Carbon dioxide is a colourless, odourless gas under ordinary atmospheric conditions with the chemical formula CO2, meaning that each molecule contains one carbon atom bonded to two oxygen atoms. It is naturally present in Earth's atmosphere and participates in the global carbon cycle through exchanges among the atmosphere, oceans, soils, rocks, vegetation and living organisms.
Carbon dioxide is produced by respiration, decomposition, combustion and volcanic activity and is removed from the atmosphere by photosynthesis, ocean absorption and longer term geological processes. Although it makes up only a small fraction of the atmosphere, its physical and chemical properties give it major importance for climate and life. Plants use carbon dioxide as a carbon source during photosynthesis, combining it with water using energy from sunlight to produce organic compounds and release oxygen. Carbon dioxide is also a greenhouse gas because its molecular structure allows it to absorb infrared radiation. Human activities have substantially increased atmospheric carbon dioxide concentrations, making it a central driver of modern climate change.
The carbon dioxide molecule has a linear structure in which the carbon atom is positioned between two oxygen atoms. Its molecular geometry and distribution of electric charge allow it to interact with infrared radiation through molecular vibrational modes. In particular, carbon dioxide absorbs infrared radiation at wavelengths important to Earth's thermal emission. This property makes it a greenhouse gas.
Carbon dioxide does not absorb most incoming visible sunlight strongly, which allows substantial solar radiation to reach the surface, but it interacts with outgoing thermal radiation emitted by the warmed Earth. Atmospheric carbon dioxide therefore participates in Earth's radiative energy balance. The amount of carbon dioxide in the atmosphere is commonly expressed as a concentration in parts per million by volume.
Even relatively small changes in concentration can have significant climatic consequences because the relevant process depends on radiative absorption rather than simply on the percentage of the atmosphere occupied by the gas. Its climatic importance is consequently much greater than its small volumetric abundance might suggest.
Carbon dioxide is an essential component of photosynthesis and therefore plays a fundamental role in terrestrial and aquatic ecosystems. Plants, algae and certain microorganisms absorb carbon dioxide and use light energy to convert carbon dioxide and water into organic molecules, with oxygen released as a by product in oxygenic photosynthesis. Carbon is subsequently transferred through food webs when organisms consume plants or other organisms.
Respiration returns carbon dioxide to the atmosphere or water, while decomposition releases carbon from dead organic matter. Oceans also exchange carbon dioxide continuously with the atmosphere. Dissolved carbon dioxide participates in a chemical system involving carbonic acid, bicarbonate and carbonate ions. These forms of dissolved inorganic carbon are important to marine chemistry and influence seawater acidity. Carbon dioxide is therefore not merely an atmospheric gas but a central participant in interconnected biological, chemical and geological cycles operating across the Earth system.
Human activities have changed the atmospheric carbon dioxide cycle primarily by transferring carbon from long term geological reservoirs and terrestrial ecosystems into the atmosphere. Burning coal, oil and natural gas converts fossil carbon into carbon dioxide. Cement manufacture releases carbon dioxide both from fuel use and from the chemical conversion of limestone into clinker.
Deforestation and other land use changes can release carbon stored in vegetation and soils and can reduce the amount of carbon that ecosystems remove from the atmosphere. The atmosphere does not retain all human emitted carbon dioxide. Oceans and terrestrial ecosystems absorb a substantial portion, which is sometimes described as a natural carbon sink. However, the continued accumulation of atmospheric carbon dioxide demonstrates that these sinks do not remove all anthropogenic emissions. Their capacity can also change with climate, land use and other environmental pressures. The relationship among emissions, atmospheric accumulation and natural sinks is a central component of the global carbon budget.
Carbon dioxide has a particularly important relationship with climate because its atmospheric concentration affects the strength of the greenhouse effect. When carbon dioxide concentration increases, the atmosphere absorbs more outgoing infrared radiation and the effective level from which some radiation escapes to space changes. The resulting radiative imbalance causes warming until the Earth system reaches a new energy balance.
Carbon dioxide differs from short lived weather influences because a significant fraction of an emission can influence atmospheric composition for centuries or longer through exchanges among carbon reservoirs. The exact atmospheric lifetime cannot be represented by one simple number because carbon dioxide is progressively redistributed among the atmosphere, ocean, biosphere and geological reservoirs. Some of the emitted carbon can be absorbed relatively quickly, while another fraction remains influential for much longer. This cumulative behaviour means that total carbon dioxide emissions strongly affect long term warming, even though the atmospheric concentration is also controlled by natural sinks.
Carbon dioxide also has important effects on oceans. When atmospheric carbon dioxide dissolves in seawater, it participates in chemical reactions that increase the concentration of hydrogen ions and reduce seawater pH. This process is known as ocean acidification, although seawater generally remains alkaline on the conventional pH scale. Changes in carbonate chemistry can reduce the availability of carbonate ions, which are important to many organisms that construct calcium carbonate shells or skeletons.
The sensitivity varies among species and environments, and biological responses also depend on temperature, oxygen, nutrients and other factors. Ocean uptake of carbon dioxide therefore has a dual significance. It removes part of the human emitted carbon dioxide from the atmosphere and slows atmospheric warming, but it also alters marine chemistry. The ocean carbon cycle includes physical dissolution, biological uptake, transport, mixing and chemical transformations, making its response to rising atmospheric carbon dioxide complex.
Carbon dioxide concentrations have varied substantially throughout Earth's geological history. Ice core records preserve ancient atmospheric gases in bubbles trapped within polar ice, providing direct measurements of atmospheric carbon dioxide over hundreds of thousands of years. Geological evidence extends the record further through proxies involving rocks, fossils, isotopes and other indicators. Modern atmospheric monitoring shows a pronounced increase associated with industrialisation and fossil fuel use.
Carbon isotope measurements provide evidence that much of the recent increase originates from fossil carbon because fossil fuels have characteristic isotopic signatures and contain little carbon 14 after geological isolation. Oxygen measurements and global carbon budget analyses provide additional evidence for combustion and land use contributions. The convergence of atmospheric chemistry, emissions records, ocean observations, ecosystem measurements and palaeoclimate evidence makes the modern carbon dioxide increase one of the most extensively documented changes in atmospheric composition.
Carbon dioxide is not inherently a toxic substance at ordinary atmospheric concentrations and is essential for photosynthesis and the natural greenhouse effect. However, at sufficiently high concentrations it can be dangerous or fatal because elevated carbon dioxide affects human physiology and can displace oxygen in enclosed environments. Industrial carbon dioxide is used in carbonated beverages, fire suppression, refrigeration systems, food packaging and numerous chemical processes.
It is also used in controlled environments such as greenhouses to enhance plant growth under suitable conditions, although excessive concentrations can harm plants and people. From a climate perspective, the key distinction is between the natural presence of carbon dioxide and the rapid anthropogenic increase in its atmospheric concentration. A common misconception is that because plants need carbon dioxide, additional atmospheric carbon dioxide must always be beneficial.
Plant responses depend on species, nutrients, water availability, temperature, pests and other environmental conditions, while increased carbon dioxide also affects climate and ocean chemistry. Carbon dioxide is therefore simultaneously a fundamental biological resource, a naturally occurring atmospheric constituent, an industrial material and a major driver of modern climate change.
