Carbon dioxide, commonly written as CO2, is a colourless gas consisting of one carbon atom bonded to two oxygen atoms. It is naturally present in Earths atmosphere and is an essential component of the carbon cycle. Plants, algae, and some microorganisms use carbon dioxide during photosynthesis to produce organic compounds and release oxygen. Carbon dioxide is also produced naturally by respiration, decomposition, volcanic activity, ocean processes, and other geological and biological mechanisms. Despite its relatively low atmospheric concentration compared with nitrogen and oxygen, carbon dioxide has major climatic importance because its molecular structure allows it to absorb infrared radiation.
Carbon dioxide participates in a global carbon cycle involving the atmosphere, oceans, vegetation, soils, rocks, and fossil carbon deposits. Plants remove atmospheric carbon dioxide through photosynthesis, while respiration and decomposition return carbon to the atmosphere. Oceans exchange carbon dioxide continuously with the atmosphere, and dissolved carbon participates in chemical reactions involving bicarbonate and carbonate ions. Over geological timescales, carbon can become stored in sediments and carbonate rocks. Volcanic activity and weathering transfer carbon between geological reservoirs and the atmosphere. Human activities have disturbed this long term balance by transferring large quantities of carbon from fossil fuel and geological stores into the atmosphere.
The largest human source of carbon dioxide emissions is the combustion of fossil fuels such as coal, oil, and natural gas. When carbon rich fuels burn, carbon combines with oxygen and forms CO2. Cement production also releases carbon dioxide through the calcination of limestone, in which calcium carbonate is converted into calcium oxide and carbon dioxide. Land use change can release carbon from vegetation and soils. Some industrial processes produce additional emissions. These sources are significant because they introduce carbon that had been isolated from the active carbon cycle for geological periods into the atmosphere over comparatively short human timescales.
Carbon dioxide is a greenhouse gas because its molecular structure allows it to absorb infrared radiation at particular wavelengths. The molecule can vibrate in ways that interact with infrared radiation emitted by Earths surface. After absorbing energy, carbon dioxide can emit infrared radiation again in different directions. This repeated absorption and emission affects the vertical and horizontal transfer of thermal energy through the atmosphere. Increasing carbon dioxide concentration therefore changes the radiative balance of the climate system. The effect is logarithmic with concentration, meaning that each additional equal increment of CO2 produces a progressively smaller direct radiative forcing than the preceding increment, although continued emissions continue to increase atmospheric concentration and warming.
Carbon dioxide is also important to ocean chemistry. When atmospheric CO2 dissolves in seawater, it participates in chemical equilibria that increase hydrogen ion concentration and reduce ocean pH. This process is commonly described as ocean acidification, although seawater generally remains alkaline rather than becoming literally acidic in the everyday sense. Increased dissolved carbon dioxide also changes the availability of carbonate ions, which are important to organisms that construct calcium carbonate shells or skeletons. Ocean uptake therefore provides an important carbon sink while simultaneously changing marine chemistry. The biological and chemical responses of marine systems depend on temperature, circulation, nutrients, ecosystems, and other environmental factors.
Carbon dioxide has numerous beneficial and essential biological roles. Plants require CO2 for photosynthesis, and atmospheric carbon dioxide concentrations have historically varied substantially over geological time. Increasing CO2 can stimulate photosynthesis in some plants under suitable conditions, an effect known as carbon dioxide fertilisation. However, the magnitude of this effect depends on water availability, nutrients, temperature, plant species, pests, and other environmental constraints. Increased CO2 does not therefore guarantee proportional increases in plant growth. In natural ecosystems, climate change, altered rainfall, heat stress, nutrient limitations, and land use changes can counteract or modify the direct physiological response to higher CO2.
Carbon dioxide differs from carbon monoxide, which contains one carbon atom and one oxygen atom. Carbon monoxide is a highly toxic gas because it interferes with oxygen transport in the human body, whereas carbon dioxide is primarily a greenhouse gas and, at sufficiently high concentrations, can also cause dangerous physiological effects through displacement of oxygen and disruption of blood chemistry. Carbon dioxide is also distinct from carbonic acid, bicarbonate, and carbonate, although these substances are chemically connected through reactions involving dissolved CO2 and water. Accurate terminology is important because similar names do not imply identical chemical or environmental properties.
Carbon dioxide is central to modern climate science because atmospheric CO2 concentration is both a measurable indicator of human influence and a major driver of long term warming. Natural sinks remove some emitted CO2, but they do not immediately remove all additional emissions. Some carbon remains in the atmosphere for very long periods while other portions are transferred among oceans, vegetation, soils, and geological reservoirs.
The long lived nature of the carbon cycle means that reducing carbon dioxide emissions is fundamental to limiting long term warming. Carbon dioxide is therefore simultaneously a natural component of life and Earth chemistry, an essential photosynthetic resource, an oceanic chemical participant, and the most important long lived anthropogenic greenhouse gas.
