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	Carbon dioxide has no liquid state at pressures below 5.1 ]. At 1 atmosphere (near mean sea level pressure), the gas ] directly to a solid at temperatures below {{convert\|-78\|C\|F K}} and the solid ] directly to a gas above −78 °C. In its solid state, carbon dioxide is commonly called ].		Carbon dioxide has no liquid state at pressures below 5.1 ]. At 1 atmosphere (near mean sea level pressure), the gas ] directly to a solid at temperatures below {{convert\|-78\|C\|F K}} and the solid ] directly to a gas above −78 °C. In its solid state, carbon dioxide is commonly called ].

	CO<sub>2</sub> is an ]: an aqueous solution turns ] from blue to pink. It is the ] of ], an acid which is unstable in aqueous solution, from which it cannot be concentrated. However it can be produced by irradiating frozen mixtures of water and carbon dioxide in a vacuum. In organisms carbonic acid production is catalysed by the ], ].		CO<sub>2</sub> is an ]: an aqueous solution turns ] from blue to pink. It is the ] of ], an acid which is unstable in aqueous solution, from which it cannot be concentrated. However it can be produced by irradiating frozen mixtures of water and carbon dioxide in vacuum. In organisms carbonic acid production is catalysed by the ], ].

	: {{chem\|CO\|2}} + {{chem\|H\|2\|O}} ⇌ {{chem\|H\|2\|CO\|3}}		: {{chem\|CO\|2}} + {{chem\|H\|2\|O}} ⇌ {{chem\|H\|2\|CO\|3}}

Revision as of 22:49, 22 May 2010

"CO2" redirects here. For the UK postal district, see CO postcode area.

Carbon dioxide

Names

IUPAC name Carbon dioxide

Other names Carbonic acid gas; carbonic anhydride; dry ice (solid)

Identifiers

CAS Number

124-38-9

3D model (JSmol)

Interactive image

204-696-9

E number

E290 (preservatives)

PubChem CID

RTECS number

FF6400000

UN number

1013
Solid (dry ice): 1845
Mixtures with Ethylene oxide: 1952, 3300

CompTox Dashboard (EPA)

DTXSID4027028

InChI

InChI=1/CO2/c2-1-3Key: CURLTUGMZLYLDI-UHFFFAOYAO

SMILES

O=C=O

Properties

Chemical formula

CO₂

Molar mass

44.010 g/mol

Appearance

colorless, odorless gas

Density

1.562 g/mL (solid at 1 atm and −78.5 °C)
0.770 g/mL (liquid at 56 atm and 20 °C)
1.977 g/L (gas at 1 atm and 0 °C)
849.6 g/L (supercritical fluid at 150 atm and 30 °C

Melting point

−78.5 °C; −109.2 °F; 194.7 K

Boiling point

−56.6 °C; −69.8 °F; 216.6 K

Solubility in water

1.45 g/L at 25 °C, 100 kPa

Acidity (pK_a)

6.35, 10.33

Refractive index (n_D)

1.1120

Viscosity

0.07 cP at −78 °C

Dipole moment

zero

Structure

Molecular shape

linear

Related compounds

Other anions

Carbon disulfide
Carbon diselenide

Other cations

Silicon dioxide
Germanium dioxide
Tin dioxide
Lead dioxide

Supplementary data page

Carbon dioxide (data page)

Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa).

Y verify (what is ?) Infobox references

Chemical compound

Carbon dioxide (chemical formula CO₂) is a 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. CO₂ is a trace gas comprising 0.039% of the atmosphere.

Carbon dioxide is used by plants during photosynthesis to make sugars, which may either be consumed in respiration or used as the raw material to produce other organic compounds needed for plant growth and development. It is produced during respiration by plants, and by all animals, fungi and microorganisms that depend either directly or indirectly on plants for food. It is thus a major component of the carbon cycle. Carbon dioxide is generated as a by-product of the combustion of fossil fuels or the burning of vegetable matter, among other chemical processes. Amounts of carbon dioxide are emitted from volcanoes and other geothermal processes such as hot springs and geysers and by the dissolution of carbonates in crustal rocks.

As of April 2010, carbon dioxide in the Earth's atmosphere is at a concentration of 391 ppm by volume. 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. Carbon dioxide is a greenhouse gas as it transmits visible light but absorbs strongly in the infrared and near-infrared.

Carbon dioxide has no liquid state at pressures below 5.1 atmospheres. At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below −78 °C (−108 °F; 195 K) and the solid sublimes directly to a gas above −78 °C. In its solid state, carbon dioxide is commonly called dry ice.

CO₂ is an acidic oxide: an aqueous solution turns litmus from blue to pink. It is the anhydride of carbonic acid, an acid which is unstable in aqueous solution, from which it cannot be concentrated. However it can be produced by irradiating frozen mixtures of water and carbon dioxide in vacuum. In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.

CO
₂ + H
₂O ⇌ H
₂CO
₃

CO₂ is toxic in higher concentrations: 1% (10,000 ppm) will make some people feel drowsy. Concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.

Chemical and physical properties

Further information: Carbon dioxide (data page)

Carbon dioxide is colorless. At low concentrations, the gas is odorless. At higher concentrations it has a sharp, acidic odor. It will act as an asphyxiant and an irritant. When inhaled at concentrations much higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the mucous membranes and saliva, forming a weak solution of carbonic acid. This sensation can also occur during an attempt to stifle a burp after drinking a carbonated beverage. Amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm (equal to 5% by volume) are considered dangerous to animal life.

At standard temperature and pressure, the density of carbon dioxide is around 1.98 kg/m, about 1.5 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole, and as it is fully oxidized, it is moderately reactive and is non-flammable, but will support the combustion of metals such as magnesium.

Above -78.51° C or -109.3° F, carbon dioxide changes directly from a solid phase to a gaseous phase through sublimation, or from gaseous to solid through deposition. Solid carbon dioxide is normally called "dry ice", a generic trademark. It was first observed in 1825 by the French chemist Charles Thilorier. Dry ice is commonly used as a cooling agent, and it is relatively inexpensive. A convenient property for this purpose is that solid carbon dioxide sublimes directly into the gas phase leaving no liquid. It can often be found in grocery stores and laboratories, and it is also used in the shipping industry. The largest non-cooling use for dry ice is blast cleaning.

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.

An alternative form of solid carbon dioxide, an amorphous glass-like form, is possible, although not at atmospheric pressure. This form of glass, called carbonia, was 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. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.

See also: Supercritical carbon dioxide, dry ice, and MO diagram

History of human understanding

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).

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. This was the invention of Soda water.

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 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.

Isolation and production

Carbon dioxide may be obtained from air distillation. However, this yields only very small quantities of CO₂. A large variety of chemical reactions yield carbon dioxide, 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+ CaCO
₃→ CaCl
₂+ H
₂CO
₃

The H
₂CO
₃ 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.

The production of quicklime (CaO) a chemical that has widespread use, from limestone by heating at about 850 °C also produces CO₂:

CaCO
₃→ CaO + CO
₂

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.

CH
₄+ 2 O
₂→ CO
₂+ 2 H
₂O

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

Fe
₂O
₃+ 3 CO → 2 Fe + 3 CO
₂

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:

C
₆H
₁₂O
₆ → 2 CO
₂+ 2 C
₂H
₅OH

All aerobic organisms produce CO
₂ 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). Photoautotrophs (i.e. plants, cyanobacteria) use another modus operandi: Plants absorb CO
₂ from the air, and, together with water, react it to form carbohydrates:

nCO₂ + nH
₂O → (CH
₂O)_n + nO
₂

Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO₂ and H
₂CO
₃ (carbonic acid). The relative concentrations of CO
₂, H
₂CO
₃, and the deprotonated forms HCO
₃ (bicarbonate) and CO
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, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble, such that air-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate per liter.

Industrial production

Industrial carbon dioxide is produced mainly from six processes:

From combustion of fossil fuels and wood;
As a by-product of hydrogen production plants, where methane is converted to CO₂;
As a by-product of fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages;
From thermal decomposition of limestone, CaCO
₃, in the manufacture of lime, CaO;
As a by-product of sodium phosphate manufacture;
Directly from natural carbon dioxide springs, where it is produced by the action of acidified water on limestone or dolomite.

Uses

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry. 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, such as the Common Clothes Moth.

Drinks

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 artificially. In the case of bottled and kegged beer, artificial carbonation is now the most common method used. With the exception of British Real Ale, draught (draft) beer is usually transferred from kegs in a cold room or cellar to dispensing taps on the bar using pressurised carbon dioxide, often mixed with nitrogen.

Foods

A candy called Pop Rocks is pressurized with carbon dioxide gas at about 40 bar (600 psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

Leavening agents produce carbon dioxide to cause dough to rise. 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.

Pneumatic systems

Carbon dioxide is one of the most commonly used compressed gases for pneumatic (pressurized gas) systems in portable pressure tools and combat robots.

Fire extinguisher

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, as it is so dry. 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, (National Fire Protection Association Code 12). 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, due to the fact that 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 (Carbon Dioxide as a Fire Suppressant: Examining the Risks, US EPA) identified 51 incidents between 1975 and the date of the report, causing 72 deaths and 145 injuries.

Welding

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.

Caffeine removal

Liquid carbon dioxide is a good solvent for many lipophilic organic compounds, and is used to remove caffeine from coffee. First, the green coffee beans are soaked in water. The beans are placed in the top of a column seventy feet (21 m) high. Then supercritical carbon dioxide in fluid form at about 93 °C enters at the bottom of the column. The caffeine diffuses out of the beans and into the carbon dioxide.

Pharmaceutical and other chemical processing

Carbon dioxide has begun to attract attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochlorides. It's used by some dry cleaners for this reason. (See green chemistry.)

In the chemical industry, carbon dioxide is used for the production of urea, carbonates and bicarbonates, and sodium salicylate.

Agricultural and biological applications

Plants require carbon dioxide to conduct photosynthesis. Greenhouses may (and of large size - must) enrich their atmospheres with additional CO₂ to sustain plant life and growth. 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 (a factor of 100 or more higher than its atmospheric concentration), 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.

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. Carbon dioxide is already increasingly used in greenhouses as the main carbon source for Spirulina algae. In medicine, up to 5% carbon dioxide (130 times the atmospheric concentration) is added to pure oxygen for stimulation of breathing after apnea and to stabilize the O
₂/CO
₂ balance in blood.

Lasers

A common type of industrial gas laser is the carbon dioxide laser.

Polymers and plastics

Carbon dioxide can also be combined with limonene oxide from orange peels or other epoxides to create polymers and plastics.

Oil recovery

Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under supercritical conditions. 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. In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

As refrigerants

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.

Liquid carbon dioxide (industry nomenclature R744 or R-744) was used as a refrigerant prior to the discovery of R-12 and is likely to enjoy a renaissance due to environmental concerns. 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.

By the end of 2007, the global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO₂ is one discussed option.(see The Cool War)

Coal bed methane recovery

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

Wine making

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 regular air. The settled carbon dioxide gas creates an 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 gasses such as nitrogen or argon are preferred for this process by professional wine makers.

pH control

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.

CO₂ is also used in the keeping of 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

Main article: Carbon dioxide in Earth's atmosphere Main article: Carbon cycle

Carbon dioxide in earth's atmosphere is considered a trace gas currently occurring at an average concentration of about 383 parts per million by volume or 582 parts per million by mass. The total mass of atmospheric carbon dioxide is 3.0×10 kg (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.

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. 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.

Up to 40% of the gas emitted by some volcanoes during subaerial eruptions is carbon dioxide. It is estimated that volcanoes release about 130-230 million tonnes (145-255 million tons) of CO₂ into the atmosphere each year. This is about 100 times smaller than the sources from human activity. 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. 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. 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.

In the oceans

Main article: Carbon cycle

There is about fifty times as much carbon dissolved in the sea water of the oceans in the form of CO₂ and carbonic acid, bicarbonate and carbonate ions 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. 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 forms carbonic acid in equilibrium with bicarbonate and carbonate ions. Some is consumed in photosynthesis by organisms in the water, and a small proportion of that sinks and leaves the carbon 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 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.
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 :
"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."

IPCC also includes in its last report that with a probability greater than 0.66: "the resilience of many ecosystems is likely to be exceeded in this century by an unprecedented combination of climate change, associated disturbances (e.g. flooding, drought, wildfire, insects, ocean acidification) and other global change drivers (e.g. landuse change, pollution, fragmentation of natural systems, overexploitation of resources)."

Some marine calcifying organisms (like 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.

Biological role

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.

Role in photosynthesis

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic compounds (cellulose, lipids, and various proteins) by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to make 3-phosphoglycerate that is used in metabolism, to construct sugars that can be used as an energy source within the plant through respiration and as the raw material for the construction of more complex organic molecules, such as polysaccharides, nucleic acids and proteins during growth.

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. Some people (for example David Bellamy) believe that as the concentration of CO₂ rises in the atmosphere that it will lead to faster plant growth and therefore increase food production. Such views are controversial. Studies have shown that increased CO₂ leads to fewer stomata developing on plants which leads to reduced water usage. Studies using FACE have shown that increases in CO₂ lead to decreased concentration 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.

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. 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." 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.

Toxicity

Human physiology

References

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Shendell, Prill, Fisk, Apte1, Blake & Faulkner, Associations between classroom CO2 concentrations and student attendance in Washington and Idaho, Indoor Air 2004. SEPPANEN, FISK AND MENDELL, Association of Ventilation Rates and CO2 Concentrations with Health and Other Responses in Commercial and Institutional Buildings, Indoor Air 1999.

External links

v t e Oxocarbons
Common oxides	CO CO₂
Exotic oxides	CO₃ CO₄ CO₅ CO₆ C₂O C₂O₂ C₂O₃ C₂O₄ (1,2-Dioxetanedione and 1,3-Dioxetanedione) C₃O C₃O₂ C₃O₃ C₃O₆ C₄O₂ C₄O₄ C₄O₆ C₅O₂ C₅O₅ C₆O₆ (Cyclohexanehexone and Ethylenetetracarboxylic dianhydride) C₈O₈ C₉O₉ C₁₀O₈ C₁₀O₁₀ C₁₂O₆ C₁₂O₉ C₁₂O₁₂
Polymers	Graphite oxide C₃O₂ CO CO₂
Compounds derived from oxides	Metal carbonyls Carbonic acid Bicarbonates Carbonates Polycarbonates (Dicarbonates and Tricarbonates) Peroxydicarbonates

Climate change

Overview
Causes of climate change Effects of climate change Climate change mitigation Climate change adaptation By country and region

Causes

Overview	Climate system Greenhouse effect (Carbon dioxide in Earth's atmosphere) Scientific consensus on climate change
Sources	Deforestation Fossil fuel Greenhouse gases Greenhouse gas emissions Carbon accounting Carbon footprint Carbon leakage from agriculture from wetlands World energy supply and consumption

History
History of climate change policy and politics History of climate change science Svante Arrhenius James Hansen Charles David Keeling United Nations Climate Change conferences Years in climate change 2019 2020 2021 2022 2023 2024

Effects and issues

Physical	Abrupt climate change Anoxic event Arctic methane emissions Arctic sea ice decline Atlantic meridional overturning circulation Drought Extreme weather Flood Coastal flooding Heat wave Marine Urban heat island Oceans acidification deoxygenation heat content sea surface temperature stratification temperature Ozone depletion Permafrost thaw Retreat of glaciers since 1850 Sea level rise Season creep Tipping points in the climate system Tropical cyclones Water cycle Wildfires
Flora and fauna	Biomes Mass mortality event Birds Extinction risk Forest dieback Invasive species Marine life Plant biodiversity
Social and economic	Agriculture Livestock United States Children Cities Civilizational collapse Crime Depopulation of settlements Destruction of cultural heritage Disability Economic impacts U.S. insurance industry Fisheries Gender Health Mental health Human rights Indigenous peoples Infectious diseases Migration Poverty Psychological impacts Security and conflict Urban flooding Water scarcity Water security
By country and region	Africa Americas Antarctica Arctic Asia Australia Caribbean Europe Middle East and North Africa Small island countries by individual country

Mitigation

Economics and finance	Carbon budget Carbon emission trading Carbon offsets and credits Gold Standard (carbon offset standard) Carbon price Carbon tax Climate debt Climate finance Climate risk insurance Co-benefits of climate change mitigation Economics of climate change mitigation Fossil fuel divestment Green Climate Fund Low-carbon economy Net zero emissions
Energy	Carbon capture and storage Energy transition Fossil fuel phase-out Nuclear power Renewable energy Sustainable energy
Preserving and enhancing carbon sinks	Blue carbon Carbon dioxide removal Carbon sequestration Direct air capture Carbon farming Climate-smart agriculture Forest management afforestation forestry for carbon sequestration REDD and REDD+ reforestation Land use, land-use change, and forestry (LULUCF and AFOLU) Nature-based solutions
Personal	Individual action on climate change Plant-based diet

Society and adaptation

Society	Business action Climate action Climate emergency declaration Climate movement School Strike for Climate Denial Ecological grief Governance Justice Litigation Politics Public opinion Women
Adaptation	Adaptation strategies on the German coast Adaptive capacity Disaster risk reduction Ecosystem-based adaptation Flood control Loss and damage Managed retreat Nature-based solutions Resilience Risk Vulnerability The Adaptation Fund National Adaptation Programme of Action
Communication	Climate Change Performance Index Climate crisis (term) Climate spiral Education Media coverage Popular culture depictions art fiction video games Warming stripes
International agreements	Glasgow Climate Pact Kyoto Protocol Paris Agreement Cooperative Mechanisms under Article 6 of the Paris Agreement Nationally determined contributions Sustainable Development Goal 13 United Nations Framework Convention on Climate Change

Background and theory

Measurements	Global surface temperature Instrumental temperature record Proxy Satellite temperature measurement
Theory	Albedo Carbon cycle atmospheric biologic oceanic permafrost Carbon sink Climate sensitivity Climate variability and change Cloud feedback Cloud forcing Fixed anvil temperature hypothesis Cryosphere Earth's energy budget Extreme event attribution Feedbacks Global warming potential Illustrative model of greenhouse effect on climate change Orbital forcing Radiative forcing
Research and modelling	Climate change scenario Climate model Coupled Model Intercomparison Project Intergovernmental Panel on Climate Change (IPCC) IPCC Sixth Assessment Report Paleoclimatology Representative Concentration Pathway Shared Socioeconomic Pathways

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