Coal, petroleum, and natural gas are referred to as fossil fuels. Their common origin is as living matter, plants, and, in particular, microorganisms that have accumulated in large quantities under favorable conditions during the earth's long history. They have been preserved (fossilized) through burial under younger sediments, to great depths and over many millions of years. The "organic" elements hydrogen (H) and carbon (C) are the primary source of their heat content (hence the derivation of the word hydrocarbons). Coal has a relatively high carbon content; petroleum and natural gas have much higher hydrogen contents. The burning of fossil fuels releases large quantities of the powerful greenhouse gas carbon dioxide (CO₂) into the atmosphere, where it remains for a long time and contributes to global warming.

Fossil fuels have powered the industrialization of the world for several centuries. During the nineteenth and early twentieth centuries, coal was the primary source of energy. Then, after World War I, petroleum and later natural gas became increasingly important and together they contribute approximately 62 percent of the primary energy sources in the United States. Coal nevertheless still provides about 23 percent, mostly by conversion into electricity at large power plants.

Internet Resource

Energy Information Administration. "Monthly Energy Review." Available from http://www.eia.doe.gov/emeu/mer.