Carbon dioxide

Carbon dioxide (chemical formula CO₂) is a chemical compound composed of one carbon atom covalently double bonded to two oxygen atoms. It is a colorless, odorless, non‑flammable gas at standard temperature and pressure and exists as a trace gas in Earth’s atmosphere. Carbon dioxide is a fundamental component of the carbon cycle, a critical substrate for photosynthesis, and the most important anthropogenic greenhouse gas driving contemporary climate change. Its concentration in the atmosphere has risen from approximately 280 ppm before the Industrial Revolution to over 420 ppm today, primarily due to the combustion of fossil fuels, cement production, and land‑use change. This entry surveys the chemical, biological, and industrial dimensions of carbon dioxide, as well as its roles in climate dynamics and health.

Chemical and physical properties

Carbon dioxide is a linear molecule (O=C=O) with a molecular weight of 44.01 g·mol⁻¹. Although the carbon–oxygen bonds are polar, the symmetric linear geometry renders the molecule non‑polar. At standard temperature and pressure (0 °C and 1 atm) CO₂ has a density of about 1.98 kg·m⁻³, roughly 1.5 times that of air. The solid form, known as dry ice, sublimates directly to the gas at −78.5 °C under atmospheric pressure. Under compression carbon dioxide can be liquefied; its critical point occurs at 31.1 °C and 73.8 bar. When dissolved in water, CO₂ reacts to form carbonic acid (H₂CO₃), which dissociates into bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions, establishing a weak acid–buffer system that governs the pH of natural waters and biological fluids.

Natural occurrence and the carbon cycle

Carbon dioxide is continuously exchanged among the atmosphere, hydrosphere, biosphere, and lithosphere on timescales ranging from seconds to millions of years. Natural sources include aerobic respiration by animals, plants, and microorganisms; decomposition of organic matter; volcanic outgassing; and the release of dissolved CO₂ from oceans. Photosynthetic organisms—plants, algae, and cyanobacteria—assimilate CO₂ using light energy, producing organic matter and oxygen. On geological timescales, CO₂ is drawn down by silicate‑rock weathering and incorporated into carbonate minerals such as limestone and dolomite. The oceans are the planet’s largest active carbon reservoir, absorbing roughly a quarter of anthropogenic CO₂ emissions annually, which leads to ocean acidification. Pre‑industrial atmospheric CO₂ levels of about 280 ppm have been exceeded; monthly averages measured at Mauna Loa have surpassed 420 ppm in the 2020s.

Biological significance

In the biosphere, carbon dioxide serves as the primary inorganic carbon source for photoautotrophs, anchoring virtually all food webs. In animals, CO₂ is a metabolic waste product of oxidative respiration; it diffuses out of cells, is transported by blood mainly in the form of bicarbonate ions, and is eliminated through the lungs. Chemoreceptors in the brainstem and peripheral arteries monitor the partial pressure of CO₂ (pCO₂) in blood; an increase (hypercapnia) triggers a strong ventilatory drive, making CO₂ the principal regulator of breathing. In plants, CO₂ concentration influences stomatal opening and can enhance photosynthetic rates in C₃ species under elevated levels, though the long‑term effects are modulated by nitrogen and water availability.

Industrial production and uses

Carbon dioxide is obtained commercially from natural CO₂ wells, as a by‑product of steam‑reforming and ammonia synthesis, and from fermentation processes. It can also be captured from flue gases or from direct air capture systems. CO₂ finds wide application: the food industry employs it as a carbonation agent in beverages, a chilling agent (dry ice), a packaging gas to inhibit spoilage, and as a leavening aid. It serves as a shielding gas in welding, a non‑flammable propellant in aerosol cans, and an extinguishing agent in fire extinguishers. In enhanced oil recovery, CO₂ is injected into reservoirs to mobilize trapped crude oil. Chemically, it is a feedstock for the synthesis of urea, methanol, salicylic acid, and other commodities. Greenhouses often employ CO₂ enrichment to boost crop yields.

Role in climate change

Carbon dioxide is the dominant long‑lived greenhouse gas emitted by human activities. It absorbs and re‑emits infrared radiation within the thermal infrared window, trapping heat in the lower atmosphere—the greenhouse effect. While natural levels of greenhouse gases maintain a habitable climate, the rapid injection of CO₂ from fossil fuel combustion, cement manufacture, and deforestation has intensified the effect, leading to global warming, sea‑level rise, and increased frequency of extreme weather events. The atmospheric lifetime of CO₂ is complex; a fast fraction is taken up by the biosphere and oceans, but a substantial portion remains in the atmosphere for centuries to millennia. Mitigation pathways include emission reductions, afforestation, soil carbon sequestration, and technological carbon capture and storage (CCS).

Health effects and safety

At ambient outdoor concentrations carbon dioxide poses no direct health risk. However, in enclosed or poorly ventilated spaces levels can accumulate. Exposure to CO₂ concentrations of 1–2 % (10 000–20 000 ppm) can cause drowsiness and increased respiration. Concentrations above 5 % may provoke hypercapnia, acidosis, headache, and confusion; levels exceeding 10 % can rapidly lead to loss of consciousness and death from asphyxiation. Because CO₂ is odorless and colorless, it presents a particular hazard in confined spaces such as fermentation cellars, grain silos, and submarine compartments. Contact with liquid CO₂ or dry ice can cause severe cold burns. Occupational exposure limits in many jurisdictions are set at 5 000 ppm as a time‑weighted average over an eight‑hour workday.

History of discovery and research

Flemish chemist Jan Baptist van Helmont first noted a gas distinct from air that resulted from fermentation and charcoal combustion in the early 17th century; he named it “gas sylvestre.” The systematic isolation of carbon dioxide was accomplished around 1756 by Joseph Black, who produced the gas by heating limestone and called it “fixed air,” demonstrating its role in respiration and its lethality at high concentrations. Antoine Lavoisier later confirmed its composition of carbon and oxygen. The gas’s function in photosynthesis was progressively unveiled by Jan Ingenhousz, Nicolas‑Théodore de Saussure, and others through the late 18th and 19th centuries. In 1896, Svante Arrhenius produced the first quantitative estimate of how changes in atmospheric CO₂ concentration could alter Earth’s surface temperature, laying the foundation for modern climate science.