Peak Oil

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Peak Oil


Peak oil occurs when the production of petroleum (crude oil) from all sources in the world reaches a maximum. Historically, oil production has increased steadily since the late nineteenth century. However, oil is a fossil fuel, and the amount waiting to be pumped is limited. The nature of pumping oil means that it is not possible to pump an ever-increasing or even a steady amount of oil until the underground reservoir suddenly runs dry; instead, when half or more of the oil has been removed from a given underground reservoir, the rate at which oil can be extracted begins to decline. Oil production for any given reservoir therefore follows a predictable pattern of rising, peaking, and declining. This fate is predicted for the world's oil reserves as a whole.

A few advocates of the peak-oil concept argue that the oil production peak began in 2005, although optimistic oil-company forecasts put the peak at about 2050. Many estimates, including some from the oil companies, agree approximately with the Association for the Study of Peak Oil and Gas' prediction that peak oil will occur

around 2015. The peaking of oil supplies is relevant to global warming because burning oil-derived fuels such as gasoline accounts for a large share of global greenhouse-gas emissions. Yet a peak in oil production would probably not mean a lessened greenhouse effect. On the contrary, it is possible that most nations, desperate to keep their economies going despite the decline in oil supplies, would turn to even dirtier energy resources— such as oil shale, tar sands, and coal—in order to make up for the shortfall. Obtaining and burning these resources might actually increase greenhouse-gas emissions overall while emissions from burning oil fall.

Historical Background and Scientific Foundations

The peak oil concept was first proposed by geophysicist M. King Hubbert (1903-1989), who from 1956 to 1982 published a number of papers analyzing trends in U.S. oil discovery and production. Hubbert successfully predicted that U.S. oil production would peak around 1970, which it did. Few experts doubt that a Hubbert peak in world oil is bound to occur, but its exact timing is disputed. The peak will only definitely be known in hindsight, when records show that oil supplies have already begun to decline.

Oil peaks because oil is finite. All petroleum in the ground was formed tens of millions of years ago by heat and chemical reactions that altered thick sea-floor sediments consisting of the bodies of trillions of tiny marine organisms. Most of the petroleum and natural gas (the two tend to form together) produced by these natural processes leaked away millions of years ago, but sometimes special geological circumstances trap the fuels in sealed underground pockets. These sealed pockets are the oil reservoirs that exist in special abundance under the territories of oil-producing nations such as Saudi Arabia, Russia, the United States, Iraq, and Venezuela, which supply the liquid fuels that modern industrial civilization has depended on for over a century. Most petroleum is used for transportation in the form of gasoline and diesel fuel, but petroleum is also used for plastics manufacturing and is burned as heating oil.


CARBON SEQUESTRATION: The uptake and storage of carbon. Trees and plants, for example, absorb carbon dioxide, release the oxygen, and store the carbon. Fossil fuels were at one time biomass and continue to store the carbon until burned.

CELLULOSIC ETHANOL: Ethanol (a liquid fuel, also found in alcoholic beverages) produced by chemical or biological digestion of cellulose, the main structural material of plant tissues. Commercial-scale production of cellulosic ethanol (not yet viable as of 2007) would allow fuel production from plant materials that presently cannot be used for that purpose because they consist almost entirely of cellulose, such as corn cobs and wood chips.

COAL-TO-LIQUIDS: The production of liquid fuels from coal. A technology favored for development by U.S. energy policy but having many environmental drawbacks, including high greenhouse emissions.

GREENHOUSE-GAS EMISSIONS: Releases of greenhouses gases into the atmosphere. To simplify discussion of how much warming is being caused by emissions of various greenhouse gases—each of which causes a different amount of warming, ton for ton—it is customary to translate emissions of gases other than carbon dioxide into the number of tons of CO2 that would produce the same amount of warming, i.e., units of “tons CO2 equivalent.”

OIL FIELD: Surface region above underground reservoirs of petroleum where wells have been drilled to extract the petroleum.

OIL SHALE: Sedimentary rock, not necessarily shale, permeated by a gooey mixture of hydrocarbons termed kerogen. Kerogen can be separated from the rock and distilled to yield petroleum products. Almost two thirds of world oil shale reserves are in the United States. Extracting petroleum from oil shale requires strip mining and large amounts of water.

TAR SANDS: Naturally occurring mixtures of thick crude oil (bitumen) and small mineral particles (e.g., clay or sand). Also called oil sands or bituminous sands. Large deposits exist in the Canadian province of Alberta, with smaller deposits in Venezuela and the United States. Petroleum can be extracted from tar sands, but only by strip-mining the landscapes underlain by the deposits. Proposals to extract the tar sands of Alberta would entail strip-mining an area about the size of the state of Florida.

Large quantities of petroleum also exist in hard-to-extract forms, mixed with rocky material rather than conveniently collected in natural underground pockets. For example, oil shale is a type of sedimentary rock containing a hydrocarbon mixture that can be transformed into synthetic crude oil. Up to several trillion barrels (oil equivalent) of oil shale may exist. Tar sands, a mixture of sand, water, and heavy crude oil, exist in large quantities in Canada (with lesser amounts elsewhere). As with oil shale, crude oil can be extracted from the tar sand deposits, but so much energy and other resources must be invested in the extraction that it has not been profitable to mine in the past. Liquid fuels can also be produced from coal, which exists in large deposits in the United States, China, and many other countries.

Every oil field has a definite life cycle. First, it is discovered and its size (how much oil it holds) is estimated by geologists. Second, it is exploited: wells are drilled and oil is pumped out. As more wells are drilled, production from the field increases. Eventually, when about half the oil in the reservoir is pumped out, production begins to decline. In order to force oil out of the underground reservoir, water must be pumped in under pressure; this water mixes with the remaining oil and some is pumped back out again with it. As the amount of oil in the reservoir decreases, the percentage of water being pumped increases and the flow of oil is reduced.

For example, the world's largest oil field, Ghawar in Saudi Arabia, first tapped in 1935, once produced six million barrels of oil per day (a barrel is equivalent to 42 U.S. gallons or 159 liters). By the early 2000s, the liquid being pumped out of Ghawar was about 30% water. Carbon dioxide is sometimes used instead of water to maintain oil flow, and steam may also be injected to make the oil flow more easily, recovering even more oil. Eventually, however, no matter what technologies are used, the oil production of any field or collection of fields must peak.

The oil supply of the world is in a constant state of change as new fields are discovered, wells are drilled, oil is pumped out and consumed, and individual fields peak and decline as others are brought into production. The production of oil from non-conventional resources such as tar sands and oil shale, and the extraction of more oil from wells using advanced drilling and pumping technology, also complicate estimates of when peak oil shall occur. About 1.1 trillion barrels have been pumped since the beginning of oil extraction in the late nineteenth century, and estimates of what remains vary from about 1 trillion to almost 4 trillion barrels. (The higher estimate includes unconventional sources.) There is no doubt, however, that the pace of new oil-resource discoveries began to lag oil consumption many years ago, or that world oil demand is rising rapidly as China and India industrialize and as major oil importers such as the United States continue to postpone any serious curtailment of their own demand.

Impacts and Issues

When peak oil does occur, oil consumption will drop in strict step with decreasing production. But this drop will be forced instead of chosen, requiring cutbacks in transportation and industrial use. Elementary economics predict that when the supply of any commodity is exceeded by inflexible demand for that commodity, prices must rise steeply. As oil prices skyrocket, richer countries take an even bigger share of what oil is available, driving poorer countries toward economic breakdown but also endangering their own economies. Some features of such a crisis may appear even before peak oil, if oil demand rises more rapidly than supply can follow.

Petroleum usage and global climate change are linked. In the early 2000s in the United States, for example, about 42% of greenhouse-gas emissions came from petroleum fuels (mostly for transportation), about 37% from coal (mostly for electricity), and about 21% from natural gas (heating and electricity). The goals of decreasing U.S. dependency on imported oil and on oil itself have moved the U.S. government to support, since the early 1990s, research into the production of liquid fuels from coal. However, this method, along with other forms of oil substitution that peak oil might drive the world to embrace, is in direct conflict with the need to mitigate global climate change by decreasing greenhouse-gas emissions. Besides the grave environmental harms that would be caused by large-scale extraction of unconventional oil or oil substitutes (mining of Canada's tar sands is predicted to involve strip-mining an area the size of Florida), these fuel sources produce more carbon dioxide than oil itself. Coal-to-liquids, for example, releases about twice as much carbon dioxide per gallon of fuel delivered as using oil-derived fuels.

Furthermore, to offset even a small fraction of oil demand would require a vast expansion of coal mining. Peabody Energy, the world's largest coal company, has promoted a plan calling for 10% of forecast U.S. oil demand to be met in 2025 by liquid fuels from coal. The Peabody plan calls for 33 large coal-to-liquids plants to be built, each costing $6.4 billion. The plants would consume a total of 475 million tons of coal per year, about 43% more coal than was being mined in the United States in 2006. Even assuming no growth in other coal usage, this would almost double (or more than double) the amount of carbon dioxide presently being released in the United States from coal. And this does not count increased methane emissions from expanded coal mining. Other unconventional sources of liquid fossil fuels also produce significantly more carbon dioxide per unit of energy delivered than does oil.

Supporters of coal-to-liquids schemes as an answer to both oil-import dependence and the looming crisis of peak oil argue that carbon sequestration technologies could store the extra carbon released underground and prevent it from altering global climate. Opponents argue that tailpipe carbon releases would be untouched by such measures even if they were effective. They also argue that we should pre-adapt to peak oil and mitigate global warming at the same time by adopting measures such as radically increased end-use efficiency and renewable fuels such as cellulosic ethanol. Increased vehicle mileage, for example, could allow the United States to achieve exactly as much transportation in 2025 as building the 33 coal-to-liquids plants proposed by Peabody Energy, but without increasing greenhouse-gas emissions, pollution of water and air, and the destruction of landscapes involved in mining coal, tar sands, or the like.

See Also Energy Contributions; Petroleum; Petroleum: Economic Uses and Dependency.



Roberts, Paul. The End of Oil. New York: Houghton Mifflin, 2004.


Campbell, Colin J., and Jean H. Laherrere. “The End of Cheap Oil.” Scientific American (March 1998): 78-83.

Charpentier, Ronald R. “Locating the Summit of the Oil Peak.” Science 295 (2002): 1470.

Kerr, Richard A. “Bumpy Road Ahead for World's Oil.” Science 310 (2005): 1106-1108.

Kerr, Richard A. “Even Oil Optimists Expect Energy Demand to Outstrip Supply.” Science 317 (2007): 437.

Kerr, Richard A. “The Looming Oil Crisis Could Arrive Uncomfortably Soon.” Science 316 (2007): 351.

Witze, Alexandra. “That's Oil, Folks.” Nature 445 (2007): 14-17

Web Sites

Dunlop, Ian T. “Climate Change and Peak Oil: An Integrated Policy Response for Australia.” Carbon Equity: Ration the Future, March 2007. < Peak%20oil%20Dunlop.pdf> (accessed October 28, 2007).

Heinberg, Richard. “Bridging Peak Oil and Climate Change Activism.” Energy Bulletin, January 9, 2007. <> (accessed October 28, 2007).

Hirsch, Robert L. “The Inevitable Peaking of World Oil Production.” The Atlantic Council of the United States, October 2005. <> (accessed October 28, 2007).

“Manifesto: Between Peak Oil and Climate Change.” The Peakist. < manifesto> (accessed October 28, 2007).

Larry Gilman