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Carbon dioxide

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Carbon dioxide
Structural formula of carbon dioxide with bond length
Ball-and-stick model of carbon dioxide
Space-filling model of carbon dioxide
Names
Other names
  • Carbonic acid gas
  • Carbonic anhydride
  • Carbonic dioxide
  • Carbon(IV) oxide
  • R-744 (refrigerant)
  • R744 (refrigerant alternative spelling)
  • Dry ice (solid phase)
Identifiers
3D model (
JSmol
)
3DMet
1900390
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard
100.004.271 Edit this at Wikidata
EC Number
  • 204-696-9
E number E290 (preservatives)
989
KEGG
MeSH Carbon+dioxide
RTECS number
  • FF6400000
UNII
UN number 1013 (gas), 1845 (solid)
  • InChI=1S/CO2/c2-1-3 checkY
    Key: CURLTUGMZLYLDI-UHFFFAOYSA-N checkY
  • InChI=1/CO2/c2-1-3
    Key: CURLTUGMZLYLDI-UHFFFAOYAO
  • O=C=O
  • C(=O)=O
Properties
CO2
Molar mass 44.009 g·mol−1
Appearance Colorless gas
Odor
  • Low concentrations: none
  • High concentrations: sharp; acidic[1]
Density
  • 1562 kg/m3 (solid at 1 atm (100 kPa) and −78.5 °C (−109.3 °F))
  • 1101 kg/m3 (liquid at saturation −37 °C (−35 °F))
  • 1.977 kg/m3 (gas at 1 atm (100 kPa) and 0 °C (32 °F))
Critical point (T, P) 304.128(15) K[2] (30.978(15) °C), 7.3773(30) MPa[2] (72.808(30) atm)
194.6855(30) K (−78.4645(30) °C) at 1 atm (0.101325 MPa)
1.45 g/L at 25 °C (77 °F), 100 kPa (0.99 atm)
Vapor pressure 5.7292(30) MPa, 56.54(30) atm (20 °C (293.15 K))
Acidity (pKa) 6.35, 10.33
−20.5·10−6 cm3/mol
Thermal conductivity 0.01662 W·m−1·K−1 (300 K (27 °C; 80 °F))[3]
1.00045
Viscosity
  • 14.90 μPa·s at 25 °C (298 K)[4]
  • 70 μPa·s at −78.5 °C (194.7 K)
0 D
Structure
Trigonal
Linear
Thermochemistry
37.135 J/K·mol
214 J·mol−1·K−1
Std enthalpy of
formation
fH298)
−393.5 kJ·mol−1
Pharmacology
V03AN02 (WHO)
Hazards
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
90,000 ppm (human, 5 min)[6]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 5000 ppm (9000 mg/m3)[5]
REL (Recommended)
TWA 5000 ppm (9000 mg/m3), ST 30,000 ppm (54,000 mg/m3)[5]
IDLH
(Immediate danger)
40,000 ppm[5]
Safety data sheet (SDS) Sigma-Aldrich
Related compounds
Other anions
Other cations
Related carbon oxides
Related compounds
Supplementary data page
Carbon dioxide (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Carbon dioxide (chemical formula CO2) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature.

In the air, carbon dioxide is transparent to visible light but absorbs

ppm (about 0.04%) by volume, having risen from pre-industrial levels of 280 ppm.[9][10] Burning fossil fuels is the primary cause of these increased CO2 concentrations and also the primary cause of global warming and climate change.[11] Carbon dioxide is soluble in water and is found in groundwater, lakes, ice caps, and seawater. When carbon dioxide dissolves in water it forms carbonic acid (H2CO3), which causes ocean acidification as atmospheric CO2 levels increase.[12]

As the source of available carbon in the

atmospheric carbon dioxide is the primary carbon source for life on Earth. Its concentration in Earth's pre-industrial atmosphere since late in the Precambrian has been regulated by organisms and geological phenomena. Plants, algae and cyanobacteria use energy from sunlight to synthesize carbohydrates from carbon dioxide and water in a process called photosynthesis, which produces oxygen as a waste product.[13] In turn, oxygen is consumed and CO2 is released as waste by all aerobic organisms when they metabolize organic compounds to produce energy by respiration.[14] CO2 is released from organic materials when they decay
or combust, such as in forest fires. Since plants require CO2 for photosynthesis, and humans and animals depend on plants for food, CO2 is necessary for the survival of life on earth.

Carbon dioxide is 53% more dense than dry air, but is long lived and thoroughly mixes in the atmosphere. About half of excess CO2 emissions to the atmosphere are absorbed by

dissolve
in water or react with acids.

CO2 is a versatile industrial material, used, for example, as an inert gas in welding and

soda water in the mouth,[25] but at normally encountered concentrations it is odorless.[1]

History

Crystal structure of dry ice

Carbon dioxide was the first gas to be described as a discrete substance. In about 1640,[26] 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 sylvestris).[27]

The properties of carbon dioxide were further studied 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 limewater (a saturated aqueous solution of 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.[28]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.[29] The earliest description of solid carbon dioxide (dry ice) 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.[30][31]

Chemical and physical properties

Structure, bonding and molecular vibrations

The

carbonyls.[32] Since it is centrosymmetric, the molecule has no electric dipole moment
.

As a linear triatomic molecule, CO2 has four vibrational modes as shown in the diagram. In the symmetric and the antisymmetric stretching modes, the atoms move along the axis of the molecule. There are two bending modes, which are degenerate, meaning that they have the same frequency and same energy, because of the symmetry of the molecule. When a molecule touches a surface or touches another molecule, the two bending modes can differ in frequency because the interaction is different for the two modes. Some of the vibrational modes are observed in the infrared (IR) spectrum: the antisymmetric stretching mode at wavenumber 2349 cm−1 (wavelength 4.25 μm) and the degenerate pair of bending modes at 667 cm−1 (wavelength 15 μm). The symmetric stretching mode does not create an electric dipole so is not observed in IR spectroscopy, but it is detected in by Raman spectroscopy at 1388 cm−1 (wavelength 7.2 μm).[33]

In the gas phase, carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure. However, in a Coulomb explosion imaging experiment, an instantaneous image of the molecular structure can be deduced. Such an experiment[34] has been performed for carbon dioxide. The result of this experiment, and the conclusion of theoretical calculations[35] based on an ab initio potential energy surface of the molecule, is that none of the molecules in the gas phase are ever exactly linear.

In aqueous solution

Carbon dioxide is

soluble in water, in which it reversibly forms H2CO3 (carbonic acid), which is a weak acid
since its ionization in water is incomplete.

The 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 CO2, H2CO3, and the deprotonated forms HCO3 (bicarbonate) and CO2−3(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, carbonic acid has two acid dissociation constants
, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO3):

Ka1 = 2.5×10−4 mol/L; pKa1 = 3.6 at 25 °C.[32]

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 4.16×10−7 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 CO2remains as CO2 molecules, Ka1(apparent) has a much larger denominator and a much smaller value than the true Ka1.[36]

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 (CO2−3):

Ka2 = 4.69×10−11 mol/L; pKa2 = 10.329

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

Chemical reactions of CO2

CO2 is a potent

organolithium compounds react with CO2 to give carboxylates
:

where M = Li or Mg 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.[38]

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

Photoautotrophs (i.e. plants and cyanobacteria) use the energy contained in sunlight to photosynthesize simple sugars
from CO2 absorbed from the air and water:

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.[39]

Physical properties

Carbon dioxide is colorless. At low concentrations the gas is odorless; however, at sufficiently high concentrations, it has a sharp, acidic odor.

Carbon dioxide has no liquid state at pressures below 0.51795(10) MPa

sublimes directly to a gas above this temperature. In its solid state, carbon dioxide is commonly called dry ice
.

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 dioxide (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.

Table of thermal and physical properties of saturated liquid carbon dioxide:[42][43]

Temperature (°C) Density (kg/m^3) Specific heat (kJ/kg K) Kinematic viscosity (m^2/s) Conductivity (W/m K) Thermal diffusivity (m^2/s) Prandtl Number Bulk modulus (K^-1)
-50 1156.34 1.84 1.19E-07 0.0855 4.02E-08 2.96 -
-40 1117.77 1.88 1.18E-07 0.1011 4.81E-08 2.46 -
-30 1076.76 1.97 1.17E-07 0.1116 5.27E-08 2.22 -
-20 1032.39 2.05 1.15E-07 0.1151 5.45E-08 2.12 -
-10 983.38 2.18 1.13E-07 0.1099 5.13E-08 2.2 -
0 926.99 2.47 1.08E-07 0.1045 4.58E-08 2.38 -
10 860.03 3.14 1.01E-07 0.0971 3.61E-08 2.8 -
20 772.57 5 9.10E-08 0.0872 2.22E-08 4.1 1.40E-02
30 597.81 36.4 8.00E-08 0.0703 0.279E-08 28.7 -

Table of thermal and physical properties of carbon dioxide (CO2) at atmospheric pressure:[42][43]

Temperature (K) Density (kg/m^3) Specific heat (kJ/kg °C) Dynamic viscosity (kg/m s) Kinematic viscosity (m^2/s) Thermal conductivity (W/m °C) Thermal diffusivity (m^2/s) Prandtl Number
220 2.4733 0.783 1.11E-05 4.49E-06 0.010805 5.92E-06 0.818
250 2.1657 0.804 1.26E-05 5.81E-06 0.012884 7.40E-06 0.793
300 1.7973 0.871 1.50E-05 8.32E-06 0.016572 1.06E-05 0.77
350 1.5362 0.9 1.72E-05 1.12E-05 0.02047 1.48E-05 0.755
400 1.3424 0.942 1.93E-05 1.44E-05 0.02461 1.95E-05 0.738
450 1.1918 0.98 2.13E-05 1.79E-05 0.02897 2.48E-05 0.721
500 1.0732 1.013 2.33E-05 2.17E-05 0.03352 3.08E-05 0.702
550 0.9739 1.047 2.51E-05 2.57E-05 0.03821 3.75E-05 0.685
600 0.8938 1.076 2.68E-05 3.00E-05 0.04311 4.48E-05 0.668
650 0.8143 1.1 2.88E-05 3.54E-05 0.0445 4.97E-05 0.712
700 0.7564 1.13E+00 3.05E-05 4.03E-05 0.0481 5.63E-05 0.717
750 0.7057 1.15 3.21E-05 4.55E-05 0.0517 6.37E-05 0.714
800 0.6614 1.17E+00 3.37E-05 5.10E-05 0.0551 7.12E-05 0.716

Isolation and production

Carbon 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.[44]

The combustion of all carbon-based fuels, such as methane (natural gas), petroleum distillates (gasoline, 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:

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide:[45]

Carbon dioxide is a byproduct of the industrial production of hydrogen by

water gas shift reaction in ammonia production. These processes begin with the reaction of water and natural gas (mainly methane).[46] This is a major source of food-grade carbon dioxide for use in carbonation of beer 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.[47]

Carbonates

It is produced by thermal decomposition of limestone, CaCO3 by heating (

calcining) at about 850 °C (1,560 °F), in the manufacture of quicklime (calcium oxide
, CaO), a compound that has many industrial uses:

Acids liberate CO2 from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxide

springs, where it is produced by the action of acidified water on limestone or dolomite. The reaction between hydrochloric acid
and calcium carbonate (limestone or chalk) is shown below:

The carbonic acid (H2CO3) 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.

Fermentation

Carbon dioxide is a by-product of the

bioethanol. Yeast metabolizes sugar to produce CO2 and ethanol
, also known as alcohol, as follows:

All aerobic organisms produce CO2 when they oxidize carbohydrates, 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:

Anaerobic organisms decompose organic material producing methane and carbon dioxide together with traces of other compounds.[48] Regardless of the type of organic material, the production of gases follows well defined 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.[49]

Applications

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry.[44] 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 chemicals

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of

Kolbe-Schmitt reaction.[51]

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.[52]

Agriculture

Plants 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.

whiteflies and spider mites in a greenhouse.[55]

Foods

Carbon dioxide is a

INS number
290).

A candy called Pop Rocks is pressurized with carbon dioxide gas[59] at about 4,000 kPa (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.

baking soda release carbon dioxide when heated or if exposed to acids
.

Beverages

Carbon dioxide is used to produce

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.

Winemaking

Carbon dioxide in the form of

grape must, and thus the 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 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 animals

Carbon dioxide is often used to "stun" animals before slaughter.[62] "Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress.[63][64]

Inert gas

Carbon dioxide 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

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.

Carbon dioxide 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 

scanning electron microscopy[65] and in the decaffeination of coffee beans
.

Fire extinguisher

Carbon 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

atmospheric oxygen. They are mainly used in server rooms.[66]

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.[67] International Maritime Organization standards 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.[68]

Supercritical CO2 as solvent

Liquid carbon dioxide is a good

dry cleaners for this reason. It is used in the preparation of some aerogels
because of the properties of supercritical carbon dioxide.

Medical and pharmacological uses

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.

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.

Another medical use are the mofette, dry spas that use carbon dioxide from post-volcanic discharge for therapeutic purposes.

Energy

Supercritical CO2 is used as the working fluid in the Allam power cycle engine.

Fossil fuel recovery

Carbon dioxide is used in

crude oil, significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well.[70]
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.[71]

Bio transformation into fuel

It has been proposed that CO2 from power generation be bubbled into ponds to stimulate growth of

cyanobacterium Synechococcus elongatus has been genetically engineered to produce the fuels isobutyraldehyde and isobutanol from CO2 using photosynthesis.[73]

Researchers have developed a process called electrolysis, using enzymes isolated from bacteria to power the chemical reactions which convert CO2 into fuels.[74][75][76]

Refrigerant

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 (−109.3 °F) 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 use[

R134a, a hydrofluorocarbon (HFC) compound) contributes to climate change more than CO2 does. CO2 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 130 bars (1,900 psi; 13,000 kPa), CO2 systems require highly mechanically resistant reservoirs and 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°, CO2 (R744) operates more efficiently than systems using HFCs (e.g., R134a). 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, and heat pump water heaters, among others. Coca-Cola has fielded CO2-based beverage coolers and the U.S. Army is interested in CO2 refrigeration and heating technology.[77][78]

Minor uses

Carbon dioxide is the 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,[79] 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 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.

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. The American Veterinary Medical Association's 2020 guidelines for carbon dioxide induction state that a displacement rate of 30% to 70% of the chamber or cage volume per minute is optimal for the humane euthanasia of small rodents.[80]: 5, 31  Percentages of CO2 vary for different species, based on identified optimal percentages to minimize distress.[80]: 22 

Carbon dioxide is also used in several related cleaning and surface-preparation techniques.

In Earth's atmosphere

Keeling curve of the atmospheric CO2 concentration[81]

Carbon 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.[82][83] Atmospheric CO2 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 near the ground with much smaller variations aloft. In urban areas concentrations are generally higher[84] and indoors they can reach 10 times background levels. CO2 emissions have also led to the stratosphere contracting by 400 meters since 1980, which could affect satellite operations, GPS systems and radio communications.[85]

The concentration of carbon dioxide has risen due to human activities.

burning of fossil fuels remains in the atmosphere and is not absorbed by vegetation and the oceans.[91][92][93][94]

While transparent to

visible light, carbon dioxide is a greenhouse gas, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies (see the section "Structure and bonding" above). Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm−1,[95] 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 CO2 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.[96]

Annual CO2 flows from anthropogenic sources (left) into Earth's atmosphere, land, and ocean sinks (right) since the 1960s. Units in equivalent gigatonnes carbon per year.[88]

Increases in atmospheric concentrations of CO2 and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have 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.

fast carbon cycle.[97] This means that some fraction (a projected 20-35%) of the fossil carbon transferred thus far will persist in the atmosphere as elevated CO2 levels for many thousands of years after these carbon transfer activities begin to subside.[98][99][100]
Not only do increasing CO2 concentrations lead to increases in global surface temperature, but increasing global temperatures also cause increasing concentrations of carbon dioxide. This produces a
a particularly swift reduction occurring 49 million years ago.[102][103]

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 100 m (330 ft) 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.[104] 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.[105]

In the oceans

ocean chemistry
projected for the year 2100.

Carbon 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.[106]

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

pteropods[118]
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)[119] 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,[120] 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,[121] although there's evidence of increased shell production by certain species under increased CO2 content.[122]

The U.S. National Oceanic and Atmospheric Administration (NOAA) states in their May 2008 "State of the science fact sheet for ocean acidification"[123] 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:[124]

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, the

OSPAR Commission, the Northwest Association of Networked Ocean Observing Systems, and the IPCC, because their most current research shows that ocean acidification should be expected to impact them negatively.[125]

Carbon dioxide is also introduced into the oceans through hydrothermal vents. The Champagne hydrothermal vent, found at the Northwest Eifuku volcano in the Mariana Trench, produces almost pure liquid carbon dioxide, one of only two known sites in the world as of 2004, the other being in the Okinawa Trough.[126] The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006.[127]

Biological role

Carbon dioxide is an end product of

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, plants can absorb more carbon dioxide from the atmosphere than they release
in respiration.

Photosynthesis and carbon fixation

Carbon fixation is a biochemical process by which atmospheric carbon dioxide is incorporated by plants, algae and (cyanobacteria) into 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.

ribulose bisphosphate
, as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth.[128]

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).