World distribution of coal

General occurrence

Coal is a widespread resource of energy and chemicals. Although terrestrial plants necessary for the development of coal did not become abundant until Carboniferous time (358.9 million to 298.9 million years ago), large sedimentary basins containing rocks of Carboniferous age and younger are known on virtually every continent, including Antarctica (not shown on the map). The presence of large coal deposits in regions that now have arctic or subarctic climates (such as Alaska and Siberia) is due to climatic changes and to the tectonic motion of crustal plates that moved ancient continental masses over Earth’s surface, sometimes through subtropical and even tropical regions. Coal is absent in some areas (such as Greenland and much of northern Canada) because the rocks found there predate the Carboniferous Period and these regions, known as continental shields, lacked the abundant terrestrial plant life needed for the formation of major coal deposits.

Resources and reserves

World coal reserves and resources are difficult to assess. Although some of the difficulty stems from the lack of accurate data for individual countries, two fundamental problems make these estimates difficult and subjective. The first problem concerns differences in the definition of terms such as proven reserves (generally only those quantities that are recoverable) and geological resources (generally the total amount of coal present, whether or not recoverable at present).

The proven reserves for any commodity should provide a reasonably accurate estimate of the amount that can be recovered under existing operating and economic conditions. To be economically mineable, a coal bed must have a minimum thickness (about 0.6 metre; 2 feet) and be buried less than some maximum depth (roughly 2,000 metres; 6,600 feet) below Earth’s surface. These values of thickness and depth are not fixed but change with coal quality, demand, the ease with which overlying rocks can be removed (in surface mining) or a shaft sunk to reach the coal seam (in underground mining), and so forth. The development of new mining techniques may increase the amount of coal that can be extracted relative to the amount that cannot be removed. For example, in underground mining (which accounts for about 60 percent of world coal production), conventional mining methods leave behind large pillars of coal to support the overlying rocks and recover only about half of the coal present. On the other hand, longwall mining, in which the equipment removes continuous parallel bands of coal, may recover nearly all the coal present.

The second problem, which concerns the estimation of reserves, is the rate at which a commodity is consumed. When considering the worldwide reserves of coal, the number of years that coal will be available may be more important than the total amount of coal resources. At present rates of consumption, world coal reserves should last more than 300–500 years. A large amount of additional coal is present in Earth but cannot be recovered at this time. These resources, sometimes called “geologic resources,” are even more difficult to estimate, but they are thought to be as much as 15 times greater than the amount of proven reserves.

The quantities of proven coal reserves are typically shown in millions of tons of coal equivalent (MTCE). One ton of coal equivalent equals 1 metric ton (2,205 pounds) of coal with a heating value of 29.3 megajoules per kilogram (12,600 British thermal units per pound). These values suggest that the United States has the largest amount of recoverable coal. Nearly 75 percent of the world’s recoverable coal resources are controlled by five countries: the United States (about 22 percent), Russia (about 15 percent), Australia (14 percent), China (about 13 percent), and India (about 10 percent).