Use of fossil fuels as a source of energy contributes significantly to air pollution in the form of: sulphur dioxide, nitrogen oxides, carbon monoxide, suspended particles (fly ash), and various volatile organic compounds. All of these may be injurious to health and to the environment. Combustion of such fuels contributes to the increase of carbon dioxide in the atmosphere and thus increases the risks of global warming.
Fossil fuels vary in the amount of carbon released per unit of energy gained under efficient combustion processes. For each joule of energy produced, gas yields about 20% less carbon dioxide than oil, 40% less than coal. Some fuel technologies are particularly polluting with respect to carbon dioxide production, for example the use of low-grade coal, the production of oil from shales or tar sands, or the burning of undried wood.
The International Energy Agency in 1993 predicted that fossil fuels will the mainstay of world power generation well into the next century, and will go on developing faster than either nuclear or hydroelectric power, in OECD countries by 2.1% compared with 1.2% for other other power sources. Fossil fuels, in particular coal and gas, will account for 65.5% of total energy usage in the OECD countries in 2010, compared to 61.2% in 1990. Carbon emission in the OECD area will rise over the same period from 2.83 billion tonnes to 3.62 billion tonnes. The use of fossil fuels is likely to be even more marked in the countries outside the OECD area because these countries lack the money to invest in hydroelectric or nuclear plants and many of them, China in particular, have access to supplies of coal. Coal is expected to increase its share in the output mix from around 38% in 1990 to 44% in 2010.