HistoryCarbon dioxide was the first gas to be described as a discrete substance. In about 1640, the chemist observed that when he burned in a closed vessel, the mass of the resulting was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (''spiritus sylvestris''). The properties of carbon dioxide were further studied in the 1750s by the physician . He found that ( ) could be heated or treated with s to yield a gas he called "fixed air." He observed that the fixed air was denser than air and supported neither flame nor animal life. Black also found that when bubbled through [[limewater (a saturated aqueous solution of [[calcium hydroxide), it would [[Precipitation (chemistry)|precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist [[Joseph Priestley published a paper entitled ''Impregnating Water with Fixed Air'' in which he described a process of dripping [[sulfuric acid (or ''oil of vitriol'' as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas. Carbon dioxide was first liquefied (at elevated pressures) in 1823 by [[Humphry Davy and [[Michael Faraday. The earliest description of solid carbon dioxide ( ) was given by the French inventor [[Adrien-Jean-Pierre Thilorier, who in 1835 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO2.
Chemical and physical properties
Structure and bondingThe carbon dioxide molecule is linear and [[centrosymmetric at equilibrium. The [[carbon–oxygen bond length is 116.3 [[picometer|pm, noticeably shorter than the [[bond length of a C–O single bond and even shorter than most other C–O multiply-bonded functional groups. Since it is centrosymmetric, the molecule has no electrical [[dipole. As a linear triatomic molecule, CO2 has four vibrational modes as shown in the diagram. However, the symmetric stretching mode does not create a dipole and so is not observed in the IR spectrum. The two bending modes are degenerate, meaning that they correspond to only one frequency. Consequently, only two vibrational bands are observed in the [[IR spectrum – an antisymmetric stretching mode at [[wavenumber 2349 cm−1 (wavelength 4.25 μm) and a [[Degenerate energy levels|degenerate pair of bending modes at 667 cm−1 (wavelength 15 μm). There is also a symmetric stretching mode at 1388 cm−1 which is only observed in the [[Raman spectrum. As a result of the two bending modes, the molecule is only strictly linear when the amount of bending is zero. It has been shown both by theory and by Coulomb explosion imaging experiments that this is never actually true for both modes at once. In a gas phase sample of carbon dioxide, none of the molecules are linear as a result of the vibrational motions. However, the molecular geometry is still described as linear, which describes the average atomic positions corresponding to minimum potential energy. This is also true for other “linear” molecules.
In aqueous solutionCarbon dioxide is [[soluble in water, in which it reversibly forms ([[carbonic acid), which is a [[Acid strength|weak acid since its ionization in water is incomplete. : + The [[Henry's law|hydration equilibrium constant of carbonic acid is (at 25 °C). Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO2 molecules, not affecting the pH. The relative concentrations of , and the [[deprotonation|deprotonated forms ([[bicarbonate) and ([[carbonate) depend on the [[pH. As shown in a [[Bjerrum plot, in neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2–8.5, contain about 120 mg of bicarbonate per liter. Being [[diprotic acid|diprotic, carbonic acid has two [[acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO3−): :H2CO3 HCO3− + H+ :''K''a1 = ; p''K''a1 = 3.6 at 25 °C. This is the ''true'' first acid dissociation constant, defined as , where the denominator includes only covalently bound H2CO3 and does not include hydrated CO2(aq). The much smaller and often-quoted value near is an ''apparent'' value calculated on the (incorrect) assumption that all dissolved CO2 is present as carbonic acid, so that . Since most of the dissolved CO2 remains as CO2 molecules, ''K''a1(apparent) has a much larger denominator and a much smaller value than the true ''K''a1. The [[bicarbonate ion is an [[amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high [[pH, it dissociates significantly into the [[carbonate ion (CO32−): :HCO3− CO32− + H+ :''K''a2 = ; p''K''a2 = 10.329 In organisms carbonic acid production is catalysed by the [[enzyme, [[carbonic anhydrase.
Chemical reactions of CO2CO2 is a potent [[electrophile having an electrophilic reactivity that is comparable to [[benzaldehyde or strong [[α,β-unsaturated carbonyl compounds. However, unlike electrophiles of similar reactivity, the reactions of nucleophiles with CO2 are thermodynamically less favored and are often found to be highly reversible. Only very strong nucleophiles, like the [[carbanions provided by [[Grignard reagents and [[organolithium compounds react with CO2 to give [[carboxylates: :MR + CO2 → RCO2M :where M = [[Lithium|Li or [[Magnesium|Mg [[Bromine|Br and R = [[alkyl or [[aryl. In [[metal carbon dioxide complexes, CO2 serves as a [[ligand, which can facilitate the conversion of CO2 to other chemicals. The reduction of CO2 to [[Carbon monoxide|CO is ordinarily a difficult and slow reaction: :CO2 + 2 e− + 2H+ → CO + H2O [[Photoautotrophs (i.e. plants and ) use the energy contained in sunlight to [[Photosynthesis|photosynthesize simple sugars from CO2 absorbed from the air and water: : ''n'' CO2 + ''n'' → + ''n'' The [[redox potential for this reaction near pH 7 is about −0.53 V ''versus'' the [[standard hydrogen electrode. The nickel-containing enzyme [[carbon monoxide dehydrogenase catalyses this process.
Physical propertiesCarbon dioxide is colorless. At low concentrations the gas is odorless; however, at sufficiently-high concentrations, it has a sharp, acidic odor. At [[Standard conditions for temperature and pressure|standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m3, about 1.53 times that of [[Earth's atmosphere|air. Carbon dioxide has no liquid state at pressures below . At 1 atmosphere (near mean sea level pressure), the gas [[deposition (physics)|deposits directly to a solid at temperatures below and the solid [[sublimation (chemistry)|sublimes directly to a gas above −78.5 °C. In its solid state, carbon dioxide is commonly called . Liquid carbon dioxide forms only at [[pressures above 5.1 atm; the [[triple point of carbon dioxide is about 5.1 [[bar (unit)|bar (517 [[kPa) at 217 K (see phase diagram). The [[Critical point (thermodynamics)|critical point is 7.38 MPa at 31.1 °C. Another form of solid carbon dioxide observed at high pressure is an [[amorphous glass-like solid. This form of glass, called ''[[amorphous carbonia|carbonia'', is produced by [[supercooling heated CO2 at extreme pressure (40–48 [[GPa or about 400,000 atmospheres) in a [[diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like [[silicon ([[silica|silica glass) and [[germanium dioxide. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts to gas when pressure is released. At temperatures and pressures above the critical point, carbon dioxide behaves as a [[supercritical fluid known as [[supercritical carbon dioxide.
Isolation and productionCarbon dioxide can be obtained by [[distillation from air, but the method is inefficient. Industrially, carbon dioxide is predominantly an unrecovered waste product, produced by several methods which may be practiced at various scales. The [[combustion of all [[carbon-based fuels, such as [[methane ( ), petroleum distillates ([[gasoline, [[Diesel fuel|diesel, [[kerosene, [[propane), coal, wood and generic organic matter produces carbon dioxide and, except in the case of pure carbon, water. As an example, the chemical reaction between methane and oxygen: : + 2 → + 2 It is produced by thermal decomposition of limestone, by heating ([[calcining) at about , in the manufacture of [[Calcium oxide|quicklime ([[calcium oxide, ), a compound that has many industrial uses: : → + [[Iron is reduced from its oxides with [[coke (fuel)|coke in a [[blast furnace, producing [[pig iron and carbon dioxide: Carbon dioxide is a byproduct of the industrial production of hydrogen by [[steam reforming and the [[water gas shift reaction in [[ammonia production. These processes begin with the reaction of water and natural gas (mainly methane). This is a major source of food-grade carbon dioxide for use in carbonation of and [[soft drinks, and is also used for stunning animals such as [[poultry. In the summer of 2018 a shortage of carbon dioxide for these purposes arose in Europe due to the temporary shut-down of several ammonia plants for maintenance. Acids liberate CO2 from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxide [[spring (hydrosphere)|springs, where it is produced by the action of acidified water on or [[Dolomite (mineral)|dolomite. The reaction between [[hydrochloric acid and calcium carbonate (limestone or chalk) is shown below: : + 2 → + The [[carbonic acid () then decomposes to water and CO2: : → + Such reactions are accompanied by foaming or bubbling, or both, as the gas is released. They have widespread uses in industry because they can be used to neutralize waste acid streams. Carbon dioxide is a by-product of the [[Fermentation (biochemistry)|fermentation of [[sugar in the [[brewing of , [[whisky and other [[alcoholic beverages and in the production of [[bioethanol. [[Yeast metabolizes [[sugar to produce CO2 and [[ethanol, also known as alcohol, as follows: : → 2 + 2 All [[cellular respiration|aerobic organisms produce CO2 when they oxidize s, [[fatty acids, and proteins. The large number of reactions involved are exceedingly complex and not described easily. Refer to ([[cellular respiration, [[anaerobic respiration and [[photosynthesis). The equation for the respiration of glucose and other [[monosaccharides is: : + 6 → 6 + 6 [[Anaerobic organisms decompose organic material producing methane and carbon dioxide together with traces of other compounds. Regardless of the type of organic material, the production of gases follows well defined [[chemical kinetics|kinetic pattern. Carbon dioxide comprises about 40–45% of the gas that emanates from decomposition in landfills (termed "[[landfill gas"). Most of the remaining 50–55% is methane.
ApplicationsCarbon dioxide is used by the food industry, the oil industry, and the chemical industry. The compound has varied commercial uses but one of its greatest uses as a chemical is in the production of carbonated beverages; it provides the sparkle in carbonated beverages such as soda water, beer and sparkling wine.
Precursor to chemicalsIn the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of [[urea, with a smaller fraction being used to produce [[methanol and a range of other products. Some carboxylic acid derivatives such as [[sodium salicylate are prepared using CO2 by the [[Kolbe-Schmitt reaction. In addition to conventional processes using CO2 for chemical production, electrochemical methods are also being explored at a research level. In particular, the use of renewable energy for production of fuels from CO2 (such as methanol) is attractive as this could result in fuels that could be easily transported and used within conventional combustion technologies but have no net CO2 emissions.
FoodsCarbon dioxide is a [[food additive used as a propellant and acidity regulator in the food industry. It is approved for usage in the EU (listed as [[E number E290), US and Australia and New Zealand (listed by its [[INS number 290). A candy called [[Pop Rocks is pressurized with carbon dioxide gas at about . When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop. [[Leavening agents cause dough to rise by producing carbon dioxide. [[Baker's yeast produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as [[baking powder and [[baking soda release carbon dioxide when heated or if exposed to s.
BeveragesCarbon dioxide is used to produce [[carbonation|carbonated [[soft drinks and [[soda water. Traditionally, the carbonation of beer and sparkling wine came about through natural fermentation, but many manufacturers carbonate these drinks with carbon dioxide recovered from the fermentation process. In the case of bottled and kegged beer, the most common method used is carbonation with recycled carbon dioxide. With the exception of British [[cask ale#Real ale|real ale, draught beer is usually transferred from kegs in a cold room or cellar to dispensing taps on the bar using pressurized carbon dioxide, sometimes mixed with nitrogen. The taste of soda water (and related taste sensations in other carbonated beverages) is an effect of the dissolved carbon dioxide rather than the bursting bubbles of the gas. [[Carbonic anhydrase 4 converts to [[carbonic acid leading to a [[sour taste, and also the dissolved carbon dioxide induces a [[somatosensory response.
WinemakingCarbon dioxide in the form of is often used during the [[cold soak phase in [[winemaking to cool clusters of [[grapes quickly after picking to help prevent spontaneous [[Fermentation (wine)|fermentation by wild [[yeast (wine)|yeast. The main advantage of using dry ice over water ice is that it cools the grapes without adding any additional water that might decrease the [[sugar concentration in the [[grape must, and thus the [[ethanol|alcohol concentration in the finished wine. Carbon dioxide is also used to create a hypoxic environment for [[carbonic maceration, the process used to produce [[Beaujolais wine. Carbon dioxide is sometimes used to top up wine bottles or other [[storage (wine)|storage vessels such as barrels to prevent oxidation, though it has the problem that it can dissolve into the wine, making a previously still wine slightly fizzy. For this reason, other gases such as [[nitrogen or [[argon are preferred for this process by professional wine makers.
Stunning animalsCarbon dioxide is often used to "stun" animals before slaughter. "Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress.
Inert gasIt is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools. Carbon dioxide is also used as an atmosphere for [[welding, although in the welding arc, it reacts to [[oxidation|oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are more [[brittle than those made in more inert atmospheres. When used for [[MIG welding, CO2 use is sometimes referred to as MAG welding, for Metal Active Gas, as CO2 can react at these high temperatures. It tends to produce a hotter puddle than truly inert atmospheres, improving the flow characteristics. Although, this may be due to atmospheric reactions occurring at the puddle site. This is usually the opposite of the desired effect when welding, as it tends to embrittle the site, but may not be a problem for general mild steel welding, where ultimate ductility is not a major concern. It is used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately , allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. [[Aluminium capsules of CO2 are also sold as supplies of compressed gas for [[air guns, [[paintball markers/guns, inflating bicycle tires, and for making [[carbonated water. High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in of some food products and technological materials, in the preparation of specimens for [[scanning electron microscopy and in the [[decaffeination of [[coffee beans.
Fire extinguisherCarbon dioxide can be used to extinguish flames by flooding the environment around the flame with the gas. It does not itself react to extinguish the flame, but starves the flame of oxygen by displacing it. Some [[Fire extinguisher#Clean agents and carbon dioxide|fire extinguishers, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide extinguishers work well on small flammable liquid and electrical fires, but not on ordinary combustible fires, because although it excludes oxygen, it does not cool the burning substances significantly and when the carbon dioxide disperses they are free to catch fire upon exposure to atmospheric oxygen. Their desirability in electrical fire stems from the fact that, unlike water or other chemical based methods, Carbon dioxide will not cause short circuits, leading to even more damage to equipment. Because it is a gas, it is also easy to dispense large amounts of the gas automatically in IT infrastructure rooms, where the fire itself might be hard to reach with more immediate methods because it is behind rack doors and inside of cases. Carbon dioxide has also been widely used as an extinguishing agent in fixed fire protection systems for local application of specific hazards and total flooding of a protected space. [[International Maritime Organization standards also recognize carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fire protection systems have been linked to several deaths, because it can cause suffocation in sufficiently high concentrations. A review of CO2 systems identified 51 incidents between 1975 and the date of the report (2000), causing 72 deaths and 145 injuries.
Supercritical CO2 as solventLiquid carbon dioxide is a good [[solvent for many [[lipophilic [[organic compounds and is used to remove [[caffeine from [[coffee. Carbon dioxide has attracted attention in the [[pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as [[organochlorides. It is also used by some [[dry cleaning|dry cleaners for this reason (see [[green chemistry). It is used in the preparation of some [[Aerogel#Production|aerogels because of the properties of supercritical carbon dioxide.
AgriculturePlants require carbon dioxide to conduct [[photosynthesis. The atmospheres of greenhouses may (if of large size, must) be enriched with additional CO2 to sustain and increase the rate of plant growth. At very high concentrations (100 times atmospheric concentration, or greater), carbon dioxide can be toxic to animal life, so raising the concentration to 10,000 ppm (1%) or higher for several hours will eliminate pests such as [[whitefly|whiteflies and [[spider mites in a greenhouse.
Medical and pharmacological usesIn medicine, up to 5% carbon dioxide (130 times atmospheric concentration) is added to [[oxygen for stimulation of breathing after [[apnea and to stabilize the balance in blood. Carbon dioxide can be mixed with up to 50% oxygen, forming an inhalable gas; this is known as [[Carbogen and has a variety of medical and research uses.
Fossil fuel recoveryCarbon dioxide is used in [[enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under [[Supercritical fluid|supercritical conditions, when it becomes [[miscibility|miscible with the oil. This approach can increase original oil recovery by reducing residual oil saturation by between 7% to 23% additional to [[Extraction of petroleum#Primary recovery|primary extraction. It acts as both a pressurizing agent and, when dissolved into the underground [[crude oil, significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well. In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points. In [[enhanced coal bed methane recovery, carbon dioxide would be pumped into the coal seam to displace methane, as opposed to current methods which primarily rely on the removal of water (to reduce pressure) to make the coal seam release its trapped methane.
Bio transformation into fuelIt has been proposed that CO2 from power generation be bubbled into ponds to stimulate growth of algae that could then be converted into [[biodiesel fuel. A strain of the [[cyanobacterium ''[[Synechococcus|Synechococcus elongatus'' has been genetically engineered to produce the fuels [[isobutyraldehyde and [[isobutanol from CO2 using photosynthesis.
RefrigerantLiquid and solid carbon dioxide are important [[refrigerants, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment is not practical. Solid carbon dioxide is always below at regular atmospheric pressure, regardless of the air temperature. Liquid carbon dioxide (industry nomenclature R744 or R-744) was used as a refrigerant prior to the discovery of [[Dichlorodifluoromethane|R-12 and may enjoy a renaissance due to the fact that [[R134a contributes to [[climate change more than CO2 does. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to the need to operate at pressures of up to , CO2 systems require highly resistant components that have already been developed for mass production in many sectors. In automobile air conditioning, in more than 90% of all driving conditions for latitudes higher than 50°, R744 operates more efficiently than systems using R134a. Its environmental advantages ([[Global warming potential|GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, and heat pump water heaters, among others. [[Coca-Cola has fielded CO2-based beverage coolers and the [[United States Army|U.S. Army is interested in CO2 refrigeration and heating technology. The global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO2 is one discussed option.(see [[Sustainable automotive air conditioning)
Minor usesCarbon dioxide is the [[active laser medium|lasing medium in a [[carbon dioxide laser, which is one of the earliest type of lasers. Carbon dioxide can be used as a means of controlling the [[pH of swimming pools, by continuously adding gas to the water, thus keeping the pH from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintaining [[Reef aquarium|reef aquaria, where it is commonly used in [[calcium reactors to temporarily lower the pH of water being passed over in order to allow the calcium carbonate to dissolve into the water more freely where it is used by some [[corals to build their skeleton. Used as the primary coolant in the British [[advanced gas-cooled reactor for nuclear power generation. Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methods to administer CO2 include placing animals directly into a closed, prefilled chamber containing CO2, or exposure to a gradually increasing concentration of CO2. In 2013, the [[American Veterinary Medical Association issued new guidelines for carbon dioxide induction, stating that a displacement rate of 30% to 70% of the [[gas chamber volume per minute is optimal for the humane euthanization of small rodents. However, there is opposition to the practice of using carbon dioxide for this, on the grounds that it is cruel. Carbon dioxide is also used in several related [[carbon dioxide cleaning|cleaning and surface preparation techniques.
In Earth's atmosphereCarbon dioxide in [[Earth's atmosphere is a [[trace gas, having a global average concentration of 415 parts per million by volume (or 630 parts per million by mass) as of the end of year 2020.[ftp://aftp.cmdl.noaa.gov/products/trends/co2/co2_mm_gl.txt National Oceanic & Atmospheric Administration (NOAA) – Earth System Research Laboratory (ESRL), Trends in Carbon Dioxide: Globally averaged marine surface monthly mean data] Values given are dry air [[mole fractions expressed in parts per million ([[Parts per million|ppm). For an [[ideal gas mixture this is equivalent to parts per million by volume (ppmv). Atmospheric concentrations fluctuate slightly with the seasons, falling during the [[Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay. Concentrations also vary on a regional basis, most strongly [[planetary boundary layer|near the ground with much smaller variations aloft. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels. The concentration of carbon dioxide has risen due to human activities. The extraction and burning of s, using carbon that has been sequestered for many millions of years in the [[lithosphere, has caused the atmospheric concentration of to increase by about 50% since the beginning of the [[Industrial Revolution|age of industrialization up to year 2020.Friedlingstein, P., Jones, M., O'Sullivan, M., Andrew, R., Hauck, J., Peters, G., Peters, W., Pongratz, J., Sitch, S., Le Quéré, C. and 66 others (2019) "Global carbon budget 2019". ''Earth System Science Data'', 11(4): 1783–1838. . Most from human activities is released from burning coal, petroleum, and natural gas. Other large anthropogenic sources include cement production, [[deforestation, and biomass burning. Human activities emit over 30 billion tons of (9 billion tons of fossil carbon) per year, while volcanoes emit only between 0.2 and 0.3 billion tons of . Human activities have caused CO2 to increase above levels not seen in hundreds of thousands of years. Currently, about half of the carbon dioxide released from the [[Global warming|burning of fossil fuels remains in the [[atmosphere and is not absorbed by vegetation and the oceans. While transparent to [[visible light, carbon dioxide is a , absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies (see the section "[[Carbon dioxide#Structure and bonding|Structure and bonding" above). Light emission from the earth's surface is most intense in the infrared region between 200 and 2500 cm−1, as opposed to light emission from the much hotter sun which is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric traps energy near the surface, warming the surface and the lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption. Increases in atmospheric concentrations of and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have correspondingly strengthened their absorption and emission of infrared radiation, causing the rise in average global temperature since the mid-20th century. Carbon dioxide is of greatest concern because it exerts a larger overall warming influence than all of these other gases combined. It furthermore has an [[Greenhouse gas#Atmospheric lifetime|atmospheric lifetime that increases with the cumulative amount of fossil carbon extracted and burned, due to the imbalance that this activity has imposed on Earth's [[fast carbon cycle. This means that some fraction (a projected 20-35%) of the fossil carbon transferred thus far will in-effect persist in the atmosphere as elevated levels for many thousands of years after these carbon transfer activities begin to subside. Not only do increasing concentrations lead to increases in global surface temperature, but increasing global temperatures also cause increasing concentrations of carbon dioxide. This produces a [[positive feedback for changes induced by other processes such as [[Milankovitch cycles|orbital cycles. Five hundred million years ago the concentration was 20 times greater than today, decreasing to 4–5 times during the [[Jurassic period and then slowly declining with [[Azolla Event|a particularly swift reduction occurring 49 million years ago. Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain. At the Bossoleto hot spring near [[Rapolano Terme in [[Tuscany, [[Italy, situated in a bowl-shaped depression about in diameter, concentrations of CO2 rise to above 75% overnight, sufficient to kill insects and small animals. After sunrise the gas is dispersed by convection. High concentrations of CO2 produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at [[Lake Monoun, [[Cameroon in 1984 and 1700 casualties at [[Lake Nyos, Cameroon in 1986.
In the oceansCarbon dioxide dissolves in the ocean to form [[carbonic acid (H2CO3), [[bicarbonate (HCO3−) and [[carbonate (CO32−). There is about fifty times as much carbon dioxide dissolved in the oceans as exists in the atmosphere. The oceans act as an enormous [[carbon sink, and have taken up about a third of CO2 emitted by human activity. As the concentration of carbon dioxide increases in the atmosphere, the increased uptake of carbon dioxide into the oceans is causing a measurable decrease in the pH of the oceans, which is referred to as . This reduction in pH affects biological systems in the oceans, primarily oceanic [[calcification|calcifying organisms. These effects span the [[food chain from [[autotrophs to [[heterotrophs and include organisms such as [[coccolithophores, [[corals, [[foraminifera, [[echinoderms, [[crustaceans and [[mollusca|mollusks. Under normal conditions, calcium carbonate is stable in surface waters since the carbonate ion is at [[supersaturation|supersaturating concentrations. However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. Corals, coccolithophore algae, coralline algae, foraminifera, [[shellfish and [[pteropods experience reduced calcification or enhanced dissolution when exposed to elevated . Gas solubility decreases as the temperature of water increases (except when both pressure exceeds 300 bar and temperature exceeds 393 K, only found near deep geothermal vents) and therefore the rate of uptake from the atmosphere decreases as ocean temperatures rise. Most of the CO2 taken up by the ocean, which is about 30% of the total released into the atmosphere, forms carbonic acid in equilibrium with bicarbonate. Some of these chemical species are consumed by photosynthetic organisms that remove carbon from the cycle. Increased CO2 in the atmosphere has led to decreasing [[alkalinity of seawater, and there is concern that this may adversely affect organisms living in the water. In particular, with decreasing alkalinity, the availability of carbonates for forming shells decreases, although there's evidence of increased shell production by certain species under increased CO2 content. NOAA states in their May 2008 "State of the science fact sheet for " that:
Biological roleCarbon dioxide is an end product of [[cellular respiration in organisms that obtain energy by breaking down sugars, fats and [[amino acids with [[oxygen as part of their [[metabolism. This includes all plants, algae and animals and [[aerobic respiration|aerobic fungi and bacteria. In [[vertebrates, the carbon dioxide travels in the blood from the body's tissues to the skin (e.g., [[amphibians) or the gills (e.g., [[fish), from where it dissolves in the water, or to the lungs from where it is exhaled. During active photosynthesis, [[compensation point|plants can absorb more carbon dioxide from the atmosphere than they release in respiration.
Photosynthesis and carbon fixation[[File:Calvin-cycle4.svg|left|upright=1|Overview of the [[Calvin cycle and carbon fixation [[Carbon fixation is a biochemical process by which atmospheric carbon dioxide is incorporated by [[plants, and ( ) into [[fuel|energy-rich organic [[molecules such as [[glucose, thus creating their own food by [[photosynthesis. Photosynthesis uses carbon dioxide and [[water to produce [[sugars from which other [[organic compounds can be constructed, and [[oxygen is produced as a by-product. [[RuBisCO|Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly abbreviated to RuBisCO, is the [[enzyme involved in the first major step of carbon fixation, the production of two molecules of [[3-phosphoglycerate from CO2 and [[ribulose bisphosphate, as shown in the diagram at left. RuBisCO is thought to be the single most abundant protein on Earth. [[Phototrophs use the products of their photosynthesis as internal food sources and as raw material for the [[biosynthesis of more complex organic molecules, such as [[polysaccharides, [[nucleic acids and [[proteins. These are used for their own growth, and also as the basis of the [[food chains and webs that feed other organisms, including animals such as ourselves. Some important phototrophs, the [[coccolithophores synthesise hard scales. A globally significant species of coccolithophore is ''[[Emiliania huxleyi'' whose [[calcite scales have formed the basis of many [[sedimentary rocks such as , where what was previously atmospheric carbon can remain fixed for geological timescales. Plants can grow as much as 50 percent faster in concentrations of 1,000 ppm CO2 when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients. Elevated CO2 levels cause increased growth reflected in the harvestable yield of crops, with wheat, rice and soybean all showing increases in yield of 12–14% under elevated CO2 in FACE experiments. Increased atmospheric CO2 concentrations result in fewer stomata developing on plants which leads to reduced water usage and increased [[water-use efficiency. Studies using [[Free-Air Concentration Enrichment|FACE have shown that CO2 enrichment leads to decreased concentrations of micronutrients in crop plants. This may have knock-on effects on other parts of [[ecosystems as herbivores will need to eat more food to gain the same amount of protein. The concentration of secondary [[metabolites such as phenylpropanoids and flavonoids can also be altered in plants exposed to high concentrations of CO2. Plants also emit CO2 during respiration, and so the majority of plants and algae, which use [[C3 photosynthesis, are only net absorbers during the day. Though a growing forest will absorb many tons of CO2 each year, a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants. Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon and remain valuable [[carbon sinks, helping to maintain the carbon balance of Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.
ToxicityCarbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about altitude) varies between 0.036% (360 ppm) and 0.041% (412 ppm), depending on the location. CO2 is an [[asphyxiant gas and not classified as toxic or harmful in accordance with [[Globally Harmonized System of Classification and Labelling of Chemicals|Globally Harmonized System of Classification and Labelling of Chemicals standards of [[United Nations Economic Commission for Europe by using the [[OECD Guidelines for the Testing of Chemicals. In concentrations up to 1% (10,000 ppm), it will make some people feel drowsy and give the lungs a stuffy feeling. Concentrations of 7% to 10% (70,000 to 100,000 ppm) may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour. The physiological effects of acute carbon dioxide exposure are grouped together under the term [[hypercapnia, a subset of [[Asphyxiant gas|asphyxiation. Because it is heavier than air, in locations where the gas seeps from the ground (due to sub-surface volcanic or geothermal activity) in relatively high concentrations, without the dispersing effects of wind, it can collect in sheltered/pocketed locations below average ground level, causing animals located therein to be suffocated. Carrion feeders attracted to the carcasses are then also killed. Children have been killed in the same way near the city of [[Goma by CO2 emissions from the nearby volcano [[Mt. Nyiragongo. The [[Swahili language|Swahili term for this phenomenon is '[[mazuku'. Adaptation to increased concentrations of CO2 occurs in humans, including [[Respiratory adaptation|modified breathing and kidney bicarbonate production, in order to balance the effects of blood acidification ([[acidosis). Several studies suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a [[submarine) since the adaptation is physiological and reversible, as deterioration in performance or in normal physical activity does not happen at this level of exposure for five days. Yet, other studies show a decrease in cognitive function even at much lower levels. Also, with ongoing respiratory acidosis, adaptation or [[acidosis|compensatory mechanisms will be unable to reverse such condition.
Below 1%There are few studies of the health effects of long-term continuous CO2 exposure on humans and animals at levels below 1%. Occupational CO2 exposure limits have been set in the United States at 0.5% (5000 ppm) for an eight-hour period. At this CO2 concentration, [[International Space Station crew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption. Studies in animals at 0.5% CO2 have demonstrated kidney calcification and bone loss after eight weeks of exposure. A study of humans exposed in 2.5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0.1% (1000ppm) CO2 likely due to CO2 induced increases in cerebral blood flow. Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm.
VentilationPoor ventilation is one of the main causes of excessive CO2 concentrations in closed spaces. Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO2 concentration has stabilized) are sometimes used to estimate ventilation rates per person. Higher CO2 concentrations are associated with occupant health, comfort and performance degradation. [[ASHRAE Standard 62.1–2007 ventilation rates may result in indoor concentrations up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor concentration is 400 ppm, indoor concentrations may reach 2,500 ppm with ventilation rates that meet this industry consensus standard. Concentrations in poorly ventilated spaces can be found even higher than this (range of 3,000 or 4,000). Miners, who are particularly vulnerable to gas exposure due to insufficient ventilation, referred to mixtures of carbon dioxide and nitrogen as "[[blackdamp," "choke damp" or "stythe." Before more effective technologies were developed, [[miners would frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by bringing a caged [[Domestic Canary|canary with them as they worked. The canary is more sensitive to asphyxiant gases than humans, and as it became unconscious would stop singing and fall off its perch. The [[Davy lamp could also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, while [[methane, another suffocating gas and explosion risk, would make the lamp burn more brightly. In February 2020, three people died from suffocation at a party in Moscow when dry ice (frozen CO2) was added to a swimming pool to cool it down.
ContentThe body produces approximately of carbon dioxide per day per person, containing of carbon. In humans, this carbon dioxide is carried through the [[venous system and is breathed out through the lungs, resulting in lower concentrations in the [[arteries. The carbon dioxide content of the blood is often given as the [[partial pressure, which is the pressure which carbon dioxide would have had if it alone occupied the volume. In humans, the blood carbon dioxide contents is shown in the adjacent table:
Transport in the bloodCO2 is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood). * Most of it (about 70% to 80%) is converted to [[bicarbonate ions by the enzyme [[carbonic anhydrase in the red blood cells, by the reaction CO2 + → → + . * 5–10% is dissolved in the [[Blood plasma|plasma * 5–10% is bound to [[hemoglobin as [[carbamino compounds [[Hemoglobin, the main oxygen-carrying molecule in [[red blood cells, carries both oxygen and carbon dioxide. However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of [[allosteric regulation|allosteric effects on the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. This is known as the [[Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the [[Bohr effect.
Regulation of respirationCarbon dioxide is one of the mediators of local [[autoregulation of blood supply. If its concentration is high, the [[capillaries expand to allow a greater blood flow to that tissue. Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level of CO2 in their blood. Breathing that is too slow or shallow causes [[respiratory acidosis, while breathing that is too rapid leads to [[hyperventilation, which can cause [[alkalosis|respiratory alkalosis. Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing [[air hunger. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the [[oxygen mask to themselves first before helping others; otherwise, one risks losing consciousness. The respiratory centers try to maintain an arterial CO2 pressure of 40 mm Hg. With intentional hyperventilation, the CO2 content of arterial blood may be lowered to 10–20 mm Hg (the oxygen content of the blood is little affected), and the respiratory drive is diminished. This is why one can hold one's breath longer after hyperventilating than without hyperventilating. This carries the risk that unconsciousness may result before the need to breathe becomes overwhelming, which is why hyperventilation is particularly dangerous before free diving.
See also* * * * (from the atmosphere) * * * * s * * * * [[List of least carbon efficient power stations * [[List of countries by carbon dioxide emissions * * * (early work on CO2 and climate change) * * NASA's *
Further reading* * * *