Fossil fuels, which include oil, coal, and natural gas, are a non-renewable source of energy.  Once the amount of fossil fuels in the Earth is used up, these fuels cannot be replaced.  They are produced out of dead carbon-based organic material that has been deposited, compressed, and fossilized over very long periods of time.
 
Fossil fuels are the foundation of the world's current energy economy. According to the Union of Concerned Scientists, they account for more than 70% of the U.S.'s electricity generation. The problem with fossil fuels is that when they are burned to produce energy (for example, when cars burn gasoline in order to run) they release large amounts of carbon dioxide and other greenhouse gases into the atmosphere.  Electricity generated by fossil fuel is the greatest source of air pollution in the U.S. and is one of the dominant causes of global warming and climate change.