What Is Carbon Dioxide?

Carbon dioxide is a colorless, odorless gas composed of one carbon atom bonded to two oxygen atoms, with the molecular formula CO2 and a molar mass of 44.01 g/mol. At standard temperature and pressure it is denser than air and dissolves readily in water to form carbonic acid, making it relevant to both atmospheric chemistry and aquatic systems. Carbon dioxide is produced by the complete combustion of carbon-containing fuels, by cellular respiration in living organisms, and by the thermal decomposition of carbonates. It is also a product of fermentation, volcanic outgassing, and the calcination of limestone in cement production. These natural and anthropogenic sources collectively make CO2 the most important greenhouse gas by total radiative forcing contribution from human activities.

Carbon dioxide occupies a unique position in both the physical sciences and engineering: it is simultaneously a reagent in photosynthesis, the principal driver of ocean acidification, a critical factor in atmospheric radiation balance, and a useful industrial fluid in its supercritical phase.

Physical Properties and the Greenhouse Effect

CO2 absorbs and re-emits infrared radiation at wavelengths corresponding to molecular vibration modes near 4.26 and 15 micrometers, frequencies at which the atmosphere would otherwise be largely transparent to outgoing thermal radiation from Earth's surface. This selective absorption traps heat in the lower atmosphere, a mechanism first described quantitatively by Eunice Newton Foote in 1856 and later elaborated by Svante Arrhenius. NOAA's Global Monitoring Laboratory has tracked atmospheric CO2 concentrations continuously since 1958, recording a rise from about 315 parts per million at the start of the Keeling Curve record to over 420 ppm in recent measurements, an increase attributed primarily to fossil fuel combustion. NOAA notes that the current rate of increase, roughly 2 to 3 ppm per year, is approximately 100 times faster than natural glacial-interglacial transitions.

Carbon Dioxide in the Climate System

The role of CO2 in the climate system extends beyond direct radiative forcing. When CO2 dissolves in ocean water, it forms carbonic acid and reduces seawater pH, a process called ocean acidification that impairs the ability of corals, mollusks, and other calcifying organisms to build calcium carbonate shells. Carbon dioxide also participates in the long-term carbon cycle: silicate weathering removes CO2 from the atmosphere over millions of years, while volcanism releases it, maintaining a geological thermostat that has regulated Earth's temperature across deep time. NASA's vital signs monitoring of atmospheric CO2 provides continuous satellite and ground-based measurements that researchers use to attribute observed warming to specific emission sources and periods.

Carbon Dioxide in Industrial Processes

In industry, CO2 is used in its various phases for a range of purposes. Solid CO2, or dry ice, sublimates at minus 78.5 degrees Celsius at atmospheric pressure and is used as a refrigerant and cleaning agent. Supercritical CO2, which forms above 31.1 degrees Celsius and 73.8 bar, is an excellent non-toxic solvent for extraction processes including decaffeination of coffee and the production of pharmaceutical compounds; it is also the working fluid in some next-cycle power cycles. Liquid and gaseous CO2 are used to carbonate beverages, to inert tank atmospheres, and as a fire suppressant. In agriculture, elevated CO2 concentrations in enclosed greenhouse facilities accelerate plant growth. The NOAA research program on CO2 basics details both atmospheric and applied dimensions of the molecule.

Applications

Carbon dioxide has applications in a range of fields, including:

• Food and beverage processing, for carbonation and modified-atmosphere packaging
• Refrigeration and cold chain logistics using CO2 as a low-global-warming-potential refrigerant
• Fire suppression systems in data centers and electrical installations
• Enhanced oil recovery, injected into reservoirs to mobilize crude oil
• Supercritical extraction in pharmaceutical and food ingredient manufacturing