Coal is a non-renewable fossil fuel that is the largest source of electricity generation worldwide. It is formed from the remains of prehistoric vegetation that accumulated in swamps or forests and underwent fossilization over millions of years. In 1999, world carbon dioxide emissions from coal usage totaled 8 million tons. The top coal producing countries are China, the United States, the European Union, India, and Australia. Coal is burned in power plant boilers to produce steam, which spins turbines that generate electricity.