Fossil fuels were planetary natural resources, such as coal, gasoline, and petroleum, often employed for goals of industrial development in technologically primitive societies at great environmental cost.
Fossil fuels were used on Earth through the first half of the 21st century. During a trip back in time to Detroit circa 2004, Jonathan Archer told T'Pol that Humans had known since the 1970s that Earth's fossil fuel supply was limited. He began to tell her about a noteworthy event in 2061 related to this issue, but his explanation was interrupted by their mission. (ENT: "Carpenter Street")