A fossil fuel is a hydrocarbon fuel that formed in the earth's crust from the remains of dead plants and animals that were transformed by high heat and pressure over the course of millions of years. The main types of fossil fuels are coal, crude oil and natural gas.
Fossil fuels are still the world's dominant energy source, accounting for more than 80 percent of total global energy consumption and more than 60 percent of total electric power generation. Although fossil fuels have been a very convenient and cheap source of energy, the huge volume of carbon dioxide emitted by their burning has been a leading cause of the current rapid deterioration of the earth's environment.