Carbon dioxide (chemical formula CO₂) is a chemical compound of molecules with one carbon atom covalently double-bonded to two oxygen atoms. It is found in the gas state at room temperature, and as the source of available carbon in the carbon cycle, atmospheric CO₂ is the primary carbon source for life on Earth.

In the air, carbon dioxide is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. Carbon dioxide dissolves in water and forms carbonate, mainly bicarbonate (HCO−3) which causes ocean acidification as atmospheric CO₂ levels increase.