Carbon dioxide
Other names Carbonic acid gas Carbonic anhydride Carbonic oxide Carbon oxide Carbon(IV) oxide Dry ice (solid phase)
Identifiers
CAS number	124-38-9 Y
PubChem	280
ChemSpider	274 Y
UNII	142M471B3J Y
EC number	204-696-9
UN number	1013
KEGG	D00004 Y
MeSH	Carbon+dioxide
ChEBI	CHEBI:16526 Y
ChEMBL	CHEMBL1231871
RTECS number	FF6400000
ATC code	V03AN02
Beilstein Reference	1900390
Gmelin Reference	989
3DMet	B01131
Jmol-3D images	Image 1 Image 2
SMILES O=C=O C(=O)=O
InChI InChI=1S/CO2/c2-1-3 Y Key: CURLTUGMZLYLDI-UHFFFAOYSA-N Y InChI=1/CO2/c2-1-3 Key: CURLTUGMZLYLDI-UHFFFAOYAO
Properties
Molecular formula	CO2
Molar mass	44.01 g mol−1
Appearance	Colorless gas
Odor	Odorless
Density	1562 kg/m3 (solid at 1 atm and −78.5 °C) 770 kg/m3 (liquid at 56 atm and 20 °C) 1.977 kg/m3 (gas at 1 atm and 0 °C)
Melting point	-78 °C, 194.7 K, -109 °F (subl.)
Boiling point	-57 °C, 216.6 K, -70 °F (at 5.185 bar)
Solubility in water	1.45 g/L at 25 °C, 100 kPa
Acidity (pKa)	6.35, 10.33
Refractive index (nD)	1.1120
Viscosity	0.07 cP at −78.5 °C
Dipole moment	zero
Structure
Molecular shape	linear
Thermochemistry
Std enthalpy of formation ΔfHo298	−393.5 kJ·mol−1
Standard molar entropy So298	214 J·mol−1·K−1
Hazards
MSDS	External MSDS
NFPA 704	0 2 0
Related compounds
Other anions	Carbon disulfide Carbon diselenide
Other cations	Silicon dioxide Germanium dioxide Tin dioxide Lead dioxide
Related carbon oxides	Carbon monoxide Carbon suboxide Dicarbon monoxide Carbon trioxide
Related compounds	Carbonic acid Carbonyl sulfide
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Carbon dioxide (chemical formula CO₂) is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state, as a trace gas at a concentration of 0.039% by volume.

As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, light, and water to produce carbohydrate energy for themselves and oxygen as a waste product.^[1] But in darkness photosynthesis cannot occur, and during the resultant respiration small amounts of carbon dioxide are produced.^[2] Carbon dioxide is also produced by combustion of coal or hydrocarbons, the fermentation of liquids and the breathing of humans and animals. In addition it is emitted from volcanoes, hot springs, geysers and other places where the earth’s crust is thin; and is freed from carbonate rocks by dissolution. CO₂ is also found in lakes at depth under the sea, and commingled with oil and gas deposits.^[3]

As of November 2011^[update], carbon dioxide in the Earth's atmosphere is at a concentration of approximately 390 ppm by volume.^[4] Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by seasonal plant growth in the Northern Hemisphere. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay. Taking all this into account, the concentration of CO₂ grew by about 2 ppm in 2009.^[5] Carbon dioxide is a greenhouse gas as it transmits visible light but absorbs strongly in the infrared and near-infrared, before slowly re-emitting the infrared at the same wavelength as what was absorbed.^[6]

Before the advent of human-caused release of carbon dioxide to the atmosphere, concentrations tended to increase with increasing global temperatures, acting as a positive feedback for changes induced by other processes such as orbital cycles.^[7] There is a seasonal cycle in CO₂ concentration associated primarily with the Northern Hemisphere growing season.^[8]

Carbon dioxide has no liquid state at pressures below 5.1 standard atmospheres (520 kPa). At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below −78.5 °C (−109.3 °F; 194.7 K) and the solid sublimes directly to a gas above −78.5 °C. In its solid state, carbon dioxide is commonly called dry ice.

CO₂ dissolves in water forming carbonic acid,^[9] which is a weak acid, because CO₂ molecule ionization in water is incomplete. The hydration equilibrium constant K_h (at 25 °C) of carbonic acid is [H₂CO₃]/[CO₂] = 1.70×10⁻³: Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO₂ molecules not affecting the pH. It is an amphoteric substance that can act as an acid or as a base, depending on pH of the solution.

CO₂ is an asphyxiant gas and not classified as toxic or harmful in accordance with 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 higher concentrations 1% (10,000 ppm) will make some people feel drowsy.^[10] Concentrations of 7% to 10% may cause suffocation, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.^[11]

In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.

CO₂ + H₂O H₂CO₃

Chemical and physical properties[link]

Structure and bonding[link]

The carbon dioxide molecule is linear and centrosymmetric. The two C-O bonds are equivalent and are short (116.3 pm), consistent with double bonding.^[12] Since it is centrosymmetric, the molecule has no electrical dipole. Consistent with this fact, only two vibrational bands are observed in the IR spectrum – an antisymmetic stretching mode at 2349 cm⁻¹ and a bending mode near 666 cm⁻¹. There is also a symmetric stretching mode at 1388 cm⁻¹ which is only observed in the Raman spectrum.

In aqueous solution[link]

Carbon dioxide is soluble in water, in which it reversibly converts to H₂CO₃ (carbonic acid). The relative concentrations of CO₂, H₂CO₃, and the deprotonated forms HCO−
3 (bicarbonate) and CO2−
3(carbonate) depend on the pH. 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, 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 = 2.5×10⁻⁴ ; pK_a1 = 3.6 at 25 °C.^[12]

At high pH, the bicarbonate ion dissociates significantly into the carbonate ion (CO₃²⁻):

HCO₃⁻ CO₃²⁻ + H⁺

K_a2 = 4.69×10⁻¹¹ ; pK_a2 = 10.329

[edit] Chemical reactions of CO₂

Overall, CO₂ is a weak electrophile. Its reaction with basic water illustrates this property, in which case hydroxide is the nucleophile. Other nucleophiles react as well. For example, carbanions as provided by Grignard reagents and organolithium compounds react with CO₂ to give carboxylates:

MR + CO₂ → RCO₂M (where M = Li or MgBr 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.^[13]

The reduction of CO₂ to CO is ordinarily a difficult and slow reaction:

CO₂ + 2 e^- + 2H⁺ → CO + H₂O

The redox potential for this reaction near pH 7 is about −0.53 V vs NHE. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process.^[14]

Physical properties[link]

Carbon dioxide pressure-temperature phase diagram showing the triple point and critical point of carbon dioxide

Sample of solid carbon dioxide or "dry ice" pellets

Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor.

At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m³, about 1.5 times that of air. At atmospheric pressure and a temperature of −78.51 °C (−109.32 °F), carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseous to solid through deposition.

Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 518 kPa at −56.6 °C (see phase diagram, above). The critical point is 7.38 MPa at 31.1 °C.^[15] Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid.^[16] This form of glass, called 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 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.

History[link]

Crystal structure of dry ice

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash 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 sylvestre).^{[citation needed]}

The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids 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 an aqueous solution of lime (calcium hydroxide), it would 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.^[17] This was the invention of Soda water.

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.^[18] The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 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.^[19]

Isolation and production[link]

Carbon dioxide is mainly produced as an unrecovered side product of four technologies: combustion of fossil fuels, production of hydrogen by steam reforming, ammonia synthesis, and fermentation. It can be obtained by or from air distillation, however, this method is inefficient.

The combustion of all carbon-containing fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), but also of coal and wood, will yield carbon dioxide and, in most cases, water. As an example the chemical reaction between methane and oxygen is given below.

CH₄+ 2 O₂→ CO₂+ 2 H₂O

The production of quicklime (CaO), a compound that enjoys widespread use, involves the heating (calcining) of limestone at about 850 °C:

CaCO₃→ CaO + CO₂

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:^[20]

Fe₂O₃+ 3 CO → 2 Fe + 3 CO₂

Yeast metabolizes sugar to produce carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits, but also in the production of bioethanol:

C₆H₁₂O₆ → 2 CO₂+ 2 C₂H₅OH

All aerobic organisms produce CO₂ when they oxidize carbohydrates, fatty acids, and proteins in the mitochondria of cells. 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 monosachharides is:

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O

Photoautotrophs (i.e. plants, cyanobacteria) use another modus operandi: Plants absorb CO₂ from the air, and, together with water, react it to form carbohydrates:

nCO₂ + nH₂O → (CH₂O)_n + nO₂

Laboratory methods[link]

A variety of chemical routes to carbon dioxide are known, such as the reaction between most acids and most metal carbonates. For example, the reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is depicted below:

2 HCl+ CaCO₃→ CaCl₂+ H₂CO₃

The carbonic acid (H₂CO₃) then decomposes to water and CO₂. Such reactions are accompanied by foaming or bubbling, or both. In industry such reactions are widespread because they can be used to neutralize waste acid streams.

Industrial production[link]

Industrial carbon dioxide can be produced by several methods, many of which are practiced at various scales.^[21] In its dominant route, carbon dioxide is produced as a side product of the industrial production of ammonia and hydrogen. These processes begin with the reaction of water and natural gas (mainly methane).^[22]

Although carbon dioxide is not often recovered, carbon dioxide results from combustion of fossil fuels and wood as well fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages. It also results from thermal decomposition of limestone, CaCO₃, in the manufacture of lime (Calcium oxide, CaO). Directly from natural carbon dioxide springs, where it is produced by the action of acidified water on limestone or dolomite.

Uses[link]

Carbon dioxide bubbles in a soft drink.

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry.^[21]

Precursor to chemicals[link]

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of urea and methanol. Metal carbonates and bicarbonates, as well as some carboxylic acids derivatives (e.g., sodium salicylate) are prepared from CO₂.

Foods[link]

Carbon dioxide is a food additive used as a propellant and acidity regulator in the food industry. It is approved for usage in the EU^[23] (listed as E number E290), USA^[24] and Australia and New Zealand^[25] (listed by its INS number 290).

A candy called Pop Rocks is pressurized with carbon dioxide gas at about 40 bar (580 psi). 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 acids.

Beverages[link]

Carbon dioxide is used to produce carbonated soft drinks and soda water. Traditionally, the carbonation in 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, recycled carbon dioxide carbonation is the most common method used. With the exception of British 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.

Wine making[link]

Carbon dioxide in the form of dry ice is often used in the wine making process to cool down bunches of grapes quickly after picking to help prevent spontaneous fermentation by wild yeasts. The main advantage of using dry ice over regular water ice is that it cools the grapes without adding any additional water that may decrease the sugar concentration in the grape must, and therefore also decrease the alcohol concentration in the finished wine.

Dry ice is also used during the cold soak phase of the wine making process to keep grapes cool. The carbon dioxide gas that results from the sublimation of the dry ice tends to settle to the bottom of tanks because it is heavier than air. The settled carbon dioxide gas creates a hypoxic environment which helps to prevent bacteria from growing on the grapes until it is time to start the fermentation with the desired strain of yeast.

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 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.

Inert gas[link]

It is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools. Carbon dioxide also finds use as an atmosphere for welding, although in the welding arc, it reacts to 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, and that such weld joints deteriorate over time because of the formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than more inert gases such as argon or helium. 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 60 bar (870 psi, 59 atm), 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. Aluminum capsules of CO₂ are also sold as supplies of compressed gas for airguns, paintball markers, inflating bicycle tires, and for making carbonated water. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in supercritical drying of some food products and technological materials, in the preparation of specimens for scanning electron microscopy and in the decaffeination of coffee beans.

Fire extinguisher[link]

Carbon dioxide extinguishes flames, and some 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. 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.^[26] 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 does not support life in the concentrations used to extinguish fire (40% or so), however, it is not considered to be toxic to humans. A review of CO₂ systems identified 51 incidents between 1975 and the date of the report, causing 72 deaths and 145 injuries.^[27]

[edit] Super critical CO₂ as solvent

Liquid 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 used by some dry cleaners for this reason (see green chemistry).

Agricultural and biological applications[link]

Plants require carbon dioxide to conduct photosynthesis. Greenhouses may (if of large size, must) enrich their atmospheres with additional CO₂ to sustain and increase plant growth.^[28][29] A photosynthesis-related drop (by a factor less than two) in carbon dioxide concentration in a greenhouse compartment would kill green plants, or, at least, completely stop their 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 whiteflies and spider mites in a greenhouse.^[30] Carbon dioxide is used in greenhouses as the main carbon source for Spirulina algae.

In medicine, up to 5% carbon dioxide (130 times atmospheric concentration) is added to oxygen for stimulation of breathing after apnea and to stabilize the O₂/CO₂ balance in blood.

It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algae that could then be converted into biodiesel fuel.^[31]

Oil recovery[link]

Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under supercritical conditions. This kind of production may increase original oil recovery by 7 per cent to 23 per cent further from primary extraction.^[32]It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, enabling the oil to flow more rapidly through the earth to the removal well.^[33] In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

Refrigerant[link]

Liquid 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 −78.5 °C 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 R-12 and may enjoy a renaissance due to the fact that r134a contributes to climate change. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to its operation at pressures of up to 130 bar (1880 psi), 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 R-134a. Its environmental advantages (GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, hot water heat pumps, among others. Coca-Cola has fielded CO₂-based beverage coolers and the U.S. Army is interested in CO₂ refrigeration and heating technology.^[34][35]

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)

Coal bed methane recovery[link]

In enhanced coal bed methane recovery, carbon dioxide is pumped into the coal seam to displace methane.^[36]

Niche uses[link]

A carbon dioxide laser.

Carbon dioxide is so inexpensive and so innocuous, that it finds many small uses that represent what might be called niche uses. For example it is used in the carbon dioxide laser, which is one of the earliest type of lasers.

Carbon dioxide can be used as a mean of controlling the pH of swimming pools, by continuously adding gas to the water, thus keeping the pH level from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintaining reef aquaria, where it is commonly used in calcium reactors to temporarily lower the pH of water being passed over calcium carbonate 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.

In the Earth's atmosphere[link]

The Keeling Curve of atmospheric CO₂ concentrations measured at Mauna Loa Observatory.

Carbon dioxide in earth's atmosphere is considered a trace gas currently occurring at an average concentration of about 390 parts per million by volume or 591 parts per million by mass.^[37] The total mass of atmospheric carbon dioxide is 3.16×10¹⁵ kg (about 3,000 gigatonnes). Its concentration varies seasonally (see graph at right) and also considerably on a regional basis, especially near the ground. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels. Carbon dioxide is a greenhouse gas.

Yearly increase of atmospheric CO₂: In the 1960s, the average annual increase was 37% of the 2000–2007 average.^[38]

Five hundred million years ago carbon dioxide was 20 times more prevalent than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with a particularly swift reduction occurring 49 million years ago.^[39][40] Human activities such as the combustion of fossil fuels and deforestation have caused the atmospheric concentration of carbon dioxide to increase by about 35% since the beginning of the age of industrialization.^[41]

Up to 40% of the gas emitted by some volcanoes during subaerial eruptions is carbon dioxide.^[42] It is estimated that volcanoes release about 130–230 million tonnes (145–255 million tons) of CO₂ into the atmosphere each year. Carbon dioxide is also produced by hot springs such as those at the Bossoleto site near Rapolano Terme in Tuscany, Italy. Here, in a bowl-shaped depression of about 100 m diameter, local concentrations of CO₂ rise to above 75% overnight, sufficient to kill insects and small animals, but it warms rapidly when sunlit and the gas is dispersed by convection during the day.^[43] Locally 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.^[44] Emissions of CO₂ by human activities are currently more than 130 times greater than the quantity emitted by volcanoes, amounting to about 27 billion tonnes per year.^[45]

In the oceans[link]

Carbon dioxide dissolves in the ocean to form carbonic acid (H₂CO₃), bicarbonate (HCO₃^-) and carbonate (CO₃^2-), and there is about fifty times as much carbon dissolved in the sea water of 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.^[46]

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 ocean acidification. Although the natural absorption of CO₂ by the world's oceans helps mitigate the climatic effects of anthropogenic emissions of CO₂, results in a decrease in the pH of the oceans. This reduciton in pH impacts the biological systems in the oceans, primarily oceanic calcifying organisms. These impacts span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and molluscs. Under normal conditions, calcite and aragonite are stable in surface waters since the carbonate ion is at 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. Even if there is no change in the rate of calcification, therefore, the rate of dissolution of calcareous material increases.^[47]

Research has already found that corals,^[48][49][50] coccolithophore algae,^{[51][52][53][54]} coralline algae,^[55] foraminifera,^[56] shellfish^[57] and pteropods^[58] experience reduced calcification or enhanced dissolution when exposed to elevated CO₂.

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)^[59] 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,^[60] forms carbonic acid in equilibrium with bicarbonate. Some of these chemical species are consumed by photosynthestic 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,^[61] although there's evidence of increased shell production by certain species under increased CO₂ content.^[62]

NOAA states in their May 2008 "State of the science fact sheet for ocean acidification" that:
"The oceans have absorbed about 50% of the carbon dioxide (CO₂) released from the burning of fossil fuels, resulting in chemical reactions that lower ocean pH. This has caused an increase in hydrogen ion (acidity) of about 30% since the start of the industrial age through a process known as “ocean acidification.” A growing number of studies have demonstrated adverse impacts on marine organisms, including:

• The rate at which reef-building corals produce their skeletons decreases, while production of numerous varieties of jellyfish increases.
• The ability of marine algae and free-swimming zooplankton to maintain protective shells is reduced.
• The survival of larval marine species, including commercial fish and shellfish, is reduced."

Also, the Intergovernmental Panel on Climate Change (IPCC) writes in their Climate Change 2007: Synthesis Report:^[63]
"The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average decrease in pH of 0.1 units. Increasing atmospheric CO₂ concentrations lead to further acidification [...] While the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species."

Some marine calcifying organisms (including coral reefs) have been singled out by major research agencies, including NOAA, OSPAR commission, NANOOS and the IPCC, because their most current research shows that ocean acidification should be expected to impact them negatively.^[64]

Carbon dioxide is also introduced into the oceans through hydrothermal vents. The Champagne hydrothermal vent, found at the Northwest Eifuku volcano at Marianas Trench Marine National Monument, produces almost pure liquid carbon dioxide, one of only two known sites in the world.^[65]

Biological role[link]

Carbon dioxide is an end product in organisms that obtain energy from breaking down sugars, fats and amino acids with oxygen as part of their metabolism, in a process known as cellular respiration. This includes all plants, animals, many fungi and some bacteria. In higher animals, the carbon dioxide travels in the blood from the body's tissues to the lungs where it is exhaled. In plants using photosynthesis, carbon dioxide is absorbed from the atmosphere.

Photosynthesis and carbon fixation[link]

Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by photosynthesis, which can be respired to water and (CO₂).

Figure 2. Overview of the Calvin cycle and carbon fixation

Carbon fixation is the removal of carbon dioxide from the air and its incorporation into solid compounds. Plants, algae, and many species of bacteria (cyanobacteria) fix carbon and create their own food by photosynthesis. Photosynthesis uses carbon dioxide and water to produce sugars and occasionally other organic compounds, releasing oxygen as a waste product.

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly known by the shorter name RuBisCO, is an enzyme involved in the first major step of carbon fixation, a process by which atmospheric carbon dioxide is converted by plants to energy-rich molecules such as glucose. It is also thought to be the single most abundant protein on Earth.^[66]

These phototrophs use the products of their photosynthesis as internal food sources and as raw material for the construction of more complex organic molecules, such as polysaccharides, nucleic acids and proteins. These are used for their own growth, and also as the basis for the food chains and webs whereby other organisms, including animals such as ourselves, are fed. Some important phototrophs, the coccolithophores synthesise hard calcium carbonate scales. A globally significant species of coccolithophore is Emiliania huxleyi whose calcite scales have formed the basis of many sedimentary rocks such as limestone, where what was previously atmospheric carbon can remain fixed for geological timescales.

Plants can grow up to 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.^[67] Research has shown that 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.^[68][69]

Studies have shown that increased CO₂ leads to fewer stomata developing on plants^[70] which leads to reduced water usage.^[71] Studies using FACE have shown that increases in CO₂ lead to decreased concentration of micronutrients in crop plants.^[72] 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.^[73]

The concentration of secondary metabolites such as phenylpropanoids and flavonoids can also be altered in plants exposed to high concentrations of CO₂.^{[74] [75]}.

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, the World Bank writes that a mature forest will produce as much CO₂ from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in biosynthesis in growing plants.^[76] However six experts in biochemistry, biogeology, forestry and related areas writing in the science journal Nature that "Our results demonstrate that old-growth forests can continue to accumulate carbon, contrary to the long-standing view that they are carbon neutral." ^[77] Mature forests are valuable carbon sinks, helping maintain balance in the 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.^[78]

Toxicity[link]

Main symptoms of carbon dioxide toxicity, by increasing volume percent in air.^[10]

Carbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about 30 km altitude) varies between 0.036% (360 ppm) and 0.039% (390 ppm), depending on the location.^[79]

Adaptation to increased levels of CO₂ occurs in humans. Continuous inhalation of CO₂ can be tolerated at three percent inspired concentrations for at least one month and four percent inspired concentrations for over a week. It was 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. Decrement in performance or in normal physical activity does not happen at this level.^[80][81] However, it should be noted that submarines have carbon dioxide scrubbers which reduce a significant amount of the CO₂ present.^[82]

Acute carbon dioxide physiological effect is hypercapnia or asphyxiation sometimes known by the names given to it by miners: blackdamp (also called choke damp or stythe). Blackdamp is primarily nitrogen and carbon dioxide and kills via suffocation (having displaced oxygen). Miners would try to alert themselves to dangerous levels of blackdamp and other gasses in a mine shaft by bringing a caged canary with them as they worked. The canary is more sensitive to environmental gasses 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 collect near the floor) by burning less brightly, while methane, another suffocating gas and explosion risk would make the lamp burn more brightly).

Carbon dioxide differential above outdoor levels 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. CO₂ is considered to be a surrogate for human bio-effluents and may correlate with other indoor pollutants. Higher CO₂ concentrations are associated with occupant health, comfort and performance degradation. ASHRAE Standard 62.1–2007 ventilation rates may result in indoor levels up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor ambient is 400 ppm, indoor levels may reach 2,500 ppm with ventilation rates that meet this industry consensus standard. Levels in poorly ventilated spaces can be found even higher than this (range of 3,000 or 4,000).

Human physiology[link]

Content[link]

The body produces approximately 2.3 pounds (1 kg) of carbon dioxide per day per person,^[83] containing 0.63 pounds (290 g) of carbon.

In humans, this carbon dioxide is carried through the venous system and is breathed out through the lungs. Therefore, the carbon dioxide content in the body is high in the venous system, and decreases in the respiratory system, resulting in lower levels along any arterial system. Carbon dioxide content in this sense is often given as the partial pressure, which is the pressure which carbon dioxide would have had if it alone occupied the volume.^[84]

In humans, the carbon dioxide contents are as follows:

Transport in the blood[link]

CO₂ 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 HCO−
  3 by the enzyme carbonic anhydrase in the red blood cells,^[88] by the reaction CO₂ + H₂O → H₂CO₃ → H⁺ + HCO−
  3.
• 5% – 10% is dissolved in the plasma^[88]
• 5% – 10% is bound to hemoglobin as carbamino compounds^[88]

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 effects on the hemoglobin molecule, the binding of CO₂ decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels 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 respiration[link]

Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its levels are 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 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.^[88]

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[link]

References[link]