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RISING oil prices, believes Ali al-Naimi, Saudi Arabia's oil minister, may soon “take the wheels off an already derailed world economy”. On the face of things, this concern is absurd. The plunge of $115 in the price of oil from its peak last July to its nadir in December was the most precipitous the world has ever seen. Demand for oil is still falling, as the world economy atrophies. Rumours abound of traders hiring tankers to store their excess oil. Rich countries' stocks cover 62 days' consumption, the most since 1993 (see chart 1). The average over the past five years has been 52 days' worth.

Nor are oil firms pumping nearly as much as they could. OPEC has announced three separate rounds of production cuts since September in a bid to steady prices. In all, it has vowed to trim its output by 4.2m barrels a day (b/d). That leaves them with as much as 6m b/d of spare capacity. Despite this growing glut, however, the price of oil has been rising steadily in recent weeks (see chart 2). On Wednesday May 20th it closed above $60 a barrel for the first time in more than six months. That marks an increase of more than 75% since February. The price of futures contracts suggests that energy traders see the price rising higher still in the coming months and years. (During the day on Friday it appeared to be nearing $62 a barrel.)

The explanation is simple. Oilmen are worried because they believe that many of the factors behind the record-breaking ascent last year remain in place. Much of the world's “easy” oil has already been extracted, or is in the hands of nationalist governments that will not allow foreigners to exploit it. That leaves firms to hunt for new reserves in ever more inhospitable and inaccessible places, such as the deep waters off Africa or the frozen oceans of the Arctic. Such fields take a long time and a lot of expensive technology to develop. Worse, new discoveries tend to be smaller than in the past and to run dry faster.

So oil firms must work doubly hard to replace declining fields and to increase output. Yet the oil industry is short of equipment and manpower, thanks to underinvestment in the 1980s and 1990s, when prices were low. As soon as the world economy starts growing again, the theory runs, demand for oil will once again outstrip the industry's ability to supply it. In other words, the global recession has only interrupted the “supercycle” of which many analysts used to speak, during which the normal boom-and-bust cycle of oil and other commodities would give way to a protracted period of high prices, as ever-growing demand from emerging markets swallowed everything the extractive industries could produce.

Oil bosses, OPEC ministers and anxious bankers all agree on what is needed to prevent this scenario becoming reality: lavish investment in the development of new fields and in exploration. Yet the reverse is happening. The oil industry is cutting its spending, bringing fewer new fields into production and exploring less. The International Energy Agency reckons that overall investment will drop by 15-20% this year.

In theory, this should not be happening. Big Western oil firms (“majors” in the industry jargon) claim that they continue to invest steadily throughout the cycle, irrespective of gyrations in price. Big fields, they argue, can take a decade or more to develop, and may then produce oil or gas for several decades more. The price of oil at the time the investment is approved is irrelevant; the important thing is to make sure projects will be profitable across a range of possible future prices. If anything, given that most oilmen expect prices to rise in the medium term, you would expect them to be increasing their investment, to capitalise on the good times to come. Nonetheless, the extreme volatility of prices over the past year must have made big firms more cautious about future investments.

Then there are the state-owned firms in oil-soaked countries. These companies control the overwhelming majority of the world's oil. The better managed and funded of them plan to continue investing despite the downturn. Saudi Aramco, the world's biggest oil producer, recently completed a five-year scheme to expand its production capacity from 10m b/d to 12.5m b/d, at a cost of $70 billion. But in Russia, the world's second-biggest oil producer, output is falling largely because private capital has been scared off by a series of expropriations, while the state starves the firms it controls of sufficient cash for investment. And most oil-rich states, naturally enough, are happy to see the price rise. Many have become used to bumper revenues in recent years and have struggled to balance their budgets since the price slumped last year.

Falling costs within the industry will offset the impact of falling investment budgets to some extent. BP argues its slight cut in investment does not really represent a reduction, thanks to deflation. Yet many constraints on expansion remain. For one thing, the world still does not have as many experienced petroleum engineers and geologists as it needs, says Iain Manson of Korn/Ferry, a recruiting firm. He expects it to take a decade or more to overcome the shortage. Meanwhile, he says, wages in the oil industry are not falling by nearly as much as other costs.

Worse, there is little sign that governments are willing to grant oil companies easier access to the most promising territory for exploration. Iraq's plans to sign big new contracts with foreign firms are years behind schedule, as is its new oil law. American sanctions continue to impede investment in Iran. The Nigerian government has been unable to quell the insurgency in the Niger delta, making it difficult for oil firms to operate there. Even in America, despite years of debate, most coastal waters and much of Alaska remain off-limits to drilling.

So when demand begins to revive, a sharp rise in prices is inevitable. That does not mean that a price spike is just around the corner, however. The speed with which it arrives will depend on the strength of the global recovery. For the moment, global consumption of oil continues to fall, despite the slight brightening of the economic outlook. At the recent OPEC powwow Mr al-Naimi, the Saudi oil minister, argued that a low oil price always sowed the seeds of a future price rise, since it led to underinvestment. The only question this time is how quickly the strain will emerge.

A longer version of this article appears in the print edition of The Economist