain, Spain, and Japan do not matter much for oil in the changed world of rising Asia. America added just 7% of crude demand growth between 2004 and 2007, compared with 34% for China, 25% for the Middle East, and 17% for emerging Asia.

Goldman Sachs argues that fuel prices in most of these countries are held down by state controls, insulating demand from the effect of any global downturn.