Carbon dioxide is a chemical compound with the chemical formula CO₂. It is 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 at normally-encountered concentrations it is odorless. As the source of 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.

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It is a trace gas in Earth's atmosphere at 421 parts per million (ppm),^[a] or about 0.042% (as of May 2022) having risen from pre-industrial levels of 280 ppm or about 0.028%.^{[10] [11]} Burning fossil fuels is the main cause of these increased CO2 concentrations, which are the primary cause of climate change.[12]