Ancient organic remains (fossils) in sediments which over eons became sedimentary rock, giving rise to solid, liquid, and gaseous fuels such as coal, crude oil, and natural gas. Coal is derived from vegetable matter altered by pressure, whereas crude oil and natural gas are derived from animal and vegetable matter altered by pressure and heat. Essentially, all fossil fuels are highly concentrated forms of far-ancient sunlight trapped in organic cells. They are the primary energy source for human societies since the industrial revolution (mid 19th century to early 20th century), are non-renewable, and also a primary source of global warming.
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