Carbon dioxide (chemical formula CO2) is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state. CO2 is a trace gas comprising 0.039% of the atmosphere.
As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, sunlight, and water to produce carbohydrate energy for themselves and oxygen as a waste product. By contrast, during respiration they emit carbon dioxide, as do all other living things that depend either directly or indirectly on plants for food. Carbon dioxide is also generated as a by-product of combustion; emitted from volcanoes, hot springs, and geysers; and freed from carbonate rocks by dissolution.