Oil—the product of the burial and transformation of biomass over the last 200 million years—has historically had no equal as an energy source for its intrinsic qualities of extractability, transportability, versatility, and cost. But the total amount of oil underground is finite, and, therefore, production will one day reach a peak and then begin to decline. Such a peak may be involuntary if supply is unable to keep up with growing demand. Alternatively, a production peak could be brought about by voluntary reductions in oil consumption before physical limits to continued supply growth kick in. Not surprisingly, concerns have arisen in recent years about the relationship between (1) the growing consumption of oil and the availability of oil reserves and (2) the impact of potentially dwindling supplies and rising prices on the world’s economy and social welfare. Following a peak in world oil production, the rate of production would eventually decrease and, necessarily, so would the rate of consumption of oil. Oil can be found and produced from a variety of sources. To date, world oil production has come almost exclusively from what are considered to be “conventional sources” of oil. While there is no universally agreed-upon definition of what is meant by conventional sources, IEA states that conventional sources can be produced using today’s mainstream technologies, compared with “nonconventional sources” that require more complex or more expensive technologies to extract, such as oil sands and oil shale. Distinguishing between conventional and nonconventional oil sources is important because the additional cost and technological challenges surrounding production of nonconventional sources make these resources more uncertain. However, this distinction is further complicated because what is considered to be a mainstream technology can change over time. For example, offshore oil deposits were considered to be a nonconventional source 50 years ago; however, today they are considered conventional. For the purpose of this report, and consistent with IEA’s classification, we define nonconventional sources as including oil sands, heavy oil deposits, and oil shale. Some oil is being produced from these nonconventional sources today. For example, in 2005 Canada produced about 1.6 million barrels per day of oil from oil sands, and Venezuelan production of extra-heavy oil for 2005 was projected to be about 600,000 barrels per day. Currently, however, production from these sources is very small compared with total world oil production. According to IEA, most countries outside the Middle East have reached their peak in conventional oil production, or will do so in the near future. The United States is a case in point. Even though the United States is currently the third-largest, oil-producing nation, U.S. oil production peaked around 1970 and has been on a declining trend ever since. (See fig. 1.) Looking toward the future, EIA projects that U.S. deepwater oil production will slightly boost total U.S. production in the near term. However, this increase will end about 2016, and then U.S. production will continue to decline. Given these projections, it is clear that future increases in U.S. demand for oil will need to be fulfilled through increases in production in the rest of the world. Increasing production in other countries has to date been able to more than make up for declining U.S. production and has resulted in increasing world production. (See fig. 2.) Oil accounts for approximately one-third of all the energy used in the world. Following the record oil prices associated with the Iranian Revolution in 1979-80 and with the start of the Iran-Iraq war in 1980, there was a drop in total world oil consumption, from about 63 million barrels per day in 1980 to 59 million barrels per day in 1983. Since then, however, world consumption of petroleum products has increased, totaling about 84 million barrels per day in 2005. In the United States, consumption of petroleum products increased an average of 1.65 percent annually from 1983 to 2004, and averaged 20.6 million barrels per day in 2005, representing about one-quarter of all world consumption. EIA projects that U.S. consumption will continue to increase and will reach 27.6 million barrels per day in 2030. As figure 3 shows, the transportation sector is by far the largest U.S. consumer of petroleum, accounting for two-thirds of all U.S. consumption and relying almost entirely on petroleum to operate. Within the transportation sector, light vehicles are the largest consumers of petroleum energy, accounting for approximately 60 percent of the transportation sector’s consumption of petroleum-based energy in the United States. Figure 3 also shows that while consumption of petroleum products in other sectors has remained relatively constant or increased slightly since the early 1980s, petroleum consumption in the transportation sector has grown at a significant rate. The price of oil is determined in the world market and depends mainly on the balance between world demand and supply. Recent world production of oil has been running at near capacity to meet rising demand, which has put upward pressure on oil prices. Figure 4 shows that world oil prices in nominal terms—unadjusted for inflation—are higher than at any time since 1950, although when adjusted for inflation, the high prices of 2006 are still lower than were reached in the 1979-80 price run-up following the Iranian Revolution and the beginning of the Iran-Iraq war. All else being equal, oil consumption is inversely correlated with oil price, with higher oil prices inducing consumers to reduce their oil consumption. Specifically, increases in crude oil prices are reflected in the prices of products made from crude oil, including gasoline, diesel, home heating oil, and petrochemicals. The extent to which consumers are willing and able to reduce their consumption of oil in response to price increases depends on the cost of switching to activities and lifestyles that use less oil. Because there are more options available in the longer term, consumers respond more to changes in oil prices in the longer term than in the shorter term. For example, in the short term, consumers can reduce oil consumption by driving less or more slowly, but in the longer term, consumers can still take those actions, but can also buy more fuel-efficient automobiles or even move closer to where they work and thereby further reduce their oil consumption. Supply and demand, in turn, affect the type of oil that is produced. Conventional oil that is less expensive to extract using lower-cost drilling techniques will be produced when oil prices are lower. Conversely, oil that is expensive to produce because of the higher cost technologies involved may not be economical to produce at low oil prices. Producers are unlikely to turn to these more expensive oil sources unless oil prices are sustained at a high enough level to make such an enterprise profitable. Given the importance of oil in the world’s energy portfolio, as cheaper oil reserves are exhausted in the future, nations will need to make the transition to more and more expensive and difficult-to-access sources of oil to meet energy demands. Recently, for example, a large discovery of oil in the Gulf of Mexico made headlines; however, this potential wealth of oil is located at a depth of over 5 miles below sea level, a fact that adds significantly to the costs of extracting that oil. Most studies estimate that oil production will peak sometime between now and 2040, although many of these projections cover a wide range of time, including two studies for which the range extends into the next century. Key uncertainties in trying to determine the timing of peak oil are the (1) amount of oil throughout the world; (2) technological, cost, and environmental challenges to produce that oil; (3) political and investment risk factors that may affect oil exploration and production; and (4) future world demand for oil. The uncertainties related to exploration and production also make it difficult to estimate the rate of decline after the peak. Most studies estimate that oil production will peak sometime between now and 2040, although many of these projections cover a wide range of time, including two studies for which the range extends into the next century. Figure 5 shows the estimates of studies we examined. Studies that predict the timing of a peak use different estimates of how much oil remains in the ground, and these differences explain some of the wide ranges of these predictions. Estimates of how much oil remains in the ground are highly uncertain because much of these data are self- reported and unverified by independent auditors; many parts of the world have yet to be fully explored for oil; and there is no comprehensive assessment of oil reserves from nonconventional sources. This uncertainty surrounding estimates of oil resources in the ground comprises the uncertainty surrounding estimates of proven reserves as well as uncertainty surrounding expected increases in these reserves and estimated future oil discoveries. Oil and Gas Journal and World Oil, two primary sources of proven reserves estimates, compile data on proven reserves from national and private company sources. Some of this information is publicly available from oil companies that are subject to public reporting requirements—for example, information provided by companies that are publicly traded on U.S. stock exchanges that are subject to the filing requirements of U.S. federal securities laws. Information filed pursuant to these laws is subject to liability standards, and, therefore, there is a strong incentive for these companies to make sure their disclosures are complete and accurate. On the other hand, companies that are not subject to these federal securities laws, including companies wholly owned by various OPEC countries where the majority of reserves are located, are not subject to these filing requirements and their related liability standards. Some experts believe OPEC estimates of proven reserves to be inflated. For example, OPEC estimates increased sharply in the 1980s, corresponding to a change in OPEC’s quota rules that linked a member country’s production quota in part to its remaining proven reserves. In addition, many OPEC countries’ reported reserves remained relatively unchanged during the 1990s, even as they continued high levels of oil production.