Carbon dioxide (CO2) is a colorless, odorless, and non-flammable gas at normal temperature and pressure. It is a chemical compound with the chemical formula CO2, made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. Carbon dioxide is an end product of cellular respiration in organisms that obtain energy by breaking down sugars, fats, and amino acids with oxygen as part of their metabolism. This includes all plants, algae, animals, aerobic fungi, and bacteria. In vertebrates, carbon dioxide travels in the blood from the bodys tissues to the skin or the gills, from where it dissolves in the water, or to the lungs from where it is exhaled.

In Earths atmosphere, carbon dioxide is a trace gas that plays an integral part in the greenhouse effect, carbon cycle, photosynthesis, and oceanic carbon cycle. It is one of several greenhouse gases in the atmosphere of Earth. The current global average concentration of CO2 in the atmosphere is 421 ppm as of May 2022 (0.04%). This is an increase of 50% since the start of the Industrial Revolution, up from 280 ppm during the 10,000 years prior to the mid-18th century.

Carbon dioxide is also used as a refrigerant, in fire extinguishers, for inflating life rafts and life jackets, blasting coal, foaming rubber and plastics, promoting the growth of plants in greenhouses, immobilizing animals before slaughter, and in carbonated beverages.