Coal and Petroleum
Coal is by far the most abundant fossil fuel on earth. It is essentially carbon and is mainly used as a combustion fuel. The large-scale use of coal began with the Industrial Revolution in the 19th century. As the number of industries increased, demand for more sources of energy grew.
Coal is the product of plants, mainly trees that died tens or hundreds of millions of years ago. Due to water logging in low-lying swampy areas or in slowly sinking lagoons, dead trees and plants did not decompose as they normally would. The dead plant matter was covered with water and protected from the oxidizing effect of air. The action of certain bacteria released the oxygen and hydrogen, making the residue richer and richer in carbon. Thick layers of this carbon-rich substance, called peat, built up over thousands of years. As more material accumulated above the peat, the water was squeezed out leaving just carbon-rich plant remains. Pressure and temperature further compressed the material. This aided the process of producing coal as more gases were forced out and the proportion of carbon continued to increase. The carbon slowly metamorphosed into coal over millions of years.
Petroleum is found in porous rock formations in the upper strata of some areas of the Earth's crust. Known reserves of petroleum are typically estimated at around 1.2 trillion barrels with at least one estimate as high as 3.74 trillion barrels. Consumption is currently around 84 million barrels per day, or 31 billion barrels per year. Because of reservoir engineering difficulties, recoverable oil reserves are significantly less than total oil-in-place. At current consumption levels, current known reserves would be gone in about 32 years, around 2039.
However, this ignores any new discoveries, changes in demand, better technology, population growth, industrialization of third world countries, and other factors. While oil is expected to remain a major source of energy in coming years, alternative energy development such as solar power, wind power, butanol, ethanol, photovoltaic, nuclear power, hydrogen, or oil from oil shale, and oil sands may increase in significance. Coal may also increase in use because of its existence in vast quantities in rapidly developing countries, such as China and India, which are exempt from controls under the Kyoto Protocol.