# Carbon dioxide

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"CO2" and "CO₂" redirect here. For other uses, see [CO2 (disambiguation)](/source/CO2_(disambiguation)).

Carbon dioxide Structural formula of carbon dioxide with bond length Ball-and-stick model of carbon dioxide Space-filling model of carbon dioxide Names IUPAC name Carbon dioxide Other names Carbonic acid gas Carbonic anhydride Carbonic dioxide Carbonic oxide Carbon(IV) oxide Methanedione R-744 (refrigerant) R744 (refrigerant alternative spelling) Dry ice (solid phase) Identifiers CAS Number 124-38-9 Y 3D model (JSmol) Interactive image Interactive image Beilstein Reference 1900390 ChEBI CHEBI:16526 Y ChEMBL ChEMBL1231871 N ChemSpider 274 Y ECHA InfoCard 100.004.271 EC Number 204-696-9 E number E290 (preservatives) Gmelin Reference 989 KEGG D00004 Y MeSH Carbon+dioxide PubChem CID 280 RTECS number FF6400000 UNII 142M471B3J Y UN number 1013 (gas), 1845 (solid) CompTox Dashboard (EPA) DTXSID4027028 InChI InChI=1S/CO2/c2-1-3 Y Key: CURLTUGMZLYLDI-UHFFFAOYSA-N Y InChI=1/CO2/c2-1-3 Key: CURLTUGMZLYLDI-UHFFFAOYAO SMILES O=C=O C(=O)=O Properties Chemical formula 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) Sublimation conditions 194.6855(30) K (−78.4645(30) °C) at 1 atm (0.101325 MPa) Solubility in water 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) Carbonic acid: pKa1 = 3.6 pKa1(apparent) = 6.35 pKa2 = 10.33 Magnetic susceptibility (χ) −20.5·10−6 cm3/mol Thermal conductivity 0.01662 W·m−1·K−1 (300 K (27 °C; 80 °F))[3] Refractive index (nD) 1.00045 Viscosity 14.90 μPa·s at 25 °C (298 K)[4] 70 μPa·s at −78.5 °C (194.7 K) Dipole moment 0 D Structure Crystal structure Trigonal Point group D∞h Molecular shape Linear Thermochemistry Heat capacity (C) 37.135 J/(K·mol) Std molar entropy (S⦵298) 214 J·mol−1·K−1 Std enthalpy of formation (ΔfH⦵298) −393.5 kJ·mol−1 Pharmacology ATC code V03AN02 (WHO) Hazards GHS labelling: Pictograms Signal word Warning Hazard statements H280, H281 Precautionary statements P282, P336+P317, P403, P410+P403 NFPA 704 (fire diamond) [7][8] 2 0 0 SA Lethal dose or concentration (LD, LC): LCLo (lowest published) 90,000 ppm (162,000 mg/m3) (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 (72,000 mg/m3)[5] Safety data sheet (SDS) Sigma-Aldrich Related compounds Other anions Carbon disulfide Carbon diselenide Carbon ditelluride Other cations Silicon dioxide Germanium dioxide Tin dioxide Lead dioxide Titanium dioxide Zirconium dioxide Hafnium dioxide Cerium dioxide Thorium dioxide Related carbon oxides See Oxocarbon Related compounds Carbonic acid Carbonyl sulfide Carbonyl selenide 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 YN ?) Infobox references

Chemical compound

**Carbon dioxide** is a [chemical compound](/source/Chemical_compound) with the [chemical formula](/source/Chemical_formula) **CO2**. It is made up of [molecules](/source/Molecule) that each have one [carbon](/source/Carbon) atom [covalently](/source/Covalent_bond) [double bonded](/source/Double_bond) to two [oxygen](/source/Oxygen) atoms. It is found in a gas state at room temperature and at normally-encountered concentrations it is odorless. As the source of carbon in the [carbon cycle](/source/Carbon_cycle), atmospheric CO2 is the primary carbon source for life on Earth. In the air, carbon dioxide is transparent to visible light but absorbs [infrared radiation](/source/Infrared), acting as a [greenhouse gas](/source/Greenhouse_gas). Carbon dioxide is soluble in water and is found in [groundwater](/source/Groundwater), [lakes](/source/Lake), [ice caps](/source/Ice_cap), and [seawater](/source/Seawater).

It is a [trace gas](/source/Trace_gas) [in Earth's atmosphere](/source/Carbon_dioxide_in_Earth's_atmosphere) at 428 [parts per million](/source/Parts_per_million) (ppm),[a] or about 0.043% (as of July 2025) having risen from pre-industrial levels of 280 ppm or about 0.028%.[10][11] Burning [fossil fuels](/source/Fossil_fuel) is the main cause of these increased CO2 concentrations, which are the primary cause of [climate change](/source/Climate_change).[12]

Its [concentration](/source/Concentration) in Earth's pre-industrial atmosphere since late in the [Precambrian](/source/Precambrian) was regulated by organisms and geological features. [Plants](/source/Plant), [algae](/source/Algae) and [cyanobacteria](/source/Cyanobacteria) use [energy](/source/Energy) from [sunlight](/source/Sunlight) to synthesize [carbohydrates](/source/Carbohydrate) from carbon dioxide and water in a process called [photosynthesis](/source/Photosynthesis), which produces oxygen as a waste product.[13] In turn, oxygen is consumed and CO2 is released as waste by all [aerobic organisms](/source/Aerobic_organism) when they metabolize [organic compounds](/source/Organic_compound) to produce energy by [respiration](/source/Cellular_respiration).[14] CO2 is released from organic materials when they [decay](/source/Decomposition) or combust, such as in forest fires. When carbon dioxide dissolves in water, it forms [carbonate](/source/Carbonate) and mainly [bicarbonate](/source/Bicarbonate) (HCO3–), which causes [ocean acidification](/source/Ocean_acidification) as [atmospheric CO2](/source/Carbon_dioxide_in_Earth's_atmosphere) levels increase.[15]

Carbon dioxide is 53% denser 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 [land](/source/Carbon_fixation) and ocean [carbon sinks](/source/Carbon_sink).[16] These sinks can become saturated and are volatile, as decay and [wildfires](/source/Wildfire) result in the CO2 being released back into the atmosphere.[17] CO2, or the carbon it holds, is eventually [sequestered](/source/Carbon_sequestration) (stored for the long term) in rocks and organic deposits like [coal](/source/Coal), [petroleum](/source/Petroleum) and [natural gas](/source/Natural_gas).

Nearly all CO2 produced by humans goes into the atmosphere. Less than 1% of CO2 produced annually is put to commercial use, mostly in the fertilizer industry and in the oil and gas industry for [enhanced oil recovery](/source/Enhanced_oil_recovery). Other commercial applications include food and beverage production, metal fabrication, cooling, fire suppression and stimulating plant growth in greenhouses.[18]: 3

## Chemical and physical properties

### Structure, bonding and molecular vibrations

See also: [Molecular orbital diagram § Carbon dioxide](/source/Molecular_orbital_diagram#Carbon_dioxide)

The [symmetry](/source/Molecular_symmetry) of a carbon dioxide molecule is linear and [centrosymmetric](/source/Centrosymmetric) at its equilibrium geometry. The [length](/source/Bond_length) of the [carbon–oxygen bond](/source/Carbon%E2%80%93oxygen_bond) in carbon dioxide is 116.3 [pm](/source/Picometer), noticeably shorter than the roughly 140 pm length of a typical single C–O bond, and shorter than most other C–O multiply bonded [functional groups](/source/Functional_group) such as [carbonyls](/source/Carbonyls).[19] Since it is centrosymmetric, the molecule has no [electric dipole moment](/source/Electric_dipole_moment).

[Stretching and bending oscillations](/source/Infrared_spectroscopy#Number_of_vibrational_modes) of the CO2 molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.

As a linear triatomic molecule, CO2 has four [vibrational modes](/source/Molecular_vibration) 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](/source/Degenerate_energy_levels), 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](/source/Infrared_spectroscopy): the antisymmetric stretching mode at [wavenumber](/source/Wavenumber) 2349 cm−1 (wavelength 4.25 μm) and the degenerate pair of bending modes at 667 cm−1 (wavelength 15.0 μm). The symmetric stretching mode does not create an electric dipole so is not observed in IR spectroscopy, but it is detected in [Raman spectroscopy](/source/Raman_spectroscopy) at 1388 cm−1 (wavelength 7.20 μm), with a [Fermi resonance](/source/Fermi_resonance) doublet at 1285 cm−1.[20]

In the gas phase, carbon dioxide molecules undergo significant vibrational motions and do not keep a fixed structure. However, in a [Coulomb explosion imaging](/source/Coulomb_explosion_imaging) experiment, an instantaneous image of the molecular structure can be deduced. Such an experiment[21] has been performed for carbon dioxide. The result of this experiment, and the conclusion of theoretical calculations[22] based on an [ab initio](/source/Ab_initio_quantum_chemistry_methods) [potential energy surface](/source/Potential_energy_surface) of the molecule, is that none of the molecules in the gas phase are ever exactly linear. This counter-intuitive result is trivially due to the fact that the nuclear motion [volume element](/source/Volume_element) vanishes for linear geometries.[22] This is so for all molecules except [diatomic molecules](/source/Diatomic_molecule).

### In aqueous solution

See also: [Carbonic acid](/source/Carbonic_acid)

Carbon dioxide is [soluble](/source/Soluble) in water, in which it reversibly forms H2CO3 (carbonic acid), which is a [weak acid](/source/Acid_strength), because its ionization in water is incomplete.

- CO2 + H2O ⇌ H2CO3

The [hydration equilibrium constant](/source/Henry's_law) of carbonic acid is, at 25 °C:

- K h = [ H 2 CO 3 ] [ CO 2 ( aq ) ] = 1.70 × 10 − 3 {\displaystyle K_{\mathrm {h} }={\frac {{\ce {[H2CO3]}}}{{\ce {[CO2_{(aq)}]}}}}=1.70\times 10^{-3}}

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](/source/Deprotonation) forms HCO3– ([bicarbonate](/source/Bicarbonate)) and CO32– ([carbonate](/source/Carbonate)) depend on the [pH](/source/PH). As shown in a [Bjerrum plot](/source/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](/source/Diprotic_acid), carbonic acid has two [acid dissociation constants](/source/Acid_dissociation_constant), the first one for the dissociation into the bicarbonate (also called hydrogencarbonate) ion (HCO3–):

- H2CO3 ⇌ H+ + HCO3–

- *K*a1 = 2.5 × 10−4 mol/L; p*K*a1 = 3.6 at 25 °C.[19]

This is the *true* first acid dissociation constant, defined as

- K a 1 = [ HCO 3 − ] [ H + ] [ H 2 CO 3 ] {\displaystyle K_{\mathrm {a1} }={\frac {{\ce {[HCO3- ][H+]}}}{{\ce {[H2CO3]}}}}}

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 (or pKa1 = 6.38) is an *apparent* value calculated on the (incorrect) assumption that all dissolved CO2 is present as carbonic acid, so that

- K a 1 ( a p p a r e n t ) = [ HCO 3 − ] [ H + ] [ H 2 CO 3 ] + [ CO 2 ( aq ) ] {\displaystyle K_{\mathrm {a1} }{\rm {(apparent)}}={\frac {{\ce {[HCO3- ][H+]}}}{{\ce {[H2CO3] + [CO2_{(aq)}]}}}}}

Since most of the dissolved CO2 remains as CO2 molecules, *K*a1(apparent) has a much larger denominator and a much smaller value than the true *K*a1.[23]

The bicarbonate ion is an [amphoteric](/source/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](/source/Carbonate) ion (CO32–):

- HCO3– ⇌ CO32– + H+

- *K*a2 = 4.69 × 10−11 mol/L; p*K*a2 = 10.329

In organisms, carbonic acid production is catalysed by the [enzyme](/source/Enzyme) known as [carbonic anhydrase](/source/Carbonic_anhydrase).

In addition to altering its acidity, the presence of carbon dioxide in water also affects its electrical properties.

Electrical conductivity of carbondioxide saturated desalinated water when heated from 20 to 98 °C. The shadowed regions indicate the error bars associated with the measurements. A comparison with the temperature dependence of vented desalinated water can be found [here](https://commons.wikimedia.org/wiki/File:Electric_conduction_of_vented_and_CO2_saturated_desalinated_water_as_function_of_temperature.svg) .

When carbon dioxide dissolves in desalinated water, the electrical conductivity increases significantly from below 1 μS/cm to nearly 30 μS/cm. When heated, the water begins to gradually lose the conductivity induced by the presence of C O 2 {\displaystyle \mathrm {CO_{2}} } , especially noticeable as temperatures exceed 30 °C.

The [temperature dependence](/source/Conductivity_(electrolytic)#Conductivity_of_purified_water_in_electrochemical_experiments) of the electrical conductivity of fully deionized water without CO2 saturation is comparably low in relation to these data.

### Chemical reactions

CO2 is a potent [electrophile](/source/Electrophile) having an electrophilic reactivity that is comparable to [benzaldehyde](/source/Benzaldehyde) or strongly electrophilic [α,β-unsaturated carbonyl compounds](/source/%CE%91%2C%CE%B2-unsaturated_carbonyl_compound). However, unlike electrophiles of similar reactivity, the reactions of nucleophiles with CO2 are thermodynamically less favored and are often found to be highly reversible.[24] The reversible reaction of carbon dioxide with [amines](/source/Amine) to make [carbamates](/source/Carbamate) is used in CO2 scrubbers and has been suggested as a possible starting point for carbon capture and storage by [amine gas treating](/source/Amine_gas_treating). Only very strong nucleophiles, like the [carbanions](/source/Carbanion) provided by [Grignard reagents](/source/Grignard_reagent) and [organolithium compounds](/source/Organolithium_compound) react with CO2 to give [carboxylates](/source/Carboxylate):

- MR + CO2 → RCO2M

- where M = [Li](/source/Lithium) or [Mg](/source/Magnesium)[Br](/source/Bromine) and R = [alkyl](/source/Alkyl) or [aryl](/source/Aryl).

In [metal carbon dioxide complexes](/source/Metal_carbon_dioxide_complex), CO2 serves as a [ligand](/source/Ligand), which can facilitate the conversion of CO2 to other chemicals.[25]

The reduction of CO2 to [CO](/source/Carbon_monoxide) is ordinarily a difficult and slow reaction:

- CO2 + 2 e− + 2 H+ → CO + H2O

The [redox potential](/source/Redox_potential) for this reaction near pH 7 is about −0.53 V *versus* the [standard hydrogen electrode](/source/Standard_hydrogen_electrode). The nickel-containing enzyme [carbon monoxide dehydrogenase](/source/Carbon_monoxide_dehydrogenase) catalyses this process.[26]

[Photoautotrophs](/source/Photoautotrophs) (i.e. [plants](/source/Plant) and [cyanobacteria](/source/Cyanobacteria)) use the energy contained in sunlight to [photosynthesize](/source/Photosynthesis) simple [sugars](/source/Sugar) from CO2 absorbed from the air and water:

- *n* CO2 + *n* H2O → (CH2O)*n* + *n* O2

### Physical properties

Further information: [Carbon dioxide data](/source/Carbon_dioxide_data)

Pellets of "dry ice", a common form of solid carbon dioxide

Carbon dioxide is colorless. At low concentrations, the gas is odorless; however, at sufficiently high concentrations, it has a sharp, acidic odor.[1] At [standard temperature and pressure](/source/Standard_temperature_and_pressure), the density of carbon dioxide is around 1.98 kg/m3, about 1.53 times that of [air](/source/Earth's_atmosphere).[27]

Carbon dioxide has no liquid state at pressures below 0.51795(10) [MPa](/source/MPa)[2] (5.11177(99) [atm](/source/Standard_atmosphere_(unit))). At a pressure of 1 atm (0.101325 MPa), the gas [deposits](/source/Deposition_(physics)) directly to a solid at temperatures below 194.6855(30) K[2] (−78.4645(30) °C) and the solid [sublimes](/source/Sublimation_(chemistry)) directly to a gas above this temperature. In its solid state, carbon dioxide is commonly called [dry ice](/source/Dry_ice).

Pressure–temperature [phase diagram](/source/Phase_diagram) of carbon dioxide. Note that it is a log-lin chart.

[Liquid carbon dioxide](/source/Liquid_carbon_dioxide) forms only at [pressures](/source/Pressure) above 0.51795(10) MPa[2] (5.11177(99) atm); the [triple point](/source/Triple_point) of carbon dioxide is 216.592(3) K[2] (−56.558(3) °C) at 0.51795(10) MPa[2] (5.11177(99) atm) (see phase diagram). The [critical point](/source/Critical_point_(thermodynamics)) is 304.128(15) K[2] (30.978(15) °C) at 7.3773(30) MPa[2] (72.808(30) atm). Another form of solid carbon dioxide observed at high pressure is an [amorphous](/source/Amorphous) glass-like solid.[28] This form of glass, called *[carbonia](/source/Amorphous_carbonia)*, is produced by [supercooling](/source/Supercooling) heated CO2 at extreme pressures (40–48 [GPa](/source/GPa), or about 400,000 atmospheres) in a [diamond anvil](/source/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](/source/Silicon_dioxide) (silica glass) and [germanium dioxide](/source/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](/source/Supercritical_fluid) known as [supercritical carbon dioxide](/source/Supercritical_carbon_dioxide).

Table of thermal and physical properties of saturated liquid carbon dioxide:[29][30]

Temperature (°C) Density (kg/m3) Specific heat (kJ/(kg⋅K)) Kinematic viscosity (m2/s) Thermal conductivity (W/(m⋅K)) Thermal diffusivity (m2/s) Prandtl Number −50 1156.34 1.84 1.19 × 10−7 0.0855 4.02 × 10−8 2.96 −40 1117.77 1.88 1.18 × 10−7 0.1011 4.81 × 10−8 2.46 −30 1076.76 1.97 1.17 × 10−7 0.1116 5.27 × 10−8 2.22 −20 1032.39 2.05 1.15 × 10−7 0.1151 5.45 × 10−8 2.12 −10 983.38 2.18 1.13 × 10−7 0.1099 5.13 × 10−8 2.2 0 926.99 2.47 1.08 × 10−7 0.1045 4.58 × 10−8 2.38 10 860.03 3.14 1.01 × 10−7 0.0971 3.61 × 10−8 2.8 20 772.57 5 9.10 × 10−8 0.0872 2.22 × 10−8 4.1 30 597.81 36.4 8.00 × 10−8 0.0703 0.279 × 10−8 28.7

Table of thermal and physical properties of carbon dioxide (CO2) at atmospheric pressure:[29][30]

Temperature (K) Density (kg/m3) Specific heat (kJ/(kg⋅°C)) Dynamic viscosity (kg/(m⋅s)) Kinematic viscosity (m2/s) Thermal conductivity (W/(m⋅°C)) Thermal diffusivity (m2/s) Prandtl Number 220 2.4733 0.783 1.11 × 10−5 4.49 × 10−6 0.010805 5.92 × 10−6 0.818 250 2.1657 0.804 1.26 × 10−5 5.81 × 10−6 0.012884 7.40 × 10−6 0.793 300 1.7973 0.871 1.50 × 10−5 8.32 × 10−6 0.016572 1.06 × 10−5 0.77 350 1.5362 0.9 1.72 × 10−5 1.12 × 10−5 0.02047 1.48 × 10−5 0.755 400 1.3424 0.942 1.93 × 10−5 1.44 × 10−5 0.02461 1.95 × 10−5 0.738 450 1.1918 0.98 2.13 × 10−5 1.79 × 10−5 0.02897 2.48 × 10−5 0.721 500 1.0732 1.013 2.33 × 10−5 2.17 × 10−5 0.03352 3.08 × 10−5 0.702 550 0.9739 1.047 2.51 × 10−5 2.57 × 10−5 0.03821 3.75 × 10−5 0.685 600 0.8938 1.076 2.68 × 10−5 3.00 × 10−5 0.04311 4.48 × 10−5 0.668 650 0.8143 1.1 2.88 × 10−5 3.54 × 10−5 0.0445 4.97 × 10−5 0.712 700 0.7564 1.13 3.05 × 10−5 4.03 × 10−5 0.0481 5.63 × 10−5 0.717 750 0.7057 1.15 3.21 × 10−5 4.55 × 10−5 0.0517 6.37 × 10−5 0.714 800 0.6614 1.17 3.37 × 10−5 5.10 × 10−5 0.0551 7.12 × 10−5 0.716

## Biological role

Carbon dioxide is an end product of [cellular respiration](/source/Cellular_respiration) in organisms that obtain energy by breaking down sugars, fats and [amino acids](/source/Amino_acid) with oxygen as part of their [metabolism](/source/Metabolism). This includes all plants, algae and animals and [aerobic](/source/Aerobic_respiration) fungi and bacteria. In [vertebrates](/source/Vertebrate), the carbon dioxide travels in the blood from the body's tissues to the skin (e.g., [amphibians](/source/Amphibian)) or the gills (e.g., [fish](/source/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](/source/Compensation_point) in respiration.

### Photosynthesis and carbon fixation

Overview of the [Calvin cycle](/source/Calvin_cycle) and carbon fixation

[Carbon fixation](/source/Carbon_fixation) is a biochemical process by which atmospheric carbon dioxide is incorporated by plants, algae and cyanobacteria into [energy-rich](/source/Fuel) organic molecules such as [glucose](/source/Glucose), thus creating their own food by photosynthesis. Photosynthesis uses carbon dioxide and [water](/source/Water) to produce sugars from which other [organic compounds](/source/Organic_compound) can be constructed, and [oxygen](/source/Oxygen) is produced as a by-product.

[Ribulose-1,5-bisphosphate carboxylase oxygenase](/source/RuBisCO), commonly abbreviated to RuBisCO, is the [enzyme](/source/Enzyme) involved in the first major step of carbon fixation, the production of two molecules of [3-phosphoglycerate](/source/3-phosphoglycerate) from CO2 and [ribulose bisphosphate](/source/Ribulose_bisphosphate), as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth.[31]

[Phototrophs](/source/Phototroph) use the products of their photosynthesis as internal food sources and as raw material for the [biosynthesis](/source/Biosynthesis) of more complex organic molecules, such as [polysaccharides](/source/Polysaccharide), [nucleic acids](/source/Nucleic_acid), and proteins. These are used for their own growth, and also as the basis of the [food chains](/source/Food_chain) and webs that feed other organisms, including animals such as ourselves. Some important phototrophs, the [coccolithophores](/source/Coccolithophore) synthesise hard [calcium carbonate](/source/Calcium_carbonate) scales.[32] A globally significant species of coccolithophore is *[Emiliania huxleyi](/source/Emiliania_huxleyi)* whose [calcite](/source/Calcite) scales have formed the basis of many [sedimentary rocks](/source/Sedimentary_rock) such as [limestone](/source/Limestone), where what was previously atmospheric carbon can remain fixed for geological timescales.

Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by [photosynthesis](/source/Photosynthesis) (green), which can be [respired](/source/Cellular_respiration) (red) to water and CO2.

Plants can grow as much as 50% 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.[33] 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.[34][35]

Increased atmospheric CO2 concentrations result in fewer stomata developing on plants[36] which leads to reduced water usage and increased [water-use efficiency](/source/Water-use_efficiency).[37] Studies using [FACE](/source/Free-Air_Concentration_Enrichment) have shown that CO2 enrichment leads to decreased concentrations of micronutrients in crop plants.[38] This may have knock-on effects on other parts of [ecosystems](/source/Ecosystem) as herbivores will need to eat more food to gain the same amount of protein.[39]

The concentration of secondary [metabolites](/source/Metabolites) such as [phenylpropanoids](/source/Phenylpropanoid) and [flavonoids](/source/Flavonoid) can also be altered in plants exposed to high concentrations of CO2.[40][41]

Plants also emit CO2 during respiration, and so the majority of plants and algae, which use [C3 photosynthesis](/source/C3_photosynthesis), are only net absorbers during the day. Though a growing forest will absorb many tons of CO2 each year, a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants.[42] Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon[43] and remain valuable [carbon sinks](/source/Carbon_sink), helping to maintain the carbon balance of Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.[44]

### Toxicity

See also: [Carbon dioxide poisoning](/source/Carbon_dioxide_poisoning)

Symptoms of carbon dioxide toxicity, by increasing [volume percent](/source/Volume_percent) in air[45]

Carbon dioxide content in fresh air (averaged between sea-level and 10 kPa level, i.e., about 30 km (19 mi) altitude) varies between 0.036% (360 ppm) and 0.041% (412 ppm), depending on the location.[46]

In humans, exposure to CO2 at concentrations greater than 5% causes the development of [hypercapnia](/source/Hypercapnia) and [respiratory acidosis](/source/Respiratory_acidosis).[47] Concentrations of 7% to 10% (70,000 to 100,000 ppm) may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.[48] Concentrations of more than 10% may cause convulsions, coma, and death. CO2 levels of more than 30% act rapidly leading to loss of consciousness in seconds.[47]

Because it is heavier than air, in locations where the gas seeps from the ground (due to sub-surface volcanic or geothermal activity) in relatively high concentrations, without the dispersing effects of wind, it can collect in sheltered/pocketed locations below average ground level, causing animals located therein to be suffocated. Carrion feeders attracted to the carcasses are then also killed. Children have been killed in the same way near the city of [Goma](/source/Goma) by CO2 emissions from the nearby volcano [Mount Nyiragongo](/source/Mount_Nyiragongo).[49] The [Swahili](/source/Swahili_language) term for this phenomenon is *[mazuku](/source/Mazuku)*.

Rising levels of CO2 threatened the [Apollo 13](/source/Apollo_13) astronauts, who had to adapt cartridges from the command module to supply the [carbon dioxide scrubber](/source/Carbon_dioxide_scrubber) in the [Apollo Lunar Module](/source/Apollo_Lunar_Module), which they used as a lifeboat.

Adaptation to increased concentrations of CO2 occurs in humans, including [modified breathing](/source/Respiratory_adaptation) and kidney bicarbonate production, in order to balance the effects of blood acidification ([acidosis](/source/Acidosis)). Several studies suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a [submarine](/source/Submarine)) since the adaptation is physiological and reversible, as deterioration in performance or in normal physical activity does not happen at this level of exposure for five days.[50][51] Yet, other studies show a decrease in cognitive function even at much lower levels.[52][53] Also, with ongoing respiratory [acidosis](/source/Acidosis), adaptation or compensatory mechanisms will be unable to reverse the condition.

#### Below 1%

There are few studies of the health effects of long-term continuous CO2 exposure on humans and animals at levels below 1%. Occupational CO2 exposure limits have been set in the United States at 0.5% (5000 ppm) for an eight-hour period.[54] At this CO2 concentration, [International Space Station](/source/International_Space_Station) crew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption.[55] Studies in animals at 0.5% CO2 have demonstrated kidney calcification and bone loss after eight weeks of exposure.[56] A study of humans exposed in 2.5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0.1% (1000 ppm) CO2 likely due to CO2 induced increases in cerebral blood flow.[52] Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm.[53]

However a review of the literature found that a reliable subset of studies on the phenomenon of carbon dioxide induced cognitive impairment to only show a small effect on high-level decision making (for concentrations below 5000 ppm). Most of the studies were confounded by inadequate study designs, environmental comfort, uncertainties in exposure doses and differing cognitive assessments used.[57] Similarly a study on the effects of the concentration of CO2 in motorcycle helmets has been criticized for having dubious methodology in not noting the self-reports of motorcycle riders and taking measurements using mannequins. Further when normal motorcycle conditions were achieved (such as highway or city speeds) or the visor was raised the concentration of CO2 declined to safe levels (0.2%).[58][59]

Typical CO2 concentration effects Concentration Note 280 ppm Pre-industrial levels 421 ppm Current (May 2022) levels ~1121 ppm ASHRAE recommendation for indoor air[60] 5,000 ppm USA 8h exposure limit[54] 10,000 ppm Cognitive impairment, Canada's long term exposure limit[45] 10,000-20,000 ppm Drowsiness[48] 20,000-50,000 ppm Headaches, sleepiness; poor concentration, loss of attention, slight nausea also possible[54]

#### Ventilation

A [carbon dioxide sensor](/source/Carbon_dioxide_sensor) that measures CO2 concentration using a [nondispersive infrared sensor](/source/Nondispersive_infrared_sensor)

Poor ventilation is one of the main causes of excessive CO2 concentrations in closed spaces, leading to poor [indoor air quality](/source/Indoor_air_quality). Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO2 concentration has stabilized) are sometimes used to estimate ventilation rates per person.[61] Higher CO2 concentrations are associated with occupant health, comfort and performance degradation.[62][63] [ASHRAE](/source/ASHRAE) Standard 62.1–2007 ventilation rates may result in indoor concentrations up to 2,100 ppm above ambient outdoor conditions. Thus if the outdoor concentration is 400 ppm, indoor concentrations may reach 2,500 ppm with ventilation rates that meet this industry consensus standard. Concentrations in poorly ventilated spaces can be found even higher than this (range of 3,000 or 4,000 ppm).

Miners, who are particularly vulnerable to gas exposure due to insufficient ventilation, referred to mixtures of carbon dioxide and nitrogen as "[blackdamp](/source/Blackdamp)", "choke damp" or "stythe". Before more effective technologies were developed, [miners](/source/Miners) would frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by bringing a caged [canary](/source/Domestic_Canary) with them as they worked. The canary is more sensitive to asphyxiant gases than humans, and as it became unconscious would stop singing and fall off its perch. The [Davy lamp](/source/Davy_lamp) could also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, while [methane](/source/Methane), another suffocating gas and explosion risk, would make the lamp burn more brightly.

In February 2020, three people died from suffocation at a party in Moscow when dry ice (frozen CO2) was added to a swimming pool to cool it down.[64] A similar accident occurred in 2018 when a woman died from CO2 fumes emanating from the large amount of dry ice she was transporting in her car.[65]

#### Indoor air

Humans spend more and more time in a confined atmosphere (around 80-90% of the time in a building or vehicle). According to the French [Agency for Food, Environmental and Occupational Health & Safety](/source/Agence_nationale_de_s%C3%A9curit%C3%A9_sanitaire_de_l'alimentation%2C_de_l'environnement_et_du_travail) (ANSES) and various actors in France, the CO2 rate in the indoor air of buildings (linked to human or animal occupancy and the presence of [combustion](/source/Combustion) installations), weighted by air renewal, is "usually between about 350 and 2,500 ppm".[66]

In homes, schools, nurseries and offices, there are no systematic relationships between the levels of CO2 and other pollutants, and indoor CO2 is statistically not a good predictor of pollutants linked to outdoor road (or air, etc.) traffic.[67] CO2 is the parameter that changes the fastest (with hygrometry and oxygen levels when humans or animals are gathered in a closed or poorly ventilated room). In poor countries, many open hearths are sources of CO2 and CO emitted directly into the living environment.[68]

#### Outdoor areas with elevated concentrations

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](/source/Rapolano_Terme) in [Tuscany](/source/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.[69] High concentrations of CO2 produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at [Lake Monoun](/source/Lake_Monoun), [Cameroon](/source/Cameroon) in 1984 and 1700 casualties at [Lake Nyos](/source/Lake_Nyos), Cameroon in 1986.[70]

## Human physiology

### Content

Reference ranges or averages for partial pressures of carbon dioxide (abbreviated pCO2) Blood compartment (kPa) (mm Hg) Venous blood carbon dioxide 5.5–6.8 41–51[71] Alveolar pulmonary gas pressures 4.8 36 Arterial blood carbon dioxide 4.7–6.0 35–45[71]

The body produces approximately 2.3 pounds (1.0 kg) of carbon dioxide per day per person,[72] containing 0.63 pounds (290 g) of carbon. In humans, this carbon dioxide is carried through the [venous system](/source/Venous_system) and is breathed out through the lungs, resulting in lower concentrations in the [arteries](/source/Arteries). The carbon dioxide content of the blood is often given as the [partial pressure](/source/Partial_pressure), which is the pressure which carbon dioxide would have had if it alone occupied the volume.[73] In humans, the blood carbon dioxide contents are shown in the adjacent table.

### Transport in the blood

CO2 is carried in blood in three different ways. Exact percentages vary between arterial and venous blood.

- Majority (about 70% to 80%) is converted to [bicarbonate](/source/Bicarbonate) ions (HCO3–) by the enzyme [carbonic anhydrase](/source/Carbonic_anhydrase) in the red blood cells,[74] by the reaction:

- CO2 + H2O → H2CO3 → H+ + HCO3–

- 5–10% is dissolved in [blood plasma](/source/Blood_plasma)[74]

- 5–10% is bound to [hemoglobin](/source/Hemoglobin) as [carbamino](/source/Carbamino) compounds[74]

[Hemoglobin](/source/Hemoglobin), the main oxygen-carrying molecule in [red blood cells](/source/Red_blood_cell), 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](/source/Allosteric_regulation) effects on the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. This is known as the [Haldane Effect](/source/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](/source/Bohr_effect).

### Regulation of respiration

Carbon dioxide is one of the mediators of local [autoregulation](/source/Autoregulation) of blood supply. If its concentration is high, the [capillaries](/source/Capillaries) expand to allow a greater blood flow to that tissue.[75]

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](/source/Respiratory_acidosis), while breathing that is too rapid leads to [hyperventilation](/source/Hyperventilation), which can cause [respiratory alkalosis](/source/Alkalosis).[76]

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](/source/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](/source/Oxygen_mask) to themselves first before helping others; otherwise, one risks losing consciousness.[74]

The respiratory centers try to maintain an arterial CO2 pressure of 40 [mmHg](/source/MmHg). With intentional hyperventilation, the CO2 content of arterial blood may be lowered to 10–20 mmHg (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.[77]

## Concentrations and role in the environment

### Atmosphere

Further information: [Carbon cycle](/source/Carbon_cycle)

This section is an excerpt from [Carbon dioxide in the atmosphere of Earth](/source/Carbon_dioxide_in_the_atmosphere_of_Earth).[[edit](https://en.wikipedia.org/w/index.php?title=Carbon_dioxide_in_the_atmosphere_of_Earth&action=edit)]

Atmospheric CO2 concentration measured at [Mauna Loa Observatory](/source/Mauna_Loa_Observatory) in Hawaii from 1958 to 2023 (also called the [Keeling Curve](/source/Keeling_Curve)). The rise in CO2 over that time period is clearly visible. The concentration is expressed as μmole per mole, or [ppm](/source/Parts-per_notation).

In the [atmosphere of Earth](/source/Atmosphere_of_Earth), carbon dioxide (CO2) is a [trace gas](/source/Trace_gas) that plays an integral part in the [greenhouse effect](/source/Greenhouse_effect), [carbon cycle](/source/Carbon_cycle), [photosynthesis](/source/Photosynthesis), and [oceanic carbon cycle](/source/Oceanic_carbon_cycle). It is one of three main [greenhouse gases](/source/Greenhouse_gas) in the atmosphere of [Earth](/source/Earth). In 2026, the concentration of carbon dioxide in the atmosphere reached 432 [ppm](/source/Parts_per_million) or 0.0432% (on a [molar basis](/source/Mole_fraction)), representing a mass of 3380 [gigatonnes](/source/Gigatonnes).[78] This is an increase of 54% since the start of the [Industrial Revolution](/source/Industrial_Revolution), up from 280 ppm during the 10,000 years prior to the mid-18th century.[79][80][81] The increase [is due to human activity](/source/Causes_of_climate_change).[82]

The current increase in CO2 concentrations is primarily driven by the burning of [fossil fuels](/source/Fossil_fuel).[83] Other significant human activities that emit CO2 include [cement](/source/Cement) production, [deforestation](/source/Deforestation), and [biomass](/source/Biomass) burning. The increase in atmospheric concentrations of CO2 and other long-lived greenhouse gases such as [methane](/source/Methane) increase the absorption and emission of infrared radiation by the atmosphere. This has led to a [rise in average global temperature](/source/Instrumental_temperature_record) and [ocean acidification](/source/Ocean_acidification). Another direct effect is the [CO2 fertilization effect](/source/CO2_fertilization_effect). The increase in atmospheric concentrations of CO2 causes a range of further [effects of climate change](/source/Effects_of_climate_change) on the environment and human living conditions.

Carbon dioxide is a greenhouse gas. It absorbs and emits [infrared radiation](/source/Infrared_radiation) at its two infrared-active vibrational frequencies. The two [wavelengths](/source/Wavelength) are 4.26 [μm](/source/%CE%9Cm) (2,347 cm−1) (antisymmetric stretching [vibrational mode](/source/Infrared_spectroscopy)) and 14.99 μm (667 cm−1) (bending vibrational mode). CO2 plays a significant role in influencing [Earth](/source/Earth)'s surface temperature through the greenhouse effect.[84] Light emission from the Earth's surface is most intense in the infrared region between 200 and 2500 cm−1,[85] as opposed to light emission from the much hotter [Sun](/source/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 of Earth and its lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption.[86]

The present atmospheric concentration of CO2 is the highest for 14 million years.[87] Concentrations of CO2 in the atmosphere were as high as 4,000 ppm during the [Cambrian period](/source/Cambrian) about 500 million years ago, and as low as 180 ppm during the [Quaternary glaciation](/source/Quaternary_glaciation) of the last two million years.[79] Reconstructed temperature records for the last 420 million years indicate that atmospheric CO2 concentrations peaked at approximately 2,000 ppm. This peak happened during the [Devonian](/source/Devonian) period (400 million years ago). Another peak occurred in the [Triassic](/source/Triassic) period (220–200 million years ago).[88]

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.[89]

### Oceans

Main articles: [Carbon cycle](/source/Carbon_cycle) and [Ocean acidification](/source/Ocean_acidification)

#### Ocean acidification

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](/source/Carbon_sink), and have taken up about a third of CO2 emitted by human activity.[90]

This section is an excerpt from [Ocean acidification](/source/Ocean_acidification).[[edit](https://en.wikipedia.org/w/index.php?title=Ocean_acidification&action=edit)]

[Ocean acidification](/source/Ocean_acidification) is the ongoing decrease in the [pH](/source/PH) of the Earth's [ocean](/source/Ocean). Between 1950 and 2020, the average pH of the ocean surface fell from approximately 8.15 to 8.05.[91] [Carbon dioxide emissions](/source/Carbon_dioxide_emissions) from human activities are the primary cause of ocean acidification, with [atmospheric carbon dioxide (CO2) levels](/source/Carbon_dioxide_in_Earth's_atmosphere) exceeding 422 ppm (as of 2024[\[update\]](https://en.wikipedia.org/w/index.php?title=Carbon_dioxide&action=edit)).[92] CO2 from the [atmosphere](/source/Atmosphere) is absorbed by the oceans. This chemical reaction produces [carbonic acid](/source/Carbonic_acid) (H2CO3) which [dissociates](/source/Dissociate) into a [bicarbonate ion](/source/Bicarbonate_ion) (HCO−3) and a [hydrogen ion](/source/Hydrogen_ion) (H+). The presence of free hydrogen ions (H+) lowers the pH of the ocean, increasing [acidity](/source/Acid) (this does not mean that [seawater](/source/Seawater) is acidic yet; it is still [alkaline](/source/Alkali), with a pH higher than 8). [Marine calcifying organisms](/source/Marine_biogenic_calcification), such as [mollusks](/source/Mollusca) and [corals](/source/Coral), are especially vulnerable because they rely on calcium carbonate to build shells and skeletons.[93]

A change in pH by 0.1 represents a 26% increase in hydrogen ion concentration in the world's oceans (the pH scale is logarithmic, so a change of one in pH units is equivalent to a tenfold change in hydrogen ion concentration). Sea-surface pH and carbonate saturation states vary depending on ocean depth and location. Colder and higher latitude waters are capable of absorbing more CO2. This can cause acidity to rise, lowering the pH and carbonate saturation levels in these areas. Several other factors influence the atmosphere-ocean CO2 exchange, and thus local ocean acidification. These include [ocean currents](/source/Ocean_current) and [upwelling](/source/Upwelling) zones, proximity to large continental rivers, [sea ice](/source/Sea_ice) coverage, and atmospheric exchange with [nitrogen](/source/Nitrogen) and [sulfur](/source/Sulfur) from [fossil fuel](/source/Fossil_fuel) burning and [agriculture](/source/Agriculture).[94][95][96]

Pterapod shell dissolved in seawater adjusted to an [ocean chemistry](/source/Ocean_chemistry) projected for the year 2100

This section is an excerpt from [Ocean acidification § Decreased calcification in marine organisms](/source/Ocean_acidification#Decreased_calcification_in_marine_organisms).[[edit](https://en.wikipedia.org/w/index.php?title=Ocean_acidification&action=edit)]

Changes in ocean chemistry can have extensive direct and indirect effects on organisms and their habitats. One of the most important repercussions of increasing ocean acidity relates to the production of shells out of [calcium carbonate](/source/Calcium_carbonate) (CaCO3).[93] This process is called calcification and is important to the biology and survival of a wide range of marine organisms. Calcification involves the [precipitation](/source/Precipitation_(chemistry)) of dissolved ions into solid CaCO3 structures, structures for many marine organisms, such as [coccolithophores](/source/Coccolithophore), [foraminifera](/source/Foraminifera), [crustaceans](/source/Crustacea), [mollusks](/source/Mollusca), etc. After they are formed, these CaCO3 structures are vulnerable to [dissolution](/source/Dissolution_(chemistry)) unless the surrounding seawater contains [saturating](/source/Saturated_solution) concentrations of carbonate ions (CO2−3).

Very little of the extra carbon dioxide that is added into the ocean remains as dissolved carbon dioxide. The majority dissociates into additional bicarbonate and free hydrogen ions. The increase in hydrogen is larger than the increase in bicarbonate,[97] creating an imbalance in the reaction:

- HCO−3 ⇌ CO2−3 + H+

To maintain chemical equilibrium, some of the carbonate ions already in the ocean combine with some of the hydrogen ions to make further bicarbonate. Thus the ocean's concentration of carbonate ions is reduced, removing an essential building block for marine organisms to build shells, or calcify:

- Ca2+ + CO2−3 ⇌ CaCO3

#### Hydrothermal vents

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](/source/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](/source/Okinawa_Trough).[98] The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006.[99]

## Sources

The burning of [fossil fuels](/source/Fossil_fuel) for energy produces 36.8 billion tonnes of CO2 per year as of 2023.[100] Nearly all of this goes into the atmosphere, where approximately half is subsequently absorbed into natural [carbon sinks](/source/Carbon_sink).[101] Less than 1% of CO2 produced annually is put to commercial use.[18]: 3

### Biological processes

Carbon dioxide is a by-product of the [fermentation](/source/Fermentation_(biochemistry)) of sugar in the [brewing](/source/Brewing) of [beer](/source/Beer), [whisky](/source/Whisky) and other [alcoholic beverages](/source/Alcoholic_beverage) and in the production of [bioethanol](/source/Bioethanol). [Yeast](/source/Yeast) metabolizes sugar to produce CO2 and [ethanol](/source/Ethanol), also known as alcohol, as follows:

- C6H12O6 → 2 CO2 + 2 CH3CH2OH

All [aerobic](/source/Cellular_respiration) organisms produce CO2 when they oxidize [carbohydrates](/source/Carbohydrate), [fatty acids](/source/Fatty_acid), and [proteins](/source/Protein). The large number of reactions involved are exceedingly complex and not described easily. Refer to [cellular respiration](/source/Cellular_respiration), [anaerobic respiration](/source/Anaerobic_respiration) and [photosynthesis](/source/Photosynthesis). The equation for the respiration of glucose and other [monosaccharides](/source/Monosaccharide) is:

- C6H12O6 + 6 O2 → 6 CO2 + 6 H2O

[Anaerobic organisms](/source/Anaerobic_organisms) decompose organic material producing methane and carbon dioxide together with traces of other compounds.[102] Regardless of the type of organic material, the production of gases follows well defined [kinetic pattern](/source/Chemical_kinetics). Carbon dioxide comprises about 40–45% of the gas that emanates from decomposition in landfills (termed "[landfill gas](/source/Landfill_gas)"). Most of the remaining 50–55% is methane.[103]

#### Combustion

The [combustion](/source/Combustion) of all [carbon-based fuels](/source/Carbon-based_fuel), such as [methane](/source/Methane) ([natural gas](/source/Natural_gas)), petroleum distillates ([gasoline](/source/Gasoline), [diesel](/source/Diesel_fuel), [kerosene](/source/Kerosene), [propane](/source/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](/source/Oxygen):

- CH4 + 2 O2 → CO2 + 2 H2O

[Iron](/source/Iron) is reduced from its oxides with [coke](/source/Coke_(fuel)) in a [blast furnace](/source/Blast_furnace), producing [pig iron](/source/Pig_iron) and carbon dioxide:[104]

- Fe2O3 + 3 CO → 3 CO2 + 2 Fe

#### By-product from hydrogen production

Carbon dioxide is a byproduct of the industrial production of hydrogen by [steam reforming](/source/Steam_reforming) and the [water gas shift reaction](/source/Water_gas_shift_reaction) in [ammonia production](/source/Ammonia_production). These processes begin with the reaction of water and natural gas (mainly methane).[105]

#### Thermal decomposition of limestone

It is produced by thermal decomposition of limestone, CaCO3 by heating ([calcining](/source/Calcining)) at about 850 °C (1,560 °F), in the manufacture of [quicklime](/source/Calcium_oxide) ([calcium oxide](/source/Calcium_oxide), CaO), a compound that has many industrial uses:

- CaCO3 → CaO + CO2

Acids liberate CO2 from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxide [springs](/source/Spring_(hydrosphere)), where it is produced by the action of acidified water on [limestone](/source/Limestone) or [dolomite](/source/Dolomite_(mineral)). The reaction between [hydrochloric acid](/source/Hydrochloric_acid) and calcium carbonate (limestone or chalk) is shown below:

- CaCO3 + 2 HCl → CaCl2 + H2CO3

The [carbonic acid](/source/Carbonic_acid) (H2CO3) then decomposes to water and CO2:

- H2CO3 → CO2 + H2O

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.

## Commercial uses

The biggest commercial uses of CO2 are in producing urea for fertilizer and in extracting oil from the ground. Beverages, food, metal fabrication, and other uses account for 3%, 3%, 2%, and 4% of commercial CO2 use, respectively.[106]

Around 230 Mt of CO2 are used each year,[107] mostly in the fertiliser industry for urea production (130 million tonnes) and in the oil and gas industry for [enhanced oil recovery](/source/Enhanced_oil_recovery) (70 to 80 million tonnes).[18]: 3 Other commercial applications include food and beverage production, metal fabrication, cooling, fire suppression and stimulating plant growth in greenhouses.[18]: 3

Technology exists to [capture CO2 from industrial flue gas](/source/Carbon_capture_and_storage) or [from the air](/source/Direct_air_capture). Research is ongoing on ways to use [captured CO2 in products](/source/Carbon_capture_and_storage#CO2_utilization_in_products) and some of these processes have been deployed commercially.[108] However, the potential to use products is very small compared to the total volume of CO2 that could foreseeably be captured.[109] The vast majority of captured CO2 is considered a waste product and sequestered in underground geologic formations.[110]

### Precursor to chemicals

This section needs expansion. You can help by making an edit requestadding missing information. (July 2014)

See also: [Sabatier reaction](/source/Sabatier_reaction)

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of [urea](/source/Urea), with a smaller fraction being used to produce [methanol](/source/Methanol) and a range of other products.[111] Some carboxylic acid derivatives such as [sodium salicylate](/source/Sodium_salicylate) are prepared using CO2 by the [Kolbe–Schmitt reaction](/source/Kolbe%E2%80%93Schmitt_reaction).[112]

Captured CO2 could be to produce [methanol](/source/Methanol) or [electrofuels](/source/Electrofuel). To be carbon-neutral, the CO2 would need to come from bioenergy production or [direct air capture](/source/Direct_air_capture).[113]: 21–24

### Fossil fuel recovery

Carbon dioxide is used in [enhanced oil recovery](/source/Enhanced_oil_recovery) where it is injected into or adjacent to producing oil wells, usually under [supercritical](/source/Supercritical_fluid) conditions, when it becomes [miscible](/source/Miscibility) with the oil. This approach can increase original oil recovery by reducing residual oil saturation by 7–23% additional to [primary extraction](/source/Extraction_of_petroleum#Primary_recovery).[114] It acts as both a pressurizing agent and, when dissolved into the underground [crude oil](/source/Crude_oil), significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well.[115]

Most CO2 injected in CO2-EOR projects comes from naturally occurring underground CO2 deposits.[116] Some CO2 used in EOR is captured from industrial facilities such as [natural gas processing plants](/source/Natural-gas_processing), using [carbon capture](/source/Carbon_capture_and_storage) technology and transported to the oilfield in pipelines.[116]

### 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.[117][118] 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](/source/Whiteflies) and [spider mites](/source/Spider_mite) in a greenhouse.[119] Some plants respond more favorably to rising carbon dioxide concentrations than others, which can lead to vegetation regime shifts like [woody plant encroachment](/source/Woody_plant_encroachment).[120]

### Foods

Carbon dioxide bubbles in a soft drink

Carbon dioxide is a [food additive](/source/Food_additive) used as a propellant and acidity regulator in the food industry. It is approved for usage in the EU[121] (listed as [E number](/source/E_number) E290), US,[122] Australia and New Zealand[123] (listed by its [INS number](/source/INS_number) 290).

A candy called [Pop Rocks](/source/Pop_Rocks) is pressurized with carbon dioxide gas[124] at about 4,000 [kPa](/source/Pascal_(unit)) (40 [bar](/source/Bar_(unit)); 580 [psi](/source/Pound_per_square_inch)). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

[Leavening agents](/source/Leavening_agent) cause dough to rise by producing carbon dioxide.[125] [Baker's yeast](/source/Baker's_yeast) produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as [baking powder](/source/Baking_powder) and [baking soda](/source/Baking_soda) release carbon dioxide when heated or if exposed to [acids](/source/Acid).

#### Beverages

Carbon dioxide is used to produce [carbonated](/source/Carbonation) [soft drinks](/source/Soft_drink) and [soda water](/source/Soda_water). Traditionally, the carbonation of beer and sparkling wine came about through natural fermentation, but many manufacturers carbonate these drinks with carbon dioxide recovered from the fermentation process. In the case of bottled and kegged beer, the most common method used is carbonation with recycled carbon dioxide. With the exception of British [real ale](/source/Cask_ale#Real_ale), draught beer is usually transferred from kegs in a cold room or cellar to dispensing taps on the bar using pressurized carbon dioxide, sometimes mixed with nitrogen.

The taste of soda water (and related taste sensations in other carbonated beverages) is an effect of the dissolved carbon dioxide rather than the bursting bubbles of the gas. [Carbonic anhydrase 4](/source/Carbonic_anhydrase_4) converts carbon dioxide to [carbonic acid](/source/Carbonic_acid) leading to a [sour](/source/Sour) taste, and also the dissolved carbon dioxide induces a [somatosensory](/source/Somatosensory) response.[126]

#### Winemaking

Dry ice used to preserve grapes after harvest

Carbon dioxide in the form of [dry ice](/source/Dry_ice) is often used during the [cold soak](https://en.wikipedia.org/w/index.php?title=Cold_soak&action=edit&redlink=1) phase in [winemaking](/source/Winemaking) to cool clusters of [grapes](/source/Grape) quickly after picking to help prevent spontaneous [fermentation](/source/Fermentation_(wine)) by wild [yeast](/source/Yeast_(wine)). The main advantage of using dry ice over water ice is that it cools the grapes without adding any additional water that might decrease the sugar concentration in the [grape must](/source/Grape_must), and thus the [alcohol](/source/Ethanol) concentration in the finished wine. Carbon dioxide is also used to create a hypoxic environment for [carbonic maceration](/source/Carbonic_maceration), the process used to produce [Beaujolais](/source/Beaujolais) wine.

Carbon dioxide is sometimes used to top up wine bottles or other [storage](/source/Storage_(wine)) vessels such as barrels to prevent oxidation, though it has the problem that it can dissolve into the wine, making a previously [still wine](/source/Still_wine) slightly fizzy. For this reason, other gases such as [nitrogen](/source/Nitrogen) or [argon](/source/Argon) are preferred for this process by professional wine makers.

#### Stunning animals

Carbon dioxide is often used to "stun" animals before slaughter.[127] "Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress.[128][129]

### 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 [welding](/source/Welding), although in the welding arc, it reacts to [oxidize](/source/Oxidation) most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are more [brittle](/source/Brittle) than those made in more inert atmospheres.[130] When used for [MIG welding](/source/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 [bar](/source/Bar_(unit)) (870 [psi](/source/Pound_per_square_inch); 59 [atm](/source/Atmosphere_(unit))), allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. [Aluminium](/source/Aluminium) capsules of CO2 are also sold as supplies of compressed gas for [air guns](/source/Air_gun), [paintball](/source/Paintball) markers/guns, inflating bicycle tires, and for making [carbonated water](/source/Carbonated_water). High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in [supercritical drying](/source/Supercritical_drying) of some food products and technological materials, in the preparation of specimens for [scanning electron microscopy](/source/Scanning_electron_microscopy)[131] and in the [decaffeination](/source/Decaffeination) of [coffee beans](/source/Coffee_bean).

### Fire extinguisher

Use of a CO2 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 [fire extinguishers](/source/Fire_extinguisher#Halons,_Halon-replacement_clean_agents_and_carbon_dioxide), especially those designed for [electrical fires](/source/Electrical_fire), 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 they do not cool the burning substances significantly, and when the carbon dioxide disperses, they can catch fire upon exposure to [atmospheric oxygen](/source/Atmospheric_oxygen). They are mainly used in server rooms.[132]

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.[133] [International Maritime Organization](/source/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.[134]

### Supercritical CO2 as solvent

See also: [Supercritical carbon dioxide](/source/Supercritical_carbon_dioxide) and [Green chemistry](/source/Green_chemistry)

Liquid carbon dioxide is a good [solvent](/source/Solvent) for many [lipophilic](/source/Lipophilic) [organic compounds](/source/Organic_compound) and is used to [decaffeinate](/source/Decaffeinate) [coffee](/source/Coffee).[135] Carbon dioxide has attracted attention in the [pharmaceutical](/source/Pharmaceutical) and other chemical processing industries as a less toxic alternative to more traditional solvents such as [organochlorides](/source/Organochloride). It is also used by some [dry cleaners](/source/Dry_cleaners) for this reason. It is used in the preparation of some [aerogels](/source/Aerogel#Production) because of the properties of supercritical carbon dioxide.

### Refrigerant

See also: [Refrigerant](/source/Refrigerant) and [Sustainable automotive air conditioning](/source/Sustainable_automotive_air_conditioning)

Comparison of the pressure–temperature phase diagrams of carbon dioxide (red) and water (blue) as a log-lin chart with phase transitions points at 1 atmosphere

Liquid and solid carbon dioxide are important [refrigerants](/source/Refrigerant), 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 of [dichlorodifluoromethane](/source/Dichlorodifluoromethane) (R12, a [chlorofluorocarbon](/source/Chlorofluorocarbon) (CFC) compound).[136] CO2 might enjoy a renaissance because one of the main substitutes to CFCs, [1,1,1,2-tetrafluoroethane](/source/1%2C1%2C1%2C2-tetrafluoroethane) ([R134a](/source/R134a), a [hydrofluorocarbon](/source/Hydrofluorocarbon) (HFC) compound) contributes to [climate change](/source/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](/source/Global_warming_potential) 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](/source/Coca-Cola) has fielded CO2-based beverage coolers and the [U.S. Army](/source/United_States_Army) is interested in CO2 refrigeration and heating technology.[137][138]

### Minor uses

A [carbon-dioxide laser](/source/Carbon-dioxide_laser)

Carbon dioxide is the [lasing medium](/source/Active_laser_medium) in a [carbon-dioxide laser](/source/Carbon-dioxide_laser), which is one of the earliest type of lasers.

Carbon dioxide can be used as a means of controlling the [pH](/source/PH) of swimming pools,[139] 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](/source/Reef_aquarium), where it is commonly used in [calcium reactors](/source/Calcium_reactor) to temporarily lower the pH of water being passed over [calcium carbonate](/source/Calcium_carbonate) in order to allow the calcium carbonate to dissolve into the water more freely, where it is used by some [corals](/source/Coral) to build their skeleton.

Used as the primary coolant in the British [advanced gas-cooled reactor](/source/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](/source/American_Veterinary_Medical_Association)'s 2020 guidelines for carbon dioxide induction claim that a displacement rate of 30–70% of the chamber or cage volume per minute is optimal for the euthanasia of small rodents.[140]: 5, 31 Percentages of CO2 vary for different species, based on identified optimal percentages claimed to minimize distress.[140]: 22

Carbon dioxide is also used in several related [cleaning and surface-preparation](/source/Carbon_dioxide_cleaning) techniques.

## History of discovery

Crystal structure of [dry ice](/source/Dry_ice)

Carbon dioxide was the first gas to be described as a discrete substance. In about 1640,[141] the [Flemish](/source/Flemish_people) chemist [Jan Baptist van Helmont](/source/Jan_Baptist_van_Helmont) observed that when he burned [charcoal](/source/Charcoal) in a closed vessel, the mass of the resulting [ash](/source/Ash_(analytical_chemistry)) 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" (from Greek "chaos") or "wild spirit" (*spiritus sylvestris*).[142]

The properties of carbon dioxide were further studied in the 1750s by the [Scottish](/source/Scotland) physician [Joseph Black](/source/Joseph_Black). He found that [limestone](/source/Limestone) ([calcium carbonate](/source/Calcium_carbonate)) could be heated or treated with [acids](/source/Acid) 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](/source/Limewater) (a saturated aqueous solution of [calcium hydroxide](/source/Calcium_hydroxide)), it would [precipitate](/source/Precipitation_(chemistry)) 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](/source/Joseph_Priestley) published a paper entitled *Impregnating Water with Fixed Air* in which he described a process of dripping [sulfuric acid](/source/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.[143]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by [Humphry Davy](/source/Humphry_Davy) and [Michael Faraday](/source/Michael_Faraday).[144] The earliest description of solid carbon dioxide ([dry ice](/source/Dry_ice)) was given by the French inventor [Adrien-Jean-Pierre Thilorier](/source/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.[145][146]

Carbon dioxide in combination with nitrogen was known from earlier times as [Blackdamp](/source/Blackdamp), stythe or choke damp.[b] Along with the other types of [damp](/source/Damp_(mining)) it was encountered in mining operations and well sinking. Slow oxidation of coal and biological processes replaced the oxygen to create a [suffocating](/source/Suffocation) mixture of nitrogen and carbon dioxide.[147]

## See also

- [Chemistry portal](https://en.wikipedia.org/wiki/Portal:Chemistry)

- [Arterial blood gas test](/source/Arterial_blood_gas_test) – Blood test that measures amounts of dissolved gas

- [Bosch reaction](/source/Bosch_reaction) – Process that is used to industrially create hydrogen

- [Carbon dioxide removal](/source/Carbon_dioxide_removal) – Removal of atmospheric carbon dioxide through human activity (from the atmosphere)

- [Gilbert Plass](/source/Gilbert_Plass) – Canadian physicist (1920–2004) (early work on CO2 and climate change)

- [Greenhouse Gases Observing Satellite](/source/Greenhouse_Gases_Observing_Satellite) – Japanese Earth observation satellite

- [List of countries by carbon dioxide emissions](/source/List_of_countries_by_carbon_dioxide_emissions)

- [List of least carbon efficient power stations](/source/List_of_least_carbon_efficient_power_stations)

- [Meromictic lake](/source/Meromictic_lake) – Permanently stratified lake with layers of water that do not intermix

- [NASA](/source/NASA)'s [Orbiting Carbon Observatory 2](/source/Orbiting_Carbon_Observatory_2) – NASA climate satellite

- [Soil gas](/source/Soil_gas) – Gases in the air space between soil components

## Notes

1. **[^](#cite_ref-10)** where "part" here means per [molecule](/source/Molecule)[9]

1. **[^](#cite_ref-148)** Sometimes spelt "choak-damp" in 19th Century texts.

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## External links

Wikimedia Commons has media related to [Carbon dioxide](https://commons.wikimedia.org/wiki/Category:Carbon_dioxide).

[Library resources](https://en.wikipedia.org/wiki/Wikipedia:The_Wikipedia_Library) about
 **Carbon dioxide**

- [Resources in your library](https://ftl.toolforge.org/cgi-bin/ftl?st=&su=Carbon+dioxide)

- [Resources in other libraries](https://ftl.toolforge.org/cgi-bin/ftl?st=&su=Carbon+dioxide&library=0CHOOSE0)

- [Current global map of carbon dioxide concentration](https://earth.nullschool.net/#current/chem/surface/level/overlay=co2sc/winkel3)

- [CDC – NIOSH Pocket Guide to Chemical Hazards – Carbon Dioxide](https://www.cdc.gov/niosh/npg/npgd0103.html)

- [Trends in Atmospheric Carbon Dioxide](https://gml.noaa.gov/ccgg/trends/) (NOAA)

- [The rediscovery of CO2: History, What is Shecco?](https://web.archive.org/web/20071007040239/http://www.shecco.com/about/history.php) - as [refrigerant](/source/Refrigerant)

v t e Salts and covalent derivatives of the oxide ion H2O He Li2O BeO B6O BO B2O3 C2O C12O9 C3O2 CO CO2 CO3 CxOy N2O NO N2O2 N2O3 N2O4 NO2 N2O5 NxOy O2−2 F Ne Na2O MgO Al2O AlO Al2O3 SiO SiO2 PO P4O6 P4O10 SO S2O2 SO2 SO3 Cl2O ClO Cl2O4 ClO2 Cl2O5? Cl2O6 Cl2O7 Ar K2O CaO Sc2O3 TiO Ti2O3 TiO2 VO V2O3 VO2 V2O5 CrO Cr2O3 CrO2 CrO3 MnO Mn3O4 Mn2O3 MnO2 Mn2O7 FeO Fe3O4 Fe2O3 CoO Co3O4 Co2O3 NiO Ni2O3 Cu2O CuO ZnO Ga2O Ga2O3 GeO GeO2 As2O3 As2O5 SeO2 SeO3 Br2O BrO BrO2 Br3O8 Kr Rb2O SrO YO Y2O3 ZrO ZrO2 NbO NbO2 Nb2O5 MoO2 MoO3 TcO2 Tc2O7 RuO2 RuO4 Rh2O3 RhO2 PdO Ag2O Ag4O4 Ag2O3 CdO In2O3 SnO SnO2 Sb2O3 Sb2O4 Sb2O5 TeO2 TeO3 I2O IO IO2 I2O5 XeO3 XeO4 Cs2O BaO * Lu2O3 HfO2 Ta2O5 W2O3 WO2 "W2O5" WO3 ReO2 ReO3 Re2O7 OsO2 OsO4 IrO2 IrO4 PtO2 Au2O3 Hg2O HgO Tl2O Tl2O3 PbO Pb3O4 PbO2 Bi2O3 Bi2O5 PoO PoO2 PoO3 At Rn Fr RaO ** Lr RfO2? Db2O5? SgO3 Bh2O7? HsO4 Mt Ds Rg Cn Nh Fl Mc Lv Ts Og * La2O3 Ce2O3 CeO2 Pr2O3 Pr6O11 PrO2 Nd2O3 Pm2O3 Sm2O3 EuO Eu2O3 Gd2O3 Tb2O3 Tb4O7 TbO2 Dy2O3 Ho2O3 Er2O3 Tm2O3 Yb2O3 ** Ac2O3 ThO ThO2 PaO PaO2 Pa2O5 UO2 U2O5 U3O8 UO3 Np2O3 NpO2 Np2O5 NpO3 Pu2O3 PuO2 PuO3 Am2O3 AmO2 Cm2O3 CmO2 BkO Bk2O3 BkO2 Cf2O3 CfO2 Es2O3 Fm Md No

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

v t e Inorganic compounds of carbon and related ions Compounds CF CO CO2 CO3 CO4 CO5 CO6 COS CS C2S2 CS2 CSe2 CSSe C3O2 C3S2 SiC Carbon ions Carbides [:C≡C:]2−, [::C::]4−, [:C=C=C:]4− Cyanide [:C≡N:]− Cyanate [:O−C≡N:]− Thiocyanate [:S−C≡N:]− Fulminate [:C≡N−O:]− Thiofulminate [:C≡N−S:]− Nanostructures Graphite intercalation compounds Fullerides Oxides and related Oxides Nitrides Metal carbonyls Carbonic acid Bicarbonates Carbonates

v t e 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 the atmosphere of Earth) 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 International Conference on Transitioning Away from Fossil Fuels Years in climate change 2019 2020 2021 2022 2023 2024 2025 2026 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 Climate sensitivity 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 Multi-breadbasket failure In the United States Children Cities Civilizational collapse Crime Depopulation of settlements Destruction of cultural heritage Disability Economic impacts U.S. insurance industry Fisheries Gender Health Infectious diseases Mental health In the United Kingdom In the Philippines Human rights Indigenous peoples 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+ reforestation Land use, land-use change, and forestry (LULUCF and AFOLU) Nature-based solutions Other Earth Strike Fridays for Future Geoengineering Individual action on climate change Positive tipping points Sustainable architecture Sustainable transport 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 Integrity Council for the Voluntary Carbon Market Taskforce on Scaling Voluntary Carbon Markets 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 International Conference on Transitioning Away from Fossil Fuels 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 Climate change portal Category Glossary Index

v t e Molecules detected in outer space Molecules Diatomic Aluminium monochloride Aluminium monofluoride Aluminium(II) oxide Argonium Carbon cation Carbon monophosphide Carbon monosulfide Carbon monoxide Cyano radical Diatomic carbon Fluoromethylidynium Helium hydride ion Hydrogen chloride Hydrogen fluoride Hydrogen (molecular) Hydroxyl radical Imidogen Iron(II) oxide Magnesium monohydride Methylidyne radical Nitric oxide Nitrogen (molecular) Oxygen (molecular) Phosphorus monoxide Phosphorus mononitride Potassium chloride Silicon carbide Silicon monoxide Silicon monosulfide Sodium chloride Sodium iodide Sulfanyl Sulfur mononitride Sulfur monoxide Titanium(II) oxide Triatomic Aluminium(I) hydroxide Aluminium isocyanide Amino radical Carbon dioxide Carbonyl sulfide CCP radical Chloronium Diazenylium Dicarbon monoxide Disilicon carbide Ethynyl radical Formyl radical Hydrogen cyanide (HCN) Hydrogen isocyanide (HNC) Hydrogen sulfide Hydroperoxyl Iron cyanide Isoformyl Magnesium cyanide Magnesium isocyanide Methylene Methylidynephosphane N2H+ Nitrous oxide Nitroxyl Ozone Potassium cyanide Sodium cyanide Sodium hydroxide Silicon carbonitride c-Silicon dicarbide SiNC Sulfur dioxide Thioformyl Thioxoethenylidene Titanium dioxide Tricarbon Trihydrogen cation Water Four atoms Acetylene Ammonia Cyanoethynyl Formaldehyde Fulminic acid HCCN Hydrogen peroxide Hydromagnesium isocyanide Isocyanic acid Isothiocyanic acid Ketenyl Methyl cation Methyl radical Methylene amidogen Propynylidyne Protonated carbon dioxide Protonated hydrogen cyanide Silicon tricarbide Thiocyanic acid Thioformaldehyde Tricarbon monosulfide Tricarbon monoxide Five atoms Ammonium ion Butadiynyl Carbodiimide Cyanamide Cyanoacetylene Cyanoformaldehyde Cyanomethyl Cyclopropenylidene Formic acid Isocyanoacetylene Ketene Methane Methoxy radical Methylenimine Propadienylidene Protonated formaldehyde Silane Silicon-carbide cluster Six atoms Acetonitrile Cyanobutadiynyl radical Cyclopropenone Diacetylene E-Cyanomethanimine Ethylene Formamide HC4N Ketenimine Methanethiol Methanol Methyl isocyanide Pentynylidyne Propynal Protonated cyanoacetylene Seven atoms Acetaldehyde Acrylonitrile Vinyl cyanide Cyanodiacetylene Ethylene oxide Glycolonitrile Hexatriynyl radical Methyl isocyanate Methylamine Propyne Vinyl alcohol Eight atoms Acetic acid Acrolein Aminoacetonitrile Cyanoallene Ethanimine Glycolaldehyde Hexapentaenylidene Methyl formate Methylcyanoacetylene Nine atoms Acetamide Cyanohexatriyne Dimethyl ether Ethanethiol Ethanol Methyldiacetylene N-Methylformamide Octatetraynyl radical Propene Propionitrile Ten atoms or more Acetone Benzene Benzonitrile Buckminsterfullerene (C60, C60+, fullerene, buckyball) Butyronitrile C70 fullerene Cyanodecapentayne Cyclopentindene Ethyl formate Ethylene glycol Heptatrienyl radical Methyl acetate Methyl-cyano-diacetylene Methyltriacetylene Propionaldehyde Pyrimidine Deuterated molecules Ammonia Ammonium ion Formaldehyde Formyl radical Heavy water Hydrogen cyanide Hydrogen deuteride Hydrogen isocyanide N2D+ Propyne Trihydrogen cation Unconfirmed Anthracene Dihydroxyacetone Glycine Graphene H2NCO+ Hemolithin Linear C5 Methoxyethane Naphthalene cation Phosphine Pyrene Silylidyne Related Abiogenesis Astrobiology Astrochemistry Atomic and molecular astrophysics Chemical formula Circumstellar dust Circumstellar envelope Cosmic dust Cosmic ray Cosmochemistry Diffuse interstellar band Earliest known life forms Extraterrestrial life Extraterrestrial liquid water Forbidden mechanism Homochirality Intergalactic dust Interplanetary medium Interstellar medium Iron–sulfur world theory Kerogen Molecules in stars Nexus for Exoplanet System Science Organic compound Outer space PAH world hypothesis Photodissociation region Polycyclic aromatic hydrocarbon (PAH) Pseudo-panspermia RNA world hypothesis Spectroscopy Tholin Category:Astrochemistry Outer space portal Astronomy portal Chemistry portal

v t e Oxygen compounds Ag4O4 Al2O3 AmO2 Am2O3 As2O3 As2O5 Au2O3 B2O3 BaO BeO Bi2O3 BiO2 Bi2O5 BrO2 Br2O3 Br2O5 Br 3O 8 CO CO2 C3O2 CaO CaO2 CdO CeO2 Ce3O4 Ce2O3 ClO2 Cl2O Cl2O2 Cl2O3 Cl2O4 Cl2O6 Cl2O7 CoO Co2O3 Co3O4 CrO3 Cr2O3 Cr2O5 Cr5O12 CsO2 Cs2O3 CuO Dy2O3 Er2O3 Eu2O3 FeO Fe2O3 Fe3O4 Ga2O Ga2O3 GeO GeO2 H2O 2H2O 3H2O H218O H2O2 HfO2 HgO Hg2O Ho2O3 IO I2O4 I2O5 I2O6 I4O9 In2O3 IrO2 KO2 K2O2 La2O3 Li2O Li2O2 Lu2O3 MgO Mg2O3 MnO MnO2 Mn2O3 Mn2O7 MoO2 MoO3 Mo2O3 NO NO2 N2O N2O3 N2O4 N2O5 NaO2 Na2O Na2O2 NbO NbO2 Nd2O3 O2F OF OF2 O2F2 O3F2 O4F2 O5F2 O6F2 O2PtF6 more...

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Adapted from the Wikipedia article [Carbon dioxide](https://en.wikipedia.org/wiki/Carbon_dioxide) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Carbon_dioxide?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.