Carbon dioxide (chemical formula CO₂) is a chemical compound made up of molecules that each have 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. When carbon dioxide dissolves in water, it forms carbonate and mainly bicarbonate (HCO⁻
₃), which causes ocean acidification as atmospheric CO₂ levels increase.

It is a trace gas in Earth's atmosphere at 421 parts per million (ppm), or about 0.04% by volume (as of May 2022), 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 climate change.

Its concentration in Earth's pre-industrial atmosphere since late in the Precambrian has been regulated by organisms and geological phenomena. Plants, algae and cyanobacteria use energy from sunlight to synthesize carbohydrates from carbon dioxide and water in a process called photosynthesis, which produces oxygen as a waste product. In turn, oxygen is consumed and CO₂ is released as waste by all aerobic organisms when they metabolize organic compounds to produce energy by respiration. CO₂ is released from organic materials when they decay or combust, such as in forest fires. Since plants require CO₂ for photosynthesis, and humans and animals depend on plants for food, CO₂ is necessary for the survival of life on earth.

Carbon dioxide is 53% more dense than dry air, but is long lived and thoroughly mixes in the atmosphere. About half of excess CO₂ emissions to the atmosphere are absorbed by land and ocean carbon sinks. These sinks can become saturated and are volatile, as decay and wildfires result in the CO₂ being released back into the atmosphere. CO₂ is eventually sequestered (stored for the long term) in rocks and organic deposits like coal, petroleum and natural gas. Sequestered CO₂ is released into the atmosphere through burning fossil fuels or naturally by volcanoes, hot springs, geysers, and when carbonate rocks dissolve in water or react with acids.

CO₂ is a versatile industrial material, used, for example, as an inert gas in welding and fire extinguishers, as a pressurizing gas in air guns and oil recovery, and as a supercritical fluid solvent in decaffeination of coffee and supercritical drying. It is a byproduct of fermentation of sugars in bread, beer and wine making, and is added to carbonated beverages like seltzer and beer for effervescence. It has a sharp and acidic odor and generates the taste of soda water in the mouth, but at normally encountered concentrations it is odorless.