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{{Chembox new
| Name = '''Carbon dioxide'''
| ImageFile = Carbon-dioxide-2D-dimensions.svg
| ImageSize = 140px
| ImageFile1 = Carbon-dioxide-3D-vdW.svg
| ImageSize1 = 140px
| ImageName = Carbon dioxide
| IUPACName = Carbon dioxide
| OtherNames = Carbonic acid gas; carbonic anhydride; [[dry ice]] (solid)
| Section1 = {{Chembox Identifiers
| CASNo = 124-38-9
| PubChem = 280
| SMILES = C(=O)=O
| EINECS = 204-696-9
| RTECS = FF6400000
| InChI = 1/CO2/c2-1-3
}}
| Section2 = {{Chembox Properties
| Formula = CO<sub>2</sub>
| MolarMass = 44.0095(14) g/mol
| Appearance = colorless gas
| Density = 1,600 g/L, solid; 1.98 g/L, gas
| Solubility = 1.45 g/L
| MeltingPt = −57 °C (216 K) (under pressure)
| BoilingPt = −78 °C (195 K), ([[Sublimation (chemistry)|sublimes]])
| pKa = 6.35 and 10.33
| Viscosity = 0.07 c[[Poise|P]] at −78 °C
| Dipole = zero
}}
| Section3 = {{Chembox Structure
| MolShape = [[Linear (chemistry)|linear]]
}}
| Section8 = {{Chembox Related
| Function = [[oxide]]s
| OtherFunctn = [[carbon monoxide]]; [[carbon suboxide]]; [[dicarbon monoxide]]; [[carbon trioxide]]}}
}}

'''Carbon dioxide''' ([[chemical formula]]: '''{{chem|CO|2}}''') is a [[chemical compound]] composed of two [[oxygen]] [[atom]]s [[covalent bond|covalently bonded]] to a single [[carbon]] atom. It is a [[gas]] at [[standard temperature and pressure]] and exists in [[Earth's atmosphere]] in this state. It is currently at a globally averaged concentration of approximately 383 [[parts per million|ppm]] by [[volume]] in the Earth's atmosphere,<ref>{{cite journal|title=Atmospheric CO<sub>2</sub> records from sites in the SIO air sampling network.|first=Keeling, CD|last=Whorf, T.P.|year=2005|work=Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tenn., U.S.A.|url=http://cdiac.ornl.gov/trends/co2/sio-mlo.htm}} Period of record: 1958-2004</ref> although this varies both by location and time. Carbon dioxide is an important [[greenhouse gas]] because it transmits [[visible spectrum|visible light]] but absorbs strongly in the [[infrared]].

Carbon dioxide is produced by all animals, plants, fungi and microorganisms during [[respiration]] and is used by plants during [[photosynthesis]]. This is to make sugars which may either be consumed again in respiration or used as the raw material for plant growth. It is, therefore, a major component of the [[carbon cycle]]. Carbon dioxide is generated as a byproduct of the combustion of [[fossil fuels]] or vegetable matter, among other chemical processes. [[Inorganic]] carbon dioxide is output by [[volcano]]es and other [[geothermal]] processes such as [[hot springs]].

Carbon dioxide has no liquid state at pressures below 5.1 [[atmosphere (unit)|atm]], but is a solid at temperatures below -78 °C. In its solid state, carbon dioxide is commonly called [[dry ice]].

CO<sub>2</sub> is an [[acidic oxide]]: an aqueous solution turns [[litmus test (chemistry)|litmus]] from blue to pink.

==Chemical and physical properties==

[[Image:Carbon dioxide pressure-temperature phase diagram.jpg|left|thumb|220px|Carbon dioxide pressure-temperature phase diagram showing the [[triple point]] of carbon dioxide]]

{{details|Carbon dioxide (data page)}}Carbon dioxide is a colorless, odorless gas. When inhaled at concentrations much higher than usual atmospheric levels, it can produce a sour taste in the mouth and a stinging sensation in the nose and throat. These effects result from the gas dissolving in the [[mucous membranes]] and [[saliva]], forming a weak solution of [[carbonic acid]]. This sensation can also occur during an attempt to stifle a burp after drinking a [[Carbonation|carbonated beverage]]. Amounts above 5,000 ppm are considered very unhealthy, and those above about 50,000 ppm (equal to 5% by volume) are considered dangerous to animal life.<ref>{{cite web | author = Staff | date= [[16 August]] [[2006]] | url = http://www.cdc.gov/niosh/idlh/124389.html | title = Carbon dioxide: IDLH Documentation | publisher = National Institute for Occupational Safety and Health | accessdate = 2007-07-05}}</ref>

At [[Standard conditions for temperature and pressure|standard temperature and pressure]], the density of carbon dioxide is around 1.98 kg/m³, about 1.5 times that of [[Earth's atmosphere|air]]. The carbon dioxide molecule (O=C=O) contains two [[covalent bond|double bonds]] and has a linear shape. It has no electrical [[dipole]], and as it is fully [[Redox|oxidized]], it is moderately [[Chemical reaction|reactive]] and is non-flammable, but will support the combustion of metals such as [[magnesium]].
{{clear}}
{| style = "float:left"
|-
|
[[Image:Dry Ice Pellets Subliming.jpg|thumb|left|220px|Small pellets of dry ice subliming in air.]]
|-
|
[[Image:Carbon-dioxide-crystal-3D-vdW.png|thumb|left|220px|Crystal structure of dry ice]]
|}
At −78.51° [[Celsius|C]] or -109.3° [[Fahrenheit|F]], carbon dioxide changes directly from a solid phase to a gaseous phase through [[sublimation (chemistry)|sublimation]], or from gaseous to solid through [[Deposition (chemistry)|deposition]]. Solid carbon dioxide is normally called "[[dry ice]]", a [[generic trademark]]. It was first observed in 1825 by the French chemist [[Charles Thilorier]]. Dry ice is commonly used as a cooling agent, and it is relatively inexpensive. A convenient property for this purpose is that solid carbon dioxide sublimes directly into the gas phase leaving no liquid. It can often be found in groceries and laboratories, and it is also used in the shipping industry. The largest non-cooling use for dry ice is blast cleaning.

Liquid carbon dioxide forms only at [[pressure]]s above 5.1 atm; the [[triple point]] of carbon dioxide is about 518 [[kPa]] at −56.6 °C (See phase diagram, above). The [[Critical point (thermodynamics)|critical point]] is 7.38 MPa at 31.1 °C.<ref>{{cite web|url=http://webbook.nist.gov/cgi/cbook.cgi?ID=C124389&Units=SI&Mask=4#Thermo-Phase|title=Phase change data for Carbon dioxide|publisher=National Institute of Standards and Technology|accessdate=2008-01-21}}</ref>

An alternative form of solid carbon dioxide, an [[amorphous]] glass-like form, is possible, although not at atmospheric pressure.<ref>{{cite journal | last=Santoro | first=M. | coauthors=et al | year=2006 | title=Amorphous silica-like carbon dioxide | url= | journal=Nature | issn=0028-0836 | volume=441 | issue=7095 | pages=857-860 | doi=10.1038/nature04879}}</ref> This form of glass, called [[amorphous carbonia|''carbonia'']], was produced by [[supercooling]] heated {{chem|CO|2}} at extreme pressure (40–48 [[GPa]] or about 400,000 atmospheres) in a [[diamond anvil]]. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like [[silicon]] ([[silica|silica glass]]) and [[germanium]]. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.

{{seealso|Supercritical carbon dioxide|dry ice}}
{{clear}}

==History of human understanding==

Carbon dioxide was one of the first gases to be described as a substance distinct from air. In the seventeenth century, the [[Flemish people|Flemish]] chemist [[Jan Baptist van Helmont]] observed that when he burned [[charcoal]] in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (''spiritus sylvestre'').

The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician [[Joseph Black]]. He found that [[limestone]] ([[calcium carbonate]]) could be heated or treated with [[acid]]s to yield a gas he called "fixed air." He observed that the fixed air was denser than air and did not support either flame or animal life. He also found that when bubbled through an aqueous solution of lime ([[calcium hydroxide]]), it would [[Precipitation (chemistry)|precipitate]] calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist [[Joseph Priestley]] published a paper entitled ''Impregnating Water with Fixed Air'' in which he described a process of dripping [[sulfuric acid]] (or ''oil of vitriol'' as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas.<ref name="Priestley">{{cite journal | first = Joseph | last = Priestley | authorlink = Joseph Priestley | title = Observations on Different Kinds of Air | journal = Philosophical Transactions | issn = 0260-7085 | volume = 62 | year = 1772 | pages = 147-264 | url = http://web.lemoyne.edu/~GIUNTA/priestley.html}}</ref>

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by [[Humphry Davy]] and [[Michael Faraday]].<ref name="Davy">{{cite journal | first = Humphry | last = Davy | authorlink = Humphry Davy | title = On the Application of Liquids Formed by the Condensation of Gases as Mechanical Agents | journal = Philosophical Transactions | issn = 0261-0523 | volume = 113 | year = 1823 | pages = 199-205 | url = http://www.journals.royalsoc.ac.uk/content/r004631789435274/fulltext.pdf }}</ref> The earliest description of solid carbon dioxide was given by [[Charles Thilorier]], who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid {{chem|CO|2}}.<ref>{{cite journal | title = Thilorier and the First Solidification of a "Permanent" Gas (1835) | last = Duane | first = H.D. Roller | coauthors = M. Thilorier | journal = Isis | issn = 0021-1753 | volume = 43 | issue = 2 | year = 1952 | pages = 109–113 | url = }}</ref>

==Isolation==

Carbon dioxide may be obtained from air [[distillation]]. However, this yields only very small quantities of {{chem|CO|2}}. A large variety of chemical reactions yield carbon dioxide, such as the reaction between most acids and most metal carbonates. For example, the reaction between [[sulfuric acid]] and calcium carbonate (limestone or chalk) is depicted below:

: {{chem|H|2|SO|4| + CaCO|3| → CaSO|4| + H|2|CO|3}}

The {{chem|H|2|CO|3}} then decomposes to water and {{chem|CO|2}}. Such reactions are accompanied by foaming or bubbling, or both. In industry such reactions are widespread because they can be used to neutralize waste acid streams.

The production of [[quicklime]] (CaO) a chemical that has widespread use, from limestone by heating at about 850 °C also produces {{chem|CO|2}}:

: {{chem|CaCO|3| → CaO + CO|2}}

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

: {{chem|CH|4| + 2 O|2| → CO|2| + 2 H|2|O}}

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

: {{chem|2 Fe|2|O|3| + 3 C → 4 Fe + 3 CO|2}}

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

: [[Glucose|{{chem|C|6|H|12|O|6}}]] → {{chem|2 CO|2| + 2 C|2|H|5|OH}}

All [[cellular respiration|aerobic]] organisms produce {{chem|CO|2}} when they oxidize [[carbohydrate]]s, [[fatty acid]]s, and proteins in the mitochondria of cells. The large number of reactions involved are exceedingly complex and not described easily. Refer to ([[cellular respiration]], [[anaerobic respiration]] and [[photosynthesis]]). [[Photoautotrophs]] (i.e. plants, [[cyanobacteria]]) use another ''modus operandi'': Plants absorb {{chem|CO|2}} from the air, and, together with water, react it to form carbohydrates:

: {{chem|''n''CO|2| + ''n''H|2|O → (CH|2|O)|n| + ''n''O|2}}

Carbon dioxide is [[soluble]] in water, in which it spontaneously interconverts between {{chem|CO|2}} and {{chem|H|2|CO|3}} ([[carbonic acid]]). The relative concentrations of {{chem|CO|2|, H|2|CO|3}}, and the deprotonated forms {{chem|HCO|3|<sup>-</sup>}} ([[bicarbonate]]) and {{chem|CO|3|<sup>2-</sup>}}([[carbonate]]) depend on the [[pH]]. In neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater, while in very alkaline water (pH > 10.4) the predominant (>50%) form is carbonate. The bicarbonate and carbonate forms are very soluble, such that air-equilibrated ocean water (mildly alkaline with typical pH = 8.2 – 8.5) contains about 120 mg of bicarbonate per liter.

===Industrial production===

Carbon dioxide is manufactured mainly from six processes:<ref name="kirk">{{cite encyclopedia | title = Carbon Dioxide | first = Ronald | last = Pierantozzi | encyclopedia = Kirk-Othmer Encyclopedia of Chemical Technology | publisher = Wiley | year = 2001 | doi = 10.1002/0471238961.0301180216090518.a01.pub2 }}</ref>

# As a byproduct in ammonia and hydrogen plants, where methane is converted to {{chem|CO|2}};
# From combustion of [[wood]] and [[fossil fuel]]s;
# As a byproduct of [[Fermentation (biochemistry)|fermentation]] of [[sugar]] in the [[brewing]] of [[beer]], [[whisky]] and other [[alcohol]]ic [[beverage]]s;
# From thermal decomposition of limestone, {{chem|CaCO|3}}, in the manufacture of [[lime]], {{chem|CaO}};
# As a byproduct of [[sodium phosphate]] manufacture;
# Directly from natural carbon dioxide [[spring (hydrosphere)|springs]], where it is produced by the action of acidified water on [[limestone]] or [[dolomite]].

==Uses==
[[Image:Soda bubbles macro.jpg|thumb|250px|Carbon dioxide bubbles in a soft drink.]]

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry.<ref name="kirk" /> It is used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately 60 [[Bar (unit)|bar]] (870 psi, 59 atm), allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Aluminum capsules are also sold as supplies of compressed gas for [[Air gun|airguns]], [[paintball]] markers, for inflating bicycle tires, and for making [[carbonated water|seltzer]]. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines. High concentrations of carbon dioxide can also be used to kill pests, such as the [[Common Clothes Moth]].

Carbon dioxide is used to produce [[carbonated]] [[soft drink]]s and [[soda water]]. Traditionally, the carbonation in beer and sparkling wine comes about through natural fermentation, but some manufacturers carbonate these drinks artificially.
A candy called [[Pop Rocks]] is pressurized with carbon dioxide gas at about 40 bar (600 psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

[[Leavening agent]]s produce carbon dioxide to cause dough to rise. [[Baker's yeast]] produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as [[baking powder]] and [[baking soda]] release carbon dioxide when heated or if exposed to [[acid]]s.

[[Image:Carbon Dioxide Laser At The Laser Effects Test Facility.jpg|thumb|right|300px|A [[carbon dioxide laser]].]]

Carbon dioxide is the most commonly used compressed gas for pneumatic systems in portable pressure tools and [[Robot combat|combat robots]].

Carbon dioxide extinguishes flames, and some [[Fire extinguisher#Carbon dioxide|fire extinguishers]], especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide also finds use as an atmosphere for [[welding]], although in the welding arc, it reacts to [[oxidation|oxidize]] most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are [[brittle]]r than those made in more inert atmospheres, and that such weld joints deteriorate over time because of the formation of carbonic acid. It is used as a welding gas primarily because it is much less expensive than more inert gases such as [[argon]] or [[helium]].

Liquid carbon dioxide is a good [[solvent]] for many [[lipophilic]] [[organic chemistry|organic compounds]], and is used to remove [[caffeine]] from [[coffee]]. First, the green coffee beans are soaked in water. The beans are placed in the top of a column seventy feet (21 meters) high. The carbon dioxide fluid at about 93 degrees Celsius enters at the bottom of the column. The caffeine diffuses out of the beans and into the carbon dioxide.

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

Plants require carbon dioxide to conduct [[photosynthesis]], and greenhouses may enrich their atmospheres with additional {{chem|CO|2}} to boost plant growth, since its low present-day atmosphere concentration is just above the "suffocation" level for green plants. A [[photosynthesis]]-related drop in carbon dioxide concentration in a greenhouse compartment can kill green plants. At high concentrations, carbon dioxide is toxic to animal life, so raising the concentration to 10,000 ppm (1%) for several hours can eliminate pests such as [[whitefly|whiteflies]] and [[spider mite]]s in a greenhouse.

It has been proposed that carbon dioxide from power generation be bubbled into ponds to grow algae that could then be converted into [[biodiesel]] fuel.<ref name='csmon'>{{cite news | first=Mark | last=Clayton | coauthors= | title=Algae - like a breath mint for smokestacks | date=[[2006-01-11]] | publisher= | url =http://www.csmonitor.com/2006/0111/p01s03-sten.html | work =[[Christian Science Monitor]] | pages = | accessdate = 2007-10-11 | language = }}</ref>
In medicine, up to 5% carbon dioxide is added to pure [[oxygen]] for stimulation of breathing after [[apnea]] and to stabilize the {{chem|O|2|/CO|2}} balance in blood.

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

Carbon dioxide can also be combined with [[limonene]] oxide from orange peels or other [[epoxides]] to create polymers and plastics.<ref>{{cite web |url=http://www.news.cornell.edu/releases/Jan05/Orangeplastic.deb.html |title=Sweet and environmentally beneficial discovery: Plastics made from orange peel and a greenhouse gas |accessdate=2007-09-09 |last=Davidson |first=Sarah |coauthors= |date=Jan. 17, 2005 |work= |publisher=Cornell News}}</ref>

Carbon dioxide is used in [[enhanced oil recovery]] where it is injected into or adjacent to producing oil wells, usually under [[Supercritical fluid|supercritical]] conditions. It acts as both a pressurizing agent and, when dissolved into the underground [[crude oil]], significantly reduces its viscosity, enabling the oil to flow more rapidly through the earth to the removal well.<ref>{{cite journal |last=Austell |first=J Michael |authorlink= |coauthors= |year=2005 |month= |title=CO2 for Enhanced Oil Recovery Needs - Enhanced Fiscal Incentives |journal=Exploration & Production: The Oil & Gas Review - |volume= |issue= |pages= |id= |url=http://www.touchoilandgas.com/enhanced-recovery-needs-enhanced-a423-1.html |accessdate= 2007-09-28 |quote= }}</ref> In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

In the chemical industry, carbon dioxide is used for the production of [[urea]], [[carbonate]]s and [[bicarbonate]]s, and [[sodium salicylate]].

Liquid and solid carbon dioxide are important [[refrigerant]]s, especially in the food industry, where they are employed during the transportation and storage of ice cream and other frozen foods. Solid carbon dioxide is called "dry ice" and is used for small shipments where refrigeration equipment is not practical.

Liquid carbon dioxide (industry nomenclature R744 / R-744) was used as a refrigerant prior to the discovery of [[Dichlorodifluoromethane|R-12]] and is likely to enjoy a renaissance due to environmental concerns. Its physical properties are highly favorable for cooling, refrigeration, and heating purposes, having a high volumetric cooling capacity. Due to its operation at pressures of up to 130 bars, {{chem|CO|2}} systems require highly resistant components that have been already developed to serial production in many sectors. In car air conditioning, in more than 90% of all driving conditions, R744 operates more efficiently than systems using [[R-134a]]. Its environmental advantages ([[Global warming potential|GWP]] of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, hot water heat pumps, among others. Some applications: Coca-Cola has fielded {{chem|CO|2}}-based beverage coolers and the [[US Army]] is interested in {{chem|CO|2}} refrigeration and heating technology.<ref name='ccref1'> {{cite web|url=http://www.thecoca-colacompany.com/presscenter/nr_20060605_corporate_hfc-free.html |title=THE COCA-COLA COMPANY ANNOUNCES ADOPTION OF HFC-FREE INSULATION IN REFRIGERATION UNITS TO COMBAT GLOBAL WARMING |accessdate=2007-10-11 |date=2006-06-05 |publisher=The Coca-Cola Company }}</ref><ref name='usforces'>{{cite news | title = Modine reinforces its CO<sub>2</sub> research efforts | url = http://www.r744.com/news/news_ida145.php | date = 2007-06-28 | publisher = R744.com}}</ref>

By the end of 2007, the global car industry is expected to decide on the next-generation refrigerant in car air conditioning. {{chem|CO|2}} is one discussed option.(see [[The Cool War]])

==In the Earth's atmosphere==

[[Image:Mauna Loa Carbon Dioxide.png|thumbnail|right|280px|Atmospheric {{chem|CO|2}} concentrations measured at [[Mauna Loa Observatory]].]]

{{further|[[Carbon dioxide in the Earth's atmosphere]]}}

Carbon dioxide in [[earth's atmosphere]] is considered a [[trace gas]] currently occurring at an average concentration of about 385 parts per million by volume or 582 parts per million by mass. The mass of the [[Earth atmosphere]] is 5.14×10<sup>18</sup> kg <ref>[http://www.agu.org/pubs/crossref/1988/87JD00743.shtml Global atmospheric mass, surface pressure, and water vapor variations<!-- Bot generated title -->]</ref>, so the total mass of atmospheric carbon dioxide is 3.0×10<sup>15</sup> kg (3,000 gigatonnes). Its concentration varies seasonally (see graph at right) and also considerably on a regional basis: in urban areas it is generally higher and indoors it can reach 10 times the background atmospheric concentration.

Carbon dioxide is a [[greenhouse gas]]; see [[greenhouse effect]] for more.

Due to human activities such as the combustion of [[fossil fuels]] and [[deforestation]], the concentration of atmospheric carbon dioxide has increased by about 35% since the beginning of the [[Industrial Revolution|age of industrialization]].<ref>[http://www.noaanews.noaa.gov/stories2005/s2412.htm NOAA News Online (Story 2412)<!-- Bot generated title -->]</ref>
In 1999, 2,244,804,000 metric tons of {{chem|CO|2}} were produced in the U.S. as a result of electric energy generation. This is an output rate of 0.6083 kg (1.341 pounds) per kWh.<ref>{{cite web | title=Carbon Dioxide Emissions from the Generation of Electric Power in the United States | url=http://www.eia.doe.gov/cneaf/electricity/page/co2_report/co2emiss.pdf }} </ref>

Five hundred million years ago carbon dioxide was 20 times more prevalent than today, decreasing to 4-5 times during the [[Jurassic]] period and then maintained a slow decline until the [[industrial revolution]], with [[Azolla Event|a particularly swift reduction]] occurring 49 million years ago.<ref>{{cite web | title = Climate and CO2 in the Atmosphere | url=http://earthguide.ucsd.edu/virtualmuseum/climatechange2/07_1.shtml| accessdate=2007-10-10 }}</ref><ref>{{cite web | title = GEOCARB III: A REVISED MODEL OF ATMOSPHERIC CO2 OVER
PHANEROZOIC TIME| url=http://www.geocraft.com/WVFossils/Reference_Docs/Geocarb_III-Berner.pdf| accessdate=2008-2-15 }}</ref>

Up to 40% of the gas emitted by some [[volcano]]es during subaerial [[volcanic eruptions]] is carbon dioxide.<ref>Sigurdsson, H. et al., (2000) ''Encyclopedia of Volcanoes'', San Diego, Academic Press</ref> According to the best estimates, volcanoes release about 130-230 million tonnes (145-255 million tons) of {{chem|CO|2}} into the atmosphere each year. Carbon dioxide is also produced by hot springs such as those at the Bossoleto site near Rapolano Terme in Tuscany, Italy. Here, in a bowl-shaped depression of about 100 m diameter, local concentrations of {{chem|CO|2}} rise to above 75% overnight, sufficient to kill insects and small animals, but warm rapidly when sunlit and disperse by convection during the day<ref>vanGardingen PR, Grace J, Jeffree CE, Byari, S.H., Miglietta, F., Raschi, A., Bettarini, I. (1997) Long-term effects of enhanced CO2 concentrations on leaf gas exchange: research opportunities using CO2 springs. In Plant responses to elevated CO2. Evidence from natural springs. Ed. A. Raschi, F. Miglietta, R. Tognetti and P.R. van Gardingen. Cambridge University Press. pp. 69-86.</ref> Locally high concentrations of {{chem|CO|2}}, produced by disturbance of deep lake water saturated with {{chem|CO|2}} are thought to have caused 37 fatalities at [[Lake Monoun]], [[Cameroon]] in 1984 and 1700 casualties at [[Lake Nyos]], Cameroon in 1986<ref> M. Martini (1997)CO2 emissions in volcanic areas: case histories and hazaards. In Plant responses to elevated CO2. Evidence from natural springs. Ed. A. Raschi, F. Miglietta, R. Tognetti and P.R. van Gardingen. Cambridge University Press. pp. 69-86.</ref>. However, emissions of {{chem|CO|2}} by human activities are currently more than 130 times greater than the quantity emitted by volcanoes, amounting to about 27 billion tonnes per year (30 billion tons).<ref>{{cite web | title = Volcanic Gases and Their Effects
| url=http://volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html| accessdate=2007-09-07 }}</ref>

==In the oceans==

There is about 50 times as much carbon dissolved in the oceans in the form of CO<sub>2</sub> and CO<sub>2</sub> hydration products as exists in the atmosphere. The oceans act as an enormous [[carbon sink]], having "absorbed about one-third of all human-generated CO<sub>2</sub> emissions to date<ref>{{cite web | last = Doney | first = Scott C. | authorlink = | coauthors = Naomi M. Levine | title = How Long Can the Ocean Slow Global Warming? | publisher = Oceanus | date = 2006-11-29 | url = http://www.whoi.edu/oceanus/viewArticle.do?id=17726 | accessdate = 2007-11-21 }}</ref>." Generally, gas solubility decreases as water temperature increases. Accordingly carbon dioxide is released from ocean water into the atmosphere as ocean temperatures rise.

==Biological role==

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

===Role in photosynthesis===


Plants remove carbon dioxide from the atmosphere by photosynthesis, also called [[carbon fixation|carbon assimilation]], which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via [[photophosphorylation]]. This energy is required for the fixation of carbon dioxide in the [[Calvin cycle]] to form sugars. These sugars can then be used for growth within the plant through respiration.
Plants remove carbon dioxide from the atmosphere by photosynthesis, also called [[carbon fixation|carbon assimilation]], which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via [[photophosphorylation]]. This energy is required for the fixation of carbon dioxide in the [[Calvin cycle]] to form sugars. These sugars can then be used for growth within the plant through respiration.

Revision as of 15:28, 17 March 2008

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to form sugars. These sugars can then be used for growth within the plant through respiration.

Even when vented, carbon dioxide must be introduced into greenhouses to maintain plant growth, as the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm (a limit of C3 carbon fixation photosynthesis[citation needed]). Plants can potentially grow up to 50 percent faster in concentrations of 1,000 ppm CO
2 when compared with ambient conditions.[1]

Plants also emit CO
2 during respiration, so it is only during growth stages that plants are net absorbers. For example a growing forest will absorb many tons of CO
2 each year, however a mature forest will produce as much CO
2 from respiration and decomposition of dead specimens (e.g. fallen branches) as used in biosynthesis in growing plants.[2] Regardless of this, mature forests are still valuable carbon sinks, helping maintain balance in the Earth's atmosphere. Additionally, and crucially to life on earth, phytoplankton photosynthesis absorbs dissolved CO
2 in the upper ocean and thereby promotes the absorption of CO
2 from the atmosphere.[3]

Toxicity

Carbon dioxide content in fresh air varies between 0.03% (300 ppm) and 0.06% (600 ppm), depending on the location (see graphical map of CO
2 in real-time).

A person's exhaled breath is approximately 4.5% carbon dioxide by volume.

It is dangerous when inhaled in high concentrations (greater than 5% by volume, or 50,000 ppm). The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an eight-hour work day is 0.5% (5,000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues is significantly less.

These figures are valid for pure carbon dioxide. In indoor spaces occupied by people the carbon dioxide concentration will reach higher levels than in pure outdoor air. Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO
2 concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO
2 itself. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The CO
2 concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.

Acute carbon dioxide toxicity is sometimes known as by the names given to it by miners: blackdamp (also called choke damp or stythe). Miners would try to alert themselves to dangerous levels of carbon dioxide in a mine shaft by bringing a caged canary with them as they worked. The canary would inevitably die before CO
2 reached levels toxic to people. Carbon dioxide caused a great loss of life at Lake Nyos in Cameroon in 1986, when an upwelling of CO
2-laden lake water quickly blanketed a large surrounding populated area..[4] The heavier carbon dioxide forced out the life-sustaining oxygen near the surface, killing nearly two thousand people.

Carbon dioxide ppm levels (CDPL) are a surrogate for measuring indoor pollutants that may cause occupants to grow drowsy, get headaches, or function at lower activity levels. To eliminate most Indoor Air Quality complaints, total indoor CDPL must be reduced to below 600. NIOSH considers that indoor air concentrations that exceed 1,000 are a marker suggesting inadequate ventilation. ASHRAE recommends they not exceed 1,000 inside a space. OSHA limits concentrations in the workplace to 5,000 for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 30,000 and considers CDPL exceeding 40,000 as "immediately dangerous to life and health." People who breathe 50,000 for more than half an hour show signs of acute hypercapnia, while breathing 70,000 – 100,000 can produce unconsciousness in only a few minutes. Accordingly, carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.

Human physiology

See also: Arterial blood gas

CO
2 is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood).

  • Most of it (about 70% – 80%) is converted to bicarbonate ions HCO3− by the enzyme carbonic anhydrase in the red blood cells,[5] by the reaction CO2 + H2O → H2CO3 → H+ + HCO3−.

The CO
2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO
2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen.

Hemoglobin, the main oxygen-carrying molecule in red blood cells, can carry both oxygen and carbon dioxide, although in quite different ways. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane Effect, and is important in the transport of carbon dioxide from the tissues to the lungs. Conversely, a rise in the partial pressure of CO
2 or a lower pH will cause offloading of oxygen from hemoglobin. This is known as the Bohr Effect.

Carbon dioxide may be one of the mediators of local autoregulation of blood supply. If its levels are high, the capillaries expand to allow a greater blood flow to that tissue.

Bicarbonate ions are crucial for regulating blood pH. A person's breathing rate influences the level of CO
2 in their blood. Breathing that is too slow or shallow can cause respiratory acidosis, while breathing that is too rapid may lead to hyperventilation, which may cause respiratory alkalosis.

Although the body requires oxygen for metabolism, low oxygen levels 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) may lead to loss of consciousness. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the oxygen mask to themselves first before helping others — otherwise one risks going unconscious without being aware of the imminent peril.[5]

According to a study by the United States Department of Agriculture, an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day.[6]

See also

References

  1. ^ Blom, T.J. (2002-12). "Carbon Dioxide In Greenhouses". Retrieved 2007-06-12. {{cite web}}: Check date values in: |date= (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  2. ^ "Global Environment Division Greenhouse Gas Assessment Handbook - A Practical Guidance Document for the Assessment of Project-level Greenhouse Gas Emissions". World Bank. Retrieved 2007-11-10.
  3. ^ Falkowski, P. (2000). "The global carbon cycle: a test of our knowledge of earth as a system". Science. 290 (5490): 291–296. doi:10.1126/science.290.5490.291. ISSN 0036-8075. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  4. ^ New York Times, "Trying to Tame the Roar of Deadly Lakes", February 27, 2001. [1].
  5. ^ a b c d "CARBON DIOXIDE". solarnavigator.net. Retrieved 2007-10-12.
  6. ^ Hannan, Jerry. "Your Role in the "Greenhouse Effect"". Retrieved 2006-04-19.
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