Carbon dioxide is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. In the air it is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. It is a trace gas in Earth's atmosphere at 417 ppm (about 0.04%) by volume, having risen from pre-industrial levels of 280 ppm. Burning fossil fuels is the primary cause of these increased CO₂ concentrations and also the primary cause of global warming and climate change.Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water it forms carbonic acid (H₂CO₃), which causes ocean acidification as atmospheric CO₂ levels increase.