Coal

Coal is a combustible black or brown sedimentary rock formed from the compacted remains of plants that grew in ancient swamps. It is mostly carbon, and it has been burned for energy for centuries, making it one of the largest sources of carbon dioxide from human activity.

Dead plant material accumulates in waterlogged swamps, where there is too little oxygen for complete decay, and forms peat. Burial, heat and pressure over millions of years drive off water and gases and concentrate the carbon, raising the rank of the coal from peat through lignite and sub-bituminous coal to bituminous coal and anthracite, which is hardest and has the most carbon. Most coal dates from the Carboniferous period, 359 to 299 million years ago, when vast forests grew. The woody polymer lignin is thought to have been an important part of the material.

Coal generates a large share of the world's electricity and is used in making steel. Burning it releases CO₂, sulfur dioxide, mercury and particles including black carbon, and it is the largest single fuel source of carbon dioxide emissions. Mining affects land and water, and fires in coal seams can burn for decades. The use of coal is a major issue for the carbon budget.

Coal beds contain a record of past ecosystems and sometimes fossils. The carbon they store was removed from the air long ago, and burning it reverses that process in a short time, shifting carbon in the carbon cycle. It is one of the fossil fuels alongside oil and gas, and phasing it out is central to the aims of the Paris Agreement and to climate policy.