Fossil fuels. Fossil fuels-including coal, oil, and natural gas-are sources of energy that humans have taken advantage of over thousands of years

Fossil fuels-including coal, oil, and natural gas-are sources of energy that humans have taken advantage of over thousands of years. About 90 percent of the world's energy consumption comes from fossil fuels. These were created by the decomposition of primitive organisms, buried in sand and mud, and compressed

under the weight of accumulating layers. Over millions of years, temperatures and pressures changed the organic matter into coal, oil, and gas. Deposits of these resources are now found below ground in many areas of the world.

Combusting fossil fuels with oxygen releases water, carbon dioxide, and other substances into the environment. In the case of coal, these substances include sulfur dioxide and nitrogen oxides, which have been shown to be responsible for acid rain. To control these emissions, today's coal-fired power plants are equipped with scrubbers, filters, collectors, electrostatic precipitators, and other devices. Natural-gas-fired power plants release virtually no sulfur dioxide, but require controls to limit their nitrogen oxide emissions.

Smog is created by a photochemical reaction of sunlight with hydrocarbons, carbon monoxide, and other molecules emitted in car exhaust. To control smog, oil companies have reformulated gasoline, and automakers have designed cars that burn gasoline more cleanly and efficiently, with better filtering mechanisms. The results: tailpipe emissions from the average new car contain 95 percent less hydrocarbons than they did in the 1960s.

Global climate change is another environmental issue linked to fossil fuel use. As a greenhouse gas, the carbon dioxide released in the combustion of fossil fuels traps infrared radiation from the earth that would otherwise radiate out to space. This effect is believed to raise the heat of the earth's atmosphere. Planting more trees is one way to remove more carbon dioxide from the atmosphere, since trees need carbon dioxide as part of photosynthesis. Other greenhouse gases-like methane and carbon dioxide-come from animals and industry practices.

Oil spills in the oceans can damage coastal and marine plants and wildlife. Double-hulled tankers and rigorous safety practices are highly effective in preventing spills and limiting damage. Since 1990, more than 99.999 percent of oil delivered by tankers to the U.S. reached its destination without incident.

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