In 1956, the geologist M. King Hubbert predicted that United States oil production would peak between 1965 and 1970. This turned out to be correct. Hubbert's theory is called "Hubbert peak theory" or simply "peak oil." According to the theory, oil production increases until it reaches a peak, at which point roughly half the original resource is remaining. Since Hubbert's initial theory, experts have debated whether worldwide production of oil will also reach a peak. Some of the same debates have also centered around coal, with analysts wondering how much longer we can power our energy-hungry civilization with finite fossil fuels.
According to the pessimists, we may have already reached worldwide peak oil production, or will reach it soon. This would eventually lead to irreversible increases in the price of oil, causing increases in the price of practically everything else, possibly leading to economic collapse, global depression, and other dire consequences, up to and including the dissolution of modern industrial civilization. They argue that we could run out of oil almost entirely in a few decades or less.
According to optimists, peak oil production will be reached between 2020 and 2030. The impact will be ameliorated by investments in alternative energy sources such as solar panels, solar thermal, wind, tidal, geothermal, and nuclear fission. Even space-based solar stations, biofuel-producing synthetic microbes, and nuclear fusion are oft-cited possibilities. These optimists argue that fears of peak oil are unfounded, and that more innovative extraction techniques will help bridge the gap. They believe oil is not likely to run out in less than 50 years, and possibly longer than a century.
Most experts agree that there is much more coal available than oil, and if oil runs out, we may switch to coal. Making coal even more useful are chemical processes that can convert it to liquid form. That way, coal could be used as a liquid fuel similar to today's petroleum. Most experts agree that we have at least a couple hundred years worth of coal remaining, perhaps more.