Ultimately, all of the world’s energy resources can be traced back to nuclear energy and, to a very small extent, gravitational energy. Gravity determines the rotation of the moon around the earth and causes the tides of the oceans. Some tidal power plants make a very small contribution to energy production. Nuclear reactions – inside the sun or inside the Earth – generate the lion’s share of our energy. Nuclear fusion within the sun is responsible for all of the solar energy that Earth receives, and nuclear fission within the interior of the Earth helps to maintain its temperature of 7000 K. One can think of the sun as a nuclear fusion reactor and the Earth’s interior as a nuclear fission reactor, see the Nuclear Power page. The supplies of these energy resources will last billions of years. However, the children of today may experience in their lifetime the exhaustion of the major sources of energy that we currently use.
The word fossil (from the Latin fossilis) was introduced by Georgius Agricola, known as the father of mineralogy. He was a student in Leipzig, Germany, from 1514-1518, and published his famous work "De Natura Fossilium" in 1546. Fossils are artifacts from the geological past (ten thousand to a billion years ago), created from the breakdown of dead plants and animals. Our current gas and oil deposits are a billion to 100 million years old. Coal originated 500 to 100 million years ago, and brown coal arose in the period of time 100 to 10 million years ago.
With the natural resources oil, natural gas, coal, and uranium, a distinction is made between reserves and resources. For the natural resources oil, natural gas, coal, and uranium, a distinction is made between reserves and resources. Reserves represent that part of resources which are commercially recoverable, while contingent resources and prospective resources are less certain, because some significant commercial or technical hurdle must be overcome prior to there being confidence in the eventual production of these quantities. However, several definitions exist.
Oil reserves grow as new sources are constantly discovered. But it is estimated that now the Earth’s stores of fossil fuels are about half empty. Volumes of oil were measured in units of barrels throughout the world. A barrel of oil corresponds exactly to 42 U.S. gallons or 158.987 liters, or a mass of 137 kg. The old barrel was a hollow cylindrical container made of vertical wooden staves and bound by wooden or metal hoops. For comparison the U.S. beer barrel is 31 U.S. gallons, while a wine barrel is 31.5 gallons. The metric oil quantity is the mass unit of one tonne, 1 t = 1000 kg and 1 × 109 t = 1 Gt (gigatonne, billion t).
The 'Statistical Review of World Energy 2021' by BP contains for the end of 2020 the world oil reserves of 244.4 billion t, the U.S. oil reserves of 8.2 billion t, and for the year 2020 the world consumption of 4.2 billion t and the U.S. consumption of 0.71 billion t. If consumption continues at this rate, world oil reserves will be depleted in this century.
Depletion of conventional natural gas supplies is expected towards the beginning of the next century. Gas is given in m3 = cubic meter. The quantity tcm (tera cubic meter) is equal to 1012 m3. The explored reserves have increased from 126 tcm in 1990 to 188 tcm in 2020. However, consumption has doubled worldwide and in 2020 was at 3.9 tcm, see BP. More than 100 tcm were already depleted.
The BGR presents data about resources and reserves. The remaining potential for crude oil (reserves and resources) is given with around 689 billion t. The quantities of non-conventional crude oil (bitumen, ultra-heavy oil, and crude oil from tight rocks like tight oil or shale oil) are around 448 billion t. The global resources of natural gas are 850 tcm with the inclusion of aquifer gas and natural gas from gas hydrate.
BGR also analyzes hard and soft varieties of coal, which we will consider together here. Reserves of about 1,000 billion t and resources of about 20,000 billion t are estimated. The annual consumption of coal decreased in 2018 to 7 billion t, but may increase significantly when oil and natural gas run out. The coal stocks, however, will last well into the next centuries.
The distinction between reserves and resources of natural uranium stocks is quite simple: reserves cost less than 80 USD per kg uranium to produce, while resources cost more than this. From a BGR study of global uranium consumption, about 60,000 tons of uranium are consumed annually worldwide. The current reserves are 1.3 million t and will last for a few decades. Resources with a price of less than 260 USD per kg are estimated at 12 million t. Even with a price of 260 USD, thermal nuclear power plants will probably continue to be operated into the next century. Another possible source of uranium is sea water, as it contains natural uranium at a mass fraction of 3 × 10−9, but recovering this uranium does not make economic sense. More likely is the use of a fast breeder reactor for substantially more effective utilization of natural uranium. This has significant environmental risks, but it could delay the depletion of uranium sources for a long time.