Fossil fuels are formed from ancient organic matter. Plants and plankton died, were buried under sediment, and over millions of years were transformed by heat and pressure into coal, oil, and natural gas. They are finite. Once burned, they cannot be replaced on human timescales.

These fuels power most of the world. Oil drives transport. Coal generates electricity and makes steel. Gas heats homes and runs power plants. They are energy-dense, easy to store, and cheap to extract in many places. That combination built modern industrial society.

They also drive climate change. Burning fossil fuels releases carbon dioxide that had been locked underground for millions of years. That carbon accumulates in the atmosphere and traps heat. Air pollution from burning coal and oil harms human health. The shift away from fossil fuels is underway, but they still supply roughly 80 percent of global primary energy.

The three main fuels

• Coal
• Oil and petroleum products
• Natural gas