Estimates of how much oil the world has left vary, as do scenarios for phasing out its use. One thing is certain: as a fossil fuel, oil is a finite resource and its end will come, sooner or later, planned for or not.

How much oil does the world have left, and how should it be used? These are currently matters of great debate, with some experts predicting that new discovery and extraction technologies will continue this current age of abundant oil and others foreseeing global shortages and rising prices in the near future, barring the discovery of massive (or many) new fields. Complicating the debate are political and environmental considerations, such as the question of the climatic effects of burning every last drop of oil on earth, even if it could be extracted. Governments may grapple in coming decades with the question of where to focus effort and resources: on discovering and extracting more oil, on developing strategies for ending oil dependence and transitioning to other energy sources before a supply crisis forces the issue, or on some mix of both.

According to the US Energy Information Administration (EIA), there are approximately 2.9 trillion barrels of oil in world reserves, with 1.28 trillion barrels of that oil in reserves currently labelled as 'proven" (that is, economically recoverable with current technology). More conservative estimates put the theoretical, non-proven worldwide supply at 2 trillion barrels. But oil reserve estimates are notoriously unreliable and are impacted by many factors, such as nations overstating their supply for political gain or understating their supply due to lack of knowledge, historically reliable fields showing precipitous drops in output as they age, and the effectiveness and economic feasibility of future discovery and extraction methods being currently unknowable. On the demand side, EIA estimates current world oil consumption at around 85 million barrels per day, rising to about 119 million barrels per day in 2025 (a 1.9% annual increase). Given a continued 1.9% annual growth in oil consumption, the world will have exhausted its 'proven' oil reserves by 2037. With current technology, 'total reserves' will be depleted anywhere between 2047 and 2060.

Meanwhile, other energy experts expect oil prices to gradually recede from the price spikes seen in 2004-05. Supply, they say, will continue to rise with demand. Saudi Aramco, for example, predicts output of 12.5 million barrels per day, up from 9.5 million barrels today, on the way to 15 million barrels per day of sustained output. Optimists also point to previous claims of oil's imminent end -- most recently, in the 1980s -- as evidence that we will overcome any potential peak with new technology and oil-rich geographies, as has happened in the past. Still, given the rapid industrialisation of China and other Asian countries, even optimists predict that the increased demand for oil will likely create a new floor for cheap oil (in the $30s a barrel instead of the upper $20s), and that oil may show a gradual long-term rise in price with continued short-term volatility.

Synthetic oil made from tar shales, which become more feasible to mine as the price of oil rises, will help mitigate the withdrawal. Some oil optimists believe that synthetic oil will replace natural oil entirely, essentially eliminating any negative effects from natural oil's end. But since creating synthetic oil is a heavily polluting activity that requires one barrel of natural gas per four barrels of oil produced -- and since synthetic oil's success would presumably correspond with high fossil fuel prices -- governments may more quickly move toward other sources of energy.

While some claim that there will be plenty of oil for the foreseeable future, others paint a different reality. One possible scenario: as realisation of 'peak oil' emerges around 2020, prices will rise markedly, consumption will fall accordingly, and the market will allocate the remaining oil to critical uses such as air travel and commercial shipping. Governments and economies that have not planned for a transition to renewable energy sources will face an accelerated crisis as oil's end moves out of slow-motion."

This will be enabled by:

Technological innovations in oil discovery and extraction that enable unearthing additional reserves and increasing production at existing sites
Rising global demand for oil

Early indicators include:

"Prediction by US geologist M. King Hubbert that US oil production would peak in the early 1970s, which was reasonably accurate
Acrimonious debate in the US over drilling for oil in the Arctic National Wildlife Refuge
Promulgation of an energy policy in the US still heavily dependent on fossil fuels
Proposal of a strategy by energy consultant Amory Lovins for the US and the world to 'win the oil endgame'"

What to watch:

"High oil prices sustained over several years severely retard economic growth and indicate that supply is having difficulty keeping up with growing demand.
Saudi Arabia cannot significantly increase oil production during periods of high prices, signaling a rough production ceiling of world oil supply.
Continued rapid economic growth, especially in China and other emerging Asian economies, strains global oil supply.