. Peak oil is the point in time when the maximum rate of global petroleum extraction is reached, after which the rate of production enters terminal decline. This concept is based on the observed production rates of individual oil wells, and the combined production rate of a field of related oil wells. The aggregate production rate from an oil field over time usually grows exponentially until the rate peaks and then declines—sometimes rapidly—until the field is depleted. This concept is derived from the Hubbert curve, and has been shown to be applicable to the sum of a nation's domestic production rate, and is similarly applied to the global rate of petroleum production. Peak oil is often confused with oil depletion; peak oil is the point of maximum production while depletion refers to a period of falling reserves and supply. M. King Hubbert created and first used the models behind peak oil in 1956 to accurately predict that United States oil production would peak between 1965 and 1970. His logistic model, now called Hubbert peak theory, and its variants have described with reasonable accuracy the peak and decline of production from oil wells, fields, regions, and countries, and has also proved useful in other limited-resource production-domains. According to the Hubbert model, the production rate of a limited resource will follow a roughly symmetrical logistic distribution curve (sometimes incorrectly compared to a bell-shaped curve) based on the limits of exploitability and market pressures. Various modified versions of his original logistic model are used, using more complex functions to allow for real world factors. While each version is applied to a specific domain, the central features of the Hubbert curve (that production stops rising and then declines) remain unchanged, albeit with different profiles. Some observers, such as petroleum industry experts Kenneth S. Deffeyes and Matthew Simmons, believe the high dependence of most modern industrial transport, agricultural, and industrial systems on the relative low cost and high availability of oil will cause the post-peak production decline and possible severe increases in the price of oil to have negative implications for the global economy. Predictions vary greatly as to what exactly these negative effects would be. If political and economic changes only occur in reaction to high prices and shortages rather than in reaction to the threat of a peak, then the degree of economic damage to importing countries will largely depend on how rapidly oil imports decline post-peak. According to the Export Land Model, oil exports drop much more quickly than production drops due to domestic consumption increases in exporting countries. Supply shortfalls would cause the price of oil to increase sharply, unless demand is mitigated with planned conservation measures and use of alternatives. .