Carbon dioxide

Last updated

Carbon dioxide
Carbon-dioxide-2D-dimensions.svg
Ball-and-stick model of carbon dioxide Carbon dioxide 3D ball.png
Ball-and-stick model of carbon dioxide
Space-filling model of carbon dioxide Carbon dioxide 3D spacefill.png
Space-filling model of carbon dioxide
Names
Other names
  • Carbonic acid gas
  • Carbonic anhydride
  • Carbonic oxide
  • Carbon oxide
  • Carbon(IV) oxide
  • Dry ice (solid phase)
Identifiers
3D model (JSmol)
3DMet
1900390
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.004.271
EC Number
  • 204-696-9
E number E290 (preservatives)
989
KEGG
MeSH Carbon+dioxide
PubChem CID
RTECS number
  • FF6400000
UNII
UN number 1013 (gas), 1845 (solid)
Properties
CO2
Molar mass 44.009 g·mol−1
AppearanceColorless gas
Odor
  • Low concentrations: none
  • High concentrations: sharp; acidic [1]
Density
  • 1562 kg/m3(solid at 1 atm and −78.5 °C)
  • 1101 kg/m3(liquid at saturation −37°C)
  • 1.977 kg/m3(gas at 1 atm and 0 °C)
Melting point −56.6 °C; −69.8 °F; 216.6 K(Triple point at 5.1 atm)
Critical point (T, P)31.1 °C (304.2 K), 7.38 megapascals (73.8 bar)
−78.5 °C; −109.2 °F; 194.7 K (1 atm)
1.45 g/L at 25 °C (77 °F), 100 kPa
Vapor pressure 5.73 MPa (20 °C)
Acidity (pKa)6.35, 10.33
−20.5·10−6 cm3/mol
Thermal conductivity 0.01662 W m-1 K-1 (300 K) [2]
1.00045
Viscosity
  • 14.90 μPa·s at 25 °C [3]
  • 70 μPa·s at −78.5 °C
0 D
Structure
trigonal
linear
Thermochemistry
37.135 J/K mol
214 J·mol−1·K−1
−393.5 kJ·mol−1
Pharmacology
V03AN02 ( WHO )
Hazards
Safety data sheet See: data page
Sigma-Aldrich
NFPA 704 (fire diamond)
Lethal dose or concentration (LD, LC):
90,000 ppm (human, 5 min) [6]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 5000 ppm (9000 mg/m3) [7]
REL (Recommended)
TWA 5000 ppm (9000 mg/m3) ST 30,000 ppm (54,000 mg/m3) [7]
IDLH (Immediate danger)
40,000 ppm [7]
Related compounds
Other anions
Other cations
Related carbon oxides
Related compounds
Supplementary data page
Refractive index (n),
Dielectric constantr), etc.
Thermodynamic
data
Phase behaviour
solidliquidgas
UV, IR, NMR, MS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)
Infobox references

Carbon dioxide (chemical formula CO
2
) is a colorless gas with a density about 60% higher than that of dry air. Carbon dioxide consists of a carbon atom covalently double bonded to two oxygen atoms. It occurs naturally in Earth's atmosphere as a trace gas. The current concentration is about 0.04% (410  ppm) by volume, having risen from pre-industrial levels of 280 ppm. [8] Natural sources include volcanoes, hot springs and geysers, and it is freed from carbonate rocks by dissolution in water and acids. Because carbon dioxide is soluble in water, it occurs naturally in groundwater, rivers and lakes, ice caps, glaciers and seawater. It is present in deposits of petroleum and natural gas. Carbon dioxide is odorless at normally encountered concentrations, but at high concentrations, it has a sharp and acidic odor. [1]

Contents

As the source of available carbon in the carbon cycle, atmospheric carbon dioxide is the primary carbon source for life on Earth and its concentration in Earth's pre-industrial atmosphere since late in the Precambrian has been regulated by photosynthetic organisms and geological phenomena. Plants, algae and cyanobacteria use light energy to photosynthesize carbohydrate from carbon dioxide and water, with oxygen produced as a waste product. [9]

CO
2
is produced by all aerobic organisms when they metabolize carbohydrates and lipids to produce energy by respiration. [10] It is returned to water via the gills of fish and to the air via the lungs of air-breathing land animals, including humans. Carbon dioxide is produced during the processes of decay of organic materials and the fermentation of sugars in bread, beer and wine making. It is produced by combustion of wood and other organic materials and fossil fuels such as coal, peat, petroleum and natural gas. It is an unwanted byproduct in many large scale oxidation processes, for example, in the production of acrylic acid (over 5 million tons/year). [11] [12] [13] [14]

It is a versatile industrial material, used, for example, as an inert gas in welding and fire extinguishers, as a pressurizing gas in air guns and oil recovery, as a chemical feedstock and as a supercritical fluid solvent in decaffeination of coffee and supercritical drying. [15] It is added to drinking water and carbonated beverages including beer and sparkling wine to add effervescence. The frozen solid form of CO
2
, known as dry ice is used as a refrigerant and as an abrasive in dry-ice blasting. It is a feedstock for the synthesis of fuels and chemicals. [16] [17] [18] [19] [20]

Carbon dioxide is the most significant long-lived greenhouse gas in Earth's atmosphere. Since the Industrial Revolution anthropogenic emissions – primarily from use of fossil fuels and deforestation – have rapidly increased its concentration in the atmosphere, leading to global warming. Carbon dioxide also causes ocean acidification because it dissolves in water to form carbonic acid. [21]

Background

Crystal structure of dry ice Carbon-dioxide-crystal-3D-vdW.png
Crystal structure of dry ice

Carbon dioxide was the first gas to be described as a discrete substance. In about 1640, [22] the Flemish chemist Jan Baptist van Helmont observed that when he burned charcoal in a closed vessel, the mass of the resulting ash was much less than that of the original charcoal. His interpretation was that the rest of the charcoal had been transmuted into an invisible substance he termed a "gas" or "wild spirit" (spiritus sylvestris). [23]

The properties of carbon dioxide were further studied in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he called "fixed air." He observed that the fixed air was denser than air and supported neither flame nor animal life. Black also found that when bubbled through limewater (a saturated aqueous solution of calcium hydroxide), it would precipitate calcium carbonate. He used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, English chemist Joseph Priestley published a paper entitled Impregnating Water with Fixed Air in which he described a process of dripping sulfuric acid (or oil of vitriol as Priestley knew it) on chalk in order to produce carbon dioxide, and forcing the gas to dissolve by agitating a bowl of water in contact with the gas. [24]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday. [25] The earliest description of solid carbon dioxide (dry ice) was given by the French inventor Adrien-Jean-Pierre Thilorier, who in 1835 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO
2
. [26] [27]

Chemical and physical properties

Stretching and bending oscillations of the CO
2 carbon dioxide molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes. Co2 vibrations.svg
Stretching and bending oscillations of the CO
2
carbon dioxide molecule. Upper left: symmetric stretching. Upper right: antisymmetric stretching. Lower line: degenerate pair of bending modes.

Structure and bonding

The carbon dioxide molecule is linear and centrosymmetric. The carbon–oxygen bond length is 116.3  pm, noticeably shorter than the bond length of a C–O single bond and even shorter than most other C–O multiply-bonded functional groups. [28] Since it is centrosymmetric, the molecule has no electrical dipole. Consequently, only two vibrational bands are observed in the IR spectrum – an antisymmetric stretching mode at wavenumber 2349 cm−1 and a degenerate pair of bending modes at 667 cm−1 (wavelength 15 μm). There is also a symmetric stretching mode at 1388 cm−1 which is only observed in the Raman spectrum. [29]

In aqueous solution

Carbon dioxide is soluble in water, in which it reversibly forms H
2
CO
3
(carbonic acid), which is a weak acid since its ionization in water is incomplete.

CO
2
+ H
2
O
H
2
CO
3

The hydration equilibrium constant of carbonic acid is (at 25 °C). Hence, the majority of the carbon dioxide is not converted into carbonic acid, but remains as CO
2
molecules, not affecting the pH.

The relative concentrations of CO
2
, H
2
CO
3
, and the deprotonated forms HCO
3
(bicarbonate) and CO2−
3
(carbonate) depend on the pH. As shown in a Bjerrum plot, in neutral or slightly alkaline water (pH > 6.5), the bicarbonate form predominates (>50%) becoming the most prevalent (>95%) at the pH of seawater. In very alkaline water (pH > 10.4), the predominant (>50%) form is carbonate. The oceans, being mildly alkaline with typical pH = 8.2–8.5, contain about 120 mg of bicarbonate per liter.

Being diprotic, carbonic acid has two acid dissociation constants, the first one for the dissociation into the bicarbonate (also called hydrogen carbonate) ion (HCO3):

H2CO3 HCO3 + H+
Ka1 = 2.5×10−4 mol/L; pKa1 = 3.6 at 25 °C. [28]

This is the true first acid dissociation constant, defined as , where the denominator includes only covalently bound H2CO3 and does not include hydrated CO
2
(aq). The much smaller and often-quoted value near 4.16×10−7 is an apparent value calculated on the (incorrect) assumption that all dissolved CO
2
is present as carbonic acid, so that . Since most of the dissolved CO
2
remains as CO
2
molecules, Ka1(apparent) has a much larger denominator and a much smaller value than the true Ka1. [30]

The bicarbonate ion is an amphoteric species that can act as an acid or as a base, depending on pH of the solution. At high pH, it dissociates significantly into the carbonate ion (CO32−):

HCO3 CO32− + H+
Ka2 = 4.69×10−11 mol/L; pKa2 = 10.329

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

Chemical reactions of CO2

CO
2
is a weak electrophile. Its reaction with basic water illustrates this property, in which case hydroxide is the nucleophile. Other nucleophiles react as well. For example, carbanions as provided by Grignard reagents and organolithium compounds react with CO
2
to give carboxylates:

MR + CO
2
→ RCO
2
M
where M = Li or Mg Br and R = alkyl or aryl.

In metal carbon dioxide complexes, CO
2
serves as a ligand, which can facilitate the conversion of CO
2
to other chemicals. [31]

The reduction of CO
2
to CO is ordinarily a difficult and slow reaction:

CO
2
+ 2 e + 2H+ → CO + H2O

Photoautotrophs (i.e. plants and cyanobacteria) use the energy contained in sunlight to photosynthesize simple sugars from CO
2
absorbed from the air and water:

nCO
2
+ nH
2
O
(CH
2
O)
n
+ nO
2

The redox potential for this reaction near pH 7 is about −0.53 V versus the standard hydrogen electrode. The nickel-containing enzyme carbon monoxide dehydrogenase catalyses this process. [32]

Physical properties

Pellets of "dry ice", a common form of solid carbon dioxide Dry Ice Pellets Subliming.jpg
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, the density of carbon dioxide is around 1.98 kg/m3, about 1.67 times that of air.

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

Pressure-temperature phase diagram of carbon dioxide Carbon dioxide pressure-temperature phase diagram.svg
Pressure–temperature phase diagram of carbon dioxide

Liquid carbon dioxide forms only at pressures above 5.1 atm; the triple point of carbon dioxide is about 5.1 bar (517 kPa) at 217 K (see phase diagram). The critical point is 7.38 MPa at 31.1 °C. [33] [34] Another form of solid carbon dioxide observed at high pressure is an amorphous glass-like solid. [35] This form of glass, called carbonia , is produced by supercooling heated 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 glass) and germanium dioxide. Unlike silica and germania glasses, however, carbonia glass is not stable at normal pressures and reverts to gas when pressure is released.

At temperatures and pressures above the critical point, carbon dioxide behaves as a supercritical fluid known as supercritical carbon dioxide. In this state it is starting (as of 2018) to be used for power generation [ citation needed ].

Isolation and production

Carbon dioxide can be obtained by distillation from air, but the method is inefficient. Industrially, carbon dioxide is predominantly an unrecovered waste product, produced by several methods which may be practiced at various scales. [36]

The combustion of all carbon-based fuels, such as methane (natural gas), petroleum distillates (gasoline, diesel, kerosene, propane), coal, wood and generic organic matter produces carbon dioxide and, except in the case of pure carbon, water. As an example, the chemical reaction between methane and oxygen:

CH
4
+ 2 O
2
→ CO
2
+ 2 H
2
O

It is produced by thermal decomposition of limestone, CaCO
3
by heating (calcining) at about 850 °C (1,560 °F), in the manufacture of quicklime (calcium oxide, CaO), a compound that has many industrial uses:

CaCO
3
→ CaO + CO
2

Iron is reduced from its oxides with coke in a blast furnace, producing pig iron and carbon dioxide: [37]

Carbon dioxide is a byproduct of the industrial production of hydrogen by steam reforming and the water gas shift reaction in ammonia production. These processes begin with the reaction of water and natural gas (mainly methane). [38] This is a major source of food-grade carbon dioxide for use in carbonation of beer and soft drinks, and is also used for stunning animals such as poultry. In the summer of 2018 a shortage of carbon dioxide for these purposes arose in Europe due to the temporary shut-down of several ammonia plants for maintenance. [39]

Acids liberate CO
2
from most metal carbonates. Consequently, it may be obtained directly from natural carbon dioxide springs, where it is produced by the action of acidified water on limestone or dolomite. The reaction between hydrochloric acid and calcium carbonate (limestone or chalk) is shown below:

CaCO
3
+ 2 HCl → CaCl
2
+ H
2
CO
3

The carbonic acid (H
2
CO
3
) then decomposes to water and CO
2
:

H
2
CO
3
→ CO
2
+ H
2
O

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.

Carbon dioxide is a by-product of the fermentation of sugar in the brewing of beer, whisky and other alcoholic beverages and in the production of bioethanol. Yeast metabolizes sugar to produce CO
2
and ethanol, also known as alcohol, as follows:

C
6
H
12
O
6
2 CO
2
+ 2 C
2
H
5
OH

All aerobic organisms produce CO
2
when they oxidize carbohydrates, fatty acids, and proteins. The large number of reactions involved are exceedingly complex and not described easily. Refer to (cellular respiration, anaerobic respiration and photosynthesis). The equation for the respiration of glucose and other monosaccharides is:

C
6
H
12
O
6
+ 6 O
2
6 CO
2
+ 6 H
2
O

Anaerobic organisms decompose organic material producing methane and carbon dioxide together with traces of other compounds. [40] Regardless of the type of organic material, the production of gases follows well defined kinetic pattern. Carbon dioxide comprises about 40–45% of the gas that emanates from decomposition in landfills (termed "landfill gas"). Most of the remaining 50–55% is methane. [41]

Applications

Carbon dioxide is used by the food industry, the oil industry, and the chemical industry. [36] The compound has varied commercial uses but one of its greatest use as a chemical is in the production of carbonated beverages; it provides the sparkle in carbonated beverages such as soda water, beer and sparkling wine.

Precursor to chemicals

In the chemical industry, carbon dioxide is mainly consumed as an ingredient in the production of urea, with a smaller fraction being used to produce methanol and a range of other products, [42] such as metal carbonates and bicarbonates.[ citation needed ] Some carboxylic acid derivatives such as sodium salicylate are prepared using CO
2
by the Kolbe-Schmitt reaction. [43]

In addition to conventional processes using CO
2
for chemical production, electrochemical methods are also being explored at a research level. In particular, the use of renewable energy for production of fuels from CO
2
(such as methanol) is attractive as this could result in fuels that could be easily transported and used within conventional combustion technologies but have no net CO
2
emissions. [44]

Foods

Carbon dioxide bubbles in a soft drink. Soda bubbles macro.jpg
Carbon dioxide bubbles in a soft drink.

Carbon dioxide is a food additive used as a propellant and acidity regulator in the food industry. It is approved for usage in the EU [45] (listed as E number E290), US [46] and Australia and New Zealand [47] (listed by its INS number 290).

A candy called Pop Rocks is pressurized with carbon dioxide gas [48] at about 4 x 106 Pa (40 bar, 580 psi). When placed in the mouth, it dissolves (just like other hard candy) and releases the gas bubbles with an audible pop.

Leavening agents cause dough to rise by producing carbon dioxide. [49] Baker's yeast produces carbon dioxide by fermentation of sugars within the dough, while chemical leaveners such as baking powder and baking soda release carbon dioxide when heated or if exposed to acids.

Beverages

Carbon dioxide is used to produce carbonated soft drinks and 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, draught beer is usually transferred from kegs in a cold room or cellar to dispensing taps on the bar using pressurized carbon dioxide, sometimes mixed with nitrogen.

Wine making

Dry ice used to preserve grapes after harvest. Dry ice used to preserve grapes after harvest.jpg
Dry ice used to preserve grapes after harvest.

Carbon dioxide in the form of dry ice is often used during the cold soak phase in wine making to cool clusters of grapes quickly after picking to help prevent spontaneous fermentation by wild yeast. 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, and thus the alcohol concentration in the finished wine. Carbon dioxide is also used to create a hypoxic environment for carbonic maceration, the process used to produce Beaujolais wine.

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

Stunning animals

Carbon dioxide is often used to "stun" animals before slaughter. [50] "Stunning" may be a misnomer, as the animals are not knocked out immediately and may suffer distress. [51] [52]

Inert gas

It 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, although in the welding arc, it reacts to oxidize most metals. Use in the automotive industry is common despite significant evidence that welds made in carbon dioxide are more brittle than those made in more inert atmospheres.[ citation needed ] It is used as a welding gas primarily because it is much less expensive than more inert gases such as argon or helium.[ citation needed ] When used for MIG welding, CO
2
use is sometimes referred to as MAG welding, for Metal Active Gas, as CO
2
can react at these high temperatures. It tends to produce a hotter puddle than truly inert atmospheres, improving the flow characteristics. Although, this may be due to atmospheric reactions occurring at the puddle site. This is usually the opposite of the desired effect when welding, as it tends to embrittle the site, but may not be a problem for general mild steel welding, where ultimate ductility is not a major concern.

It is used in many consumer products that require pressurized gas because it is inexpensive and nonflammable, and because it undergoes a phase transition from gas to liquid at room temperature at an attainable pressure of approximately 60 bar (870 psi, 59 atm), allowing far more carbon dioxide to fit in a given container than otherwise would. Life jackets often contain canisters of pressured carbon dioxide for quick inflation. Aluminium capsules of CO
2
are also sold as supplies of compressed gas for air guns, paintball markers/guns, inflating bicycle tires, and for making carbonated water. Rapid vaporization of liquid carbon dioxide is used for blasting in coal mines.[ citation needed ] High concentrations of carbon dioxide can also be used to kill pests. Liquid carbon dioxide is used in supercritical drying of some food products and technological materials, in the preparation of specimens for scanning electron microscopy [53] and in the decaffeination of coffee beans.

Fire extinguisher

Use of a CO
2 fire extinguisher. US Army 53023 Fire Prevention Week.jpg
Use of a CO
2
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, especially those designed for electrical fires, contain liquid carbon dioxide under pressure. Carbon dioxide extinguishers work well on small flammable liquid and electrical fires, but not on ordinary combustible fires, because although it excludes oxygen, it does not cool the burning substances significantly and when the carbon dioxide disperses they are free to catch fire upon exposure to atmospheric oxygen. Their desirability in electrical fire stems from the fact that, unlike water or other chemical based methods, Carbon dioxide will not cause short circuits, leading to even more damage to equipment. Because it is a gas, it is also easy to dispense large amounts of the gas automatically in IT infrastructure rooms, where the fire itself might be hard to reach with more immediate methods because it is behind rack doors and inside of cases. 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. [54] International Maritime Organization standards also recognize carbon dioxide systems for fire protection of ship holds and engine rooms. Carbon dioxide based fire protection systems have been linked to several deaths, because it can cause suffocation in sufficiently high concentrations. A review of CO
2
systems identified 51 incidents between 1975 and the date of the report (2000), causing 72 deaths and 145 injuries. [55]

Supercritical CO2 as solvent

Liquid carbon dioxide is a good solvent for many lipophilic organic compounds and is used to remove caffeine from coffee. [15] Carbon dioxide has attracted attention in the pharmaceutical and other chemical processing industries as a less toxic alternative to more traditional solvents such as organochlorides. It is also used by some dry cleaners for this reason (see green chemistry). It is used in the preparation of some aerogels because of the properties of supercritical carbon dioxide.

Agricultural and biological applications

Plants require carbon dioxide to conduct photosynthesis. The atmospheres of greenhouses may (if of large size, must) be enriched with additional CO
2
to sustain and increase the rate of plant growth. [56] [57] At very high concentrations (100 times atmospheric concentration, or greater), carbon dioxide can be toxic to animal life, so raising the concentration to 10,000 ppm (1%) or higher for several hours will eliminate pests such as whiteflies and spider mites in a greenhouse. [58]

It has been proposed that CO
2
from power generation be bubbled into ponds to stimulate growth of algae that could then be converted into biodiesel fuel. [59]

Medical and pharmacological uses

In medicine, up to 5% carbon dioxide (130 times atmospheric concentration) is added to oxygen for stimulation of breathing after apnea and to stabilize the O
2
/CO
2
balance in blood.

Carbon dioxide can be mixed with up to 50% oxygen, forming an inhalable gas; this is known as Carbogen and has a variety of medical and research uses.

Oil recovery

Carbon dioxide is used in enhanced oil recovery where it is injected into or adjacent to producing oil wells, usually under supercritical conditions, when it becomes miscible with the oil. This approach can increase original oil recovery by reducing residual oil saturation by between 7% to 23% additional to primary extraction. [60] It acts as both a pressurizing agent and, when dissolved into the underground crude oil, significantly reduces its viscosity, and changing surface chemistry enabling the oil to flow more rapidly through the reservoir to the removal well. [61] In mature oil fields, extensive pipe networks are used to carry the carbon dioxide to the injection points.

Bio transformation into fuel

A strain of the cyanobacterium Synechococcus elongatus has been genetically engineered to produce the fuels isobutyraldehyde and isobutanol from CO
2
using photosynthesis. [62]

Refrigerant

Comparison of phase diagrams of carbon dioxide (red) and water (blue) as a log-lin chart with phase transitions points at 1 atmosphere Comparison carbon dioxide water phase diagrams.svg
Comparison of 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, 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 discovery of R-12 and may enjoy a renaissance due to the fact that R134a contributes to climate change more than CO
2
does. Its 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 bar (1880 psi), CO
2
systems require highly resistant components that have already been developed for mass production in many sectors. In automobile air conditioning, in more than 90% of all driving conditions for latitudes higher than 50°, R744 operates more efficiently than systems using R134a. Its environmental advantages (GWP of 1, non-ozone depleting, non-toxic, non-flammable) could make it the future working fluid to replace current HFCs in cars, supermarkets, and heat pump water heaters, among others. Coca-Cola has fielded CO
2
-based beverage coolers and the U.S. Army is interested in CO
2
refrigeration and heating technology. [63] [64]

The global automobile industry is expected to decide on the next-generation refrigerant in car air conditioning. CO
2
is one discussed option.(see Sustainable automotive air conditioning)

Coal bed methane recovery

In enhanced coal bed methane recovery, carbon dioxide would be pumped into the coal seam to displace methane, as opposed to current methods which primarily rely on the removal of water (to reduce pressure) to make the coal seam release its trapped methane. [65]

Minor uses

A carbon dioxide laser. Carbon Dioxide Laser At The Laser Effects Test Facility.jpg
A carbon dioxide laser.

Carbon dioxide is the lasing medium in a carbon dioxide laser, which is one of the earliest type of lasers.

Carbon dioxide can be used as a means of controlling the pH of swimming pools, [66] by continuously adding gas to the water, thus keeping the pH from rising. Among the advantages of this is the avoidance of handling (more hazardous) acids. Similarly, it is also used in the maintaining reef aquaria, where it is commonly used in calcium reactors to temporarily lower the pH of water being passed over calcium carbonate in order to allow the calcium carbonate to dissolve into the water more freely where it is used by some corals to build their skeleton.

Used as the primary coolant in the British advanced gas-cooled reactor for nuclear power generation.

Carbon dioxide induction is commonly used for the euthanasia of laboratory research animals. Methods to administer CO
2
include placing animals directly into a closed, prefilled chamber containing CO
2
, or exposure to a gradually increasing concentration of CO
2
. In 2013, the American Veterinary Medical Association issued new guidelines for carbon dioxide induction, stating that a displacement rate of 10% to 30% of the gas chamber volume per minute is optimal for the humane euthanization of small rodents. [67] However, there is opposition to the practice of using carbon dioxide for this, on the grounds that it is cruel. [52]

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

In Earth's atmosphere

The Keeling Curve of atmospheric CO
2 concentrations measured at Mauna Loa Observatory Mauna Loa CO2 monthly mean concentration.svg
The Keeling Curve of atmospheric CO
2
concentrations measured at Mauna Loa Observatory

Carbon dioxide in Earth's atmosphere is a trace gas, currently (mid 2018) having a global average concentration of 409 parts per million by volume [68] [69] [70] (or 622 parts per million by mass). Atmospheric concentrations of carbon dioxide fluctuate slightly with the seasons, falling during the Northern Hemisphere spring and summer as plants consume the gas and rising during northern autumn and winter as plants go dormant or die and decay. Concentrations also vary on a regional basis, most strongly near the ground with much smaller variations aloft. In urban areas concentrations are generally higher [71] and indoors they can reach 10 times background levels.

Yearly increase of atmospheric CO
2: In the 1960s, the average annual increase was 35% of the 2009-2018 average. CO2 increase rate.png
Yearly increase of atmospheric CO
2
: In the 1960s, the average annual increase was 35% of the 2009-2018 average.

The concentration of carbon dioxide has risen due to human activities. [73] Combustion of fossil fuels and deforestation have caused the atmospheric concentration of carbon dioxide to increase by about 43% since the beginning of the age of industrialization. [74] Most carbon dioxide from human activities is released from burning coal and other fossil fuels. Other human activities, including deforestation, biomass burning, and cement production also produce carbon dioxide. Human activities emit about 29 billion tons of carbon dioxide per year, while volcanoes emit between 0.2 and 0.3 billion tons. [75] [76] Human activities have caused CO
2
to increase above levels not seen in hundreds of thousands of years. Currently, about half of the carbon dioxide released from the burning of fossil fuels remains in the atmosphere and is not absorbed by vegetation and the oceans. [77] [78] [79] [80]

While transparent to visible light, carbon dioxide is a greenhouse gas, absorbing and emitting infrared radiation at its two infrared-active vibrational frequencies (see the section "Structure and bonding" above). Light emission from the earth's surface is most intense in the infrared region between 200 and 2500 cm−1, [81] as opposed to light emission from the much hotter sun which is most intense in the visible region. Absorption of infrared light at the vibrational frequencies of atmospheric carbon dioxide traps energy near the surface, warming the surface and the lower atmosphere. Less energy reaches the upper atmosphere, which is therefore cooler because of this absorption. [82] [83] Increases in atmospheric concentrations of CO
2
and other long-lived greenhouse gases such as methane, nitrous oxide and ozone have correspondingly strengthened their absorption and emission of infrared radiation, causing the rise in average global temperature since the mid-20th century. Carbon dioxide is of greatest concern because it exerts a larger overall warming influence than all of these other gases combined and because it has a long atmospheric lifetime (hundreds to thousands of years).

CO
2 in Earth's atmosphere if half of global-warming emissions are not absorbed.
(NASA computer simulation). M15-162b-EarthAtmosphere-CarbonDioxide-FutureRoleInGlobalWarming-Simulation-20151109.jpg
CO
2
in Earth's atmosphere if half of global-warming emissions are not absorbed.
(NASA computer simulation).

Not only do increasing carbon dioxide concentrations lead to increases in global surface temperature, but increasing global temperatures also cause increasing concentrations of carbon dioxide. This produces a positive feedback for changes induced by other processes such as orbital cycles. [84] Five hundred million years ago the carbon dioxide concentration was 20 times greater than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with a particularly swift reduction occurring 49 million years ago. [85] [86]

Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain. At the Bossoleto hot spring near Rapolano Terme in Tuscany, Italy, situated in a bowl-shaped depression about 100 m (330 ft) in diameter, concentrations of CO
2
rise to above 75% overnight, sufficient to kill insects and small animals. After sunrise the gas is dispersed by convection. [87] High concentrations of CO
2
produced by disturbance of deep lake water saturated with CO
2
are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986. [88]

In the oceans

Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100. Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100.jpg
Pterapod shell dissolved in seawater adjusted to an ocean chemistry projected for the year 2100.

Carbon dioxide dissolves in the ocean to form carbonic acid (H2CO3), bicarbonate (HCO3) and carbonate (CO32−). There is about fifty times as much carbon dissolved in the oceans as exists in the atmosphere. The oceans act as an enormous carbon sink, and have taken up about a third of CO
2
emitted by human activity. [89]

As the concentration of carbon dioxide increases in the atmosphere, the increased uptake of carbon dioxide into the oceans is causing a measurable decrease in the pH of the oceans, which is referred to as ocean acidification. This reduction in pH affects biological systems in the oceans, primarily oceanic calcifying organisms. These effects span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and mollusks. Under normal conditions, calcium carbonate is stable in surface waters since the carbonate ion is at supersaturating concentrations. However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution. [90] Corals, [91] [92] [93] coccolithophore algae, [94] [95] [96] [97] coralline algae, [98] foraminifera, [99] shellfish [100] and pteropods [101] experience reduced calcification or enhanced dissolution when exposed to elevated CO
2
.

Gas solubility decreases as the temperature of water increases (except when both pressure exceeds 300 bar and temperature exceeds 393 K, only found near deep geothermal vents) [102] and therefore the rate of uptake from the atmosphere decreases as ocean temperatures rise.

Most of the CO
2
taken up by the ocean, which is about 30% of the total released into the atmosphere, [103] forms carbonic acid in equilibrium with bicarbonate. Some of these chemical species are consumed by photosynthetic organisms that remove carbon from the cycle. Increased CO
2
in the atmosphere has led to decreasing alkalinity of seawater, and there is concern that this may adversely affect organisms living in the water. In particular, with decreasing alkalinity, the availability of carbonates for forming shells decreases, [104] although there's evidence of increased shell production by certain species under increased CO
2
content. [105]

NOAA states in their May 2008 "State of the science fact sheet for ocean acidification" that:
"The oceans have absorbed about 50% of the carbon dioxide (CO
2
) released from the burning of fossil fuels, resulting in chemical reactions that lower ocean pH. This has caused an increase in hydrogen ion (acidity) of about 30% since the start of the industrial age through a process known as "ocean acidification." A growing number of studies have demonstrated adverse impacts on marine organisms, including:

Also, the Intergovernmental Panel on Climate Change (IPCC) writes in their Climate Change 2007: Synthesis Report: [106]
"The uptake of anthropogenic carbon since 1750 has led to the ocean becoming more acidic with an average decrease in pH of 0.1 units. Increasing atmospheric CO
2
concentrations lead to further acidification ... While the effects of observed ocean acidification on the marine biosphere are as yet undocumented, the progressive acidification of oceans is expected to have negative impacts on marine shell-forming organisms (e.g. corals) and their dependent species."

Some marine calcifying organisms (including coral reefs) have been singled out by major research agencies, including NOAA, OSPAR commission, NANOOS and the IPCC, because their most current research shows that ocean acidification should be expected to impact them negatively. [107]

Carbon dioxide is also introduced into the oceans through hydrothermal vents. The Champagne hydrothermal vent, found at the Northwest Eifuku volcano in the Marianas 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. [108] The finding of a submarine lake of liquid carbon dioxide in the Okinawa Trough was reported in 2006. [109]

Biological role

Carbon dioxide is an end product of cellular respiration in organisms that obtain energy by breaking down sugars, fats and amino acids with oxygen as part of their metabolism. This includes all plants, algae and animals and aerobic fungi and bacteria. In vertebrates, the carbon dioxide travels in the blood from the body's tissues to the skin (e.g., amphibians) or the gills (e.g., fish), from where it dissolves in the water, or to the lungs from where it is exhaled. During active photosynthesis, plants can absorb more carbon dioxide from the atmosphere than they release in respiration.

Photosynthesis and carbon fixation

Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by photosynthesis, which can be respired to water and (CO
2). Auto-and heterotrophs.png
Overview of photosynthesis and respiration. Carbon dioxide (at right), together with water, form oxygen and organic compounds (at left) by photosynthesis, which can be respired to water and (CO
2
).
Overview of the Calvin cycle and carbon fixation Calvin-cycle4.svg
Overview of the Calvin cycle and carbon fixation

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

Ribulose-1,5-bisphosphate carboxylase oxygenase, commonly abbreviated to RuBisCO, is the enzyme involved in the first major step of carbon fixation, the production of two molecules of 3-phosphoglycerate from CO
2
and ribulose bisphosphate, as shown in the diagram at left.

RuBisCO is thought to be the single most abundant protein on Earth. [110]

Phototrophs use the products of their photosynthesis as internal food sources and as raw material for the biosynthesis of more complex organic molecules, such as polysaccharides, nucleic acids and proteins. These are used for their own growth, and also as the basis of the food chains and webs that feed other organisms, including animals such as ourselves. Some important phototrophs, the coccolithophores synthesise hard calcium carbonate scales. [111] A globally significant species of coccolithophore is Emiliania huxleyi whose calcite scales have formed the basis of many sedimentary rocks such as limestone, where what was previously atmospheric carbon can remain fixed for geological timescales.

Plants can grow as much as 50 percent faster in concentrations of 1,000 ppm CO
2
when compared with ambient conditions, though this assumes no change in climate and no limitation on other nutrients. [112] Elevated CO
2
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 CO
2
in FACE experiments. [113] [114]

Increased atmospheric CO
2
concentrations result in fewer stomata developing on plants [115] which leads to reduced water usage and increased water-use efficiency. [116] Studies using FACE have shown that CO
2
enrichment leads to decreased concentrations of micronutrients in crop plants. [117] This may have knock-on effects on other parts of ecosystems as herbivores will need to eat more food to gain the same amount of protein. [118]

The concentration of secondary metabolites such as phenylpropanoids and flavonoids can also be altered in plants exposed to high concentrations of CO
2
. [119] [120]

Plants also emit CO
2
during respiration, and so the majority of plants and algae, which use C3 photosynthesis, are only net absorbers during the day. Though a growing forest will absorb many tons of CO
2
each year, a mature forest will produce as much CO
2
from respiration and decomposition of dead specimens (e.g., fallen branches) as is used in photosynthesis in growing plants. [121] Contrary to the long-standing view that they are carbon neutral, mature forests can continue to accumulate carbon [122] and remain valuable carbon sinks, helping to maintain the carbon balance of Earth's atmosphere. Additionally, and crucially to life on earth, photosynthesis by phytoplankton consumes dissolved CO
2
in the upper ocean and thereby promotes the absorption of CO
2
from the atmosphere. [123]

Toxicity

Main symptoms of carbon dioxide toxicity, by increasing volume percent in air. Main symptoms of carbon dioxide toxicity.svg
Main symptoms of carbon dioxide toxicity, by increasing volume percent in air.

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% (410 ppm), depending on the location. [125] [ clarification needed ]

CO
2
is an asphyxiant gas and not classified as toxic or harmful in accordance with Globally Harmonized System of Classification and Labelling of Chemicals standards of United Nations Economic Commission for Europe by using the OECD Guidelines for the Testing of Chemicals. In concentrations up to 1% (10,000 ppm), it will make some people feel drowsy and give the lungs a stuffy feeling. [124] 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. [126] The physiological effects of acute carbon dioxide exposure are grouped together under the term hypercapnia, a subset of asphyxiation.

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 by CO
2
emissions from the nearby volcano Mt. Nyiragongo. [127] The Swahili term for this phenomenon is 'mazuku'.

Rising levels of CO
2 threatened the Apollo 13 astronauts who had to adapt cartridges from the command module to supply the carbon dioxide scrubber in the lunar module, which they used as a lifeboat. Apollo13 apparatus.jpg
Rising levels of CO
2
threatened the Apollo 13 astronauts who had to adapt cartridges from the command module to supply the carbon dioxide scrubber in the lunar module, which they used as a lifeboat.

Adaptation to increased concentrations of CO
2
occurs in humans, including modified breathing and kidney bicarbonate production, in order to balance the effects of blood acidification (acidosis). Several studies suggested that 2.0 percent inspired concentrations could be used for closed air spaces (e.g. a submarine) since the adaptation is physiological and reversible, as deterioration in performance or in normal physical activity does not happen at this level of exposure for five days. [128] [129] Yet, other studies show a decrease in cognitive function even at much lower levels. [130] [131] Also, with ongoing respiratory acidosis, adaptation or compensatory mechanisms will be unable to reverse such condition.

Below 1%

There are few studies of the health effects of long-term continuous CO
2
exposure on humans and animals at levels below 1%. Occupational CO
2
exposure limits have been set in the United States at 0.5% (5000 ppm) for an eight-hour period. [132] At this CO
2
concentration, International Space Station crew experienced headaches, lethargy, mental slowness, emotional irritation, and sleep disruption. [133] Studies in animals at 0.5% CO
2
have demonstrated kidney calcification and bone loss after eight weeks of exposure. [134] A study of humans exposed in 2.5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0.1% (1000ppm) CO
2
likely due to CO
2
induced increases in cerebral blood flow. [130] Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm. [131]

Ventilation

CO
2 concentration meter using a nondispersive infrared sensor CO2Mini monitor.jpg
CO
2
concentration meter using a nondispersive infrared sensor

Poor ventilation is one of the main causes of excessive CO
2
concentrations in closed spaces. Carbon dioxide differential above outdoor concentrations at steady state conditions (when the occupancy and ventilation system operation are sufficiently long that CO
2
concentration has stabilized) are sometimes used to estimate ventilation rates per person.[ citation needed ] Higher CO
2
concentrations are associated with occupant health, comfort and performance degradation. [135] [136] 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).

Miners, who are particularly vulnerable to gas exposure due to an insufficient ventilation, referred to mixtures of carbon dioxide and nitrogen as "blackdamp," "choke damp" or "stythe." Before more effective technologies were developed, miners would frequently monitor for dangerous levels of blackdamp and other gases in mine shafts by bringing a caged 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 could also detect high levels of blackdamp (which sinks, and collects near the floor) by burning less brightly, while methane, another suffocating gas and explosion risk, would make the lamp burn more brightly.

Human physiology

Content

Reference ranges or averages for partial pressures of carbon dioxide (abbreviated pCO
2
)
kPa mmHg
Venous blood carbon dioxide5.5–6.841–51 [137]
Alveolar pulmonary
gas pressures
4.836
Arterial blood carbon dioxide 4.7–6.035–45 [137]

The body produces approximately 2.3 pounds (1.0 kg) of carbon dioxide per day per person, [138] containing 0.63 pounds (290 g) of carbon. In humans, this carbon dioxide is carried through the venous system and is breathed out through the lungs, resulting in lower concentrations in the arteries. The carbon dioxide content of the blood is often given as the partial pressure, which is the pressure which carbon dioxide would have had if it alone occupied the volume. [139] In humans, the blood carbon dioxide contents is shown in the adjacent table:

Transport in the blood

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

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, 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. This 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, which is known as the Bohr effect.

Regulation of respiration

Carbon dioxide is one of the mediators of local autoregulation of blood supply. If its concentration is 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 causes respiratory acidosis, while breathing that is too rapid leads to hyperventilation, which can cause respiratory alkalosis.

Although the body requires oxygen for metabolism, low oxygen levels normally do not stimulate breathing. Rather, breathing is stimulated by higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (such as pure nitrogen) can lead to loss of consciousness without ever experiencing air hunger. This is especially perilous for high-altitude fighter pilots. It is also why flight attendants instruct passengers, in case of loss of cabin pressure, to apply the oxygen mask to themselves first before helping others; otherwise, one risks losing consciousness. [140]

The respiratory centers try to maintain an arterial CO
2
pressure of 40 mm Hg. With intentional hyperventilation, the CO
2
content of arterial blood may be lowered to 10–20 mm Hg (the oxygen content of the blood is little affected), and the respiratory drive is diminished. This is why one can hold one's breath longer after hyperventilating than without hyperventilating. This carries the risk that unconsciousness may result before the need to breathe becomes overwhelming, which is why hyperventilation is particularly dangerous before free diving.

See also

Related Research Articles

Carbonate Salt or ester of carbonic acid

In chemistry, a carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula of CO2−
3
. The name may also refer to a carbonate ester, an organic compound containing the carbonate group C(=O)(O–)2.

Calcium carbonate Chemical compound

Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is the main component of pearls and the shells of marine organisms, snails, and eggs. Calcium carbonate is the active ingredient in agricultural lime and is created when calcium ions in hard water react with carbonate ions to create limescale. It is medicinally used as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause poor digestion.

Carbon cycle Biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere,

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth. Carbon is the main component of biological compounds as well as a major component of many minerals such as limestone. Along with the nitrogen cycle and the water cycle, the carbon cycle comprises a sequence of events that are key to make Earth capable of sustaining life. It describes the movement of carbon as it is recycled and reused throughout the biosphere, as well as long-term processes of carbon sequestration to and release from carbon sinks.

Carbonic acid is a chemical compound with the chemical formula H2CO3 (equivalently: OC(OH)2). It is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H2CO3. In physiology, carbonic acid is described as volatile acid or respiratory acid because it is the only acid excreted as a gas by the lungs. It plays an important role in the bicarbonate buffer system to maintain acid–base homeostasis.

Biological pump The oceans biologically driven sequestration of carbon from the atmosphere to the ocean interior and seafloor

The biological pump, in its simplest form, is the ocean's biologically driven sequestration of carbon from the atmosphere to the ocean interior and seafloor sediments. It is the part of the oceanic carbon cycle responsible for the cycling of organic matter formed mainly by phytoplankton during photosynthesis (soft-tissue pump), as well as the cycling of calcium carbonate (CaCO3) formed into shells by certain organisms such as plankton and mollusks (carbonate pump).

Oxygen cycle The biogeochemical cycle of oxygen within its four main reservoirs: the atmosphere, the biosphere, the hydrosphere, and the lithosphere

The oxygen cycle is the biogeochemical transitions of oxygen atoms between different oxidation states in ions, oxides, and molecules through redox reactions within and between the spheres/reservoirs of the planet Earth. The word oxygen in the literature typically refers to the most common oxygen allotrope, elemental/diatomic oxygen (O2), as it is a common product or reactant of many biogeochemical redox reactions within the cycle. Processes within the oxygen cycle are considered to be biological or geological and are evaluated as either a source (O2 production) or sink (O2 consumption).

Solubility pump A physico-chemical process that transports dissolved inorganic carbon from the oceans surface to its interior

In oceanic biogeochemistry, the solubility pump is a physico-chemical process that transports carbon from the ocean's surface to its interior.

Ocean acidification Ongoing decrease in the pH of the Earths oceans, caused by the uptake of carbon dioxide

Ocean acidification is the ongoing decrease in the pH of the Earth's oceans, caused by the uptake of carbon dioxide (CO
2
) from the atmosphere. Seawater is slightly basic (meaning pH > 7), and ocean acidification involves a shift towards pH-neutral conditions rather than a transition to acidic conditions (pH < 7). An estimated 30–40% of the carbon dioxide from human activity released into the atmosphere dissolves into oceans, rivers and lakes. To achieve chemical equilibrium, some of it reacts with the water to form carbonic acid. Some of the resulting carbonic acid molecules dissociate into a bicarbonate ion and a hydrogen ion, thus increasing ocean acidity (H+ ion concentration). Between 1751 and 1996, surface ocean pH is estimated to have decreased from approximately 8.25 to 8.14, representing an increase of almost 30% in H+ ion concentration in the world's oceans. Earth System Models project that, by around 2008, ocean acidity exceeded historical analogues and, in combination with other ocean biogeochemical changes, could undermine the functioning of marine ecosystems and disrupt the provision of many goods and services associated with the ocean beginning as early as 2100.

Carbon dioxide in Earths atmosphere Atmospheric constituent; greenhouse gas

Carbon dioxide is an important trace gas in Earth's atmosphere. It is an integral part of the carbon cycle, a biogeochemical cycle in which carbon is exchanged between the Earth's oceans, soil, rocks and the biosphere. Plants and other photoautotrophs use solar energy to produce carbohydrate from atmospheric carbon dioxide and water by photosynthesis. Almost all other organisms depend on carbohydrate derived from photosynthesis as their primary source of energy and carbon compounds. CO
2
absorbs and emits infrared radiation at wavelengths of 4.26 µm and 14.99 µm and consequently is a greenhouse gas that plays a significant role in influencing Earth's surface temperature through the greenhouse effect.

Carbonate–silicate cycle

The carbonate–silicate geochemical cycle, also known as the inorganic carbon cycle, describes the long-term transformation of silicate rocks to carbonate rocks by weathering and sedimentation, and the transformation of carbonate rocks back into silicate rocks by metamorphism and volcanism. Carbon dioxide is removed from the atmosphere during burial of weathered minerals and returned to the atmosphere through volcanism. On million-year time scales, the carbonate-silicate cycle is a key factor in controlling Earth's climate because it regulates carbon dioxide levels and therefore global temperature.

Carbon dioxide removal (CDR) refers to a group of technologies the objective of which is the large-scale removal of carbon dioxide from the atmosphere. Among such technologies are bio-energy with carbon capture and storage, biochar, ocean fertilization, enhanced weathering, and direct air capture when combined with storage. CDR is a different approach from removing CO
2
from the stack emissions of large fossil fuel point sources, such as power stations. The latter reduces emission to the atmosphere but cannot reduce the amount of carbon dioxide already in the atmosphere. As CDR removes carbon dioxide from the atmosphere, it 'creates' negative emissions that offset the emissions from small and dispersed point sources such as domestic heating systems, airplanes and vehicle exhausts. It is regarded by some as a form of climate engineering, while other commentators describe it as a form of carbon capture and storage or extreme mitigation. Whether CDR would satisfy common definitions of "climate engineering" or "geoengineering" usually depends upon the scale at which it would be undertaken.

Greenhouse gas Gas in an atmosphere that absorbs and emits radiation within the thermal infrared range

A greenhouse gas (sometimes abbreviated GHG) is a gas that absorbs and emits radiant energy within the thermal infrared range. Greenhouse gases cause the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and ozone (O3). Without greenhouse gases, the average temperature of Earth's surface would be about −18 °C (0 °F), rather than the present average of 15 °C (59 °F). The atmospheres of Venus, Mars and Titan also contain greenhouse gases.

Enhanced weathering refers to geoengineering approaches that use the dissolution of natural or artificially created minerals to remove carbon dioxide from the atmosphere. Since the carbon dioxide is usually first removed from ocean water, these approaches would attack the problem by first reducing ocean acidification.

The Revelle factor (buffer factor) is the ratio of instantaneous change in carbon dioxide (CO
2
) to the change in total dissolved inorganic carbon (DIC), and is a measure of the resistance to atmospheric CO2 being absorbed by the ocean surface layer. The buffer factor is used to examine the distribution of CO2 between the atmosphere and the ocean, and measures the amount of CO2 that can be dissolved in the mixed surface layer. It is named after the oceanographer Roger Revelle. The importance of his work was that he found human produced CO2 would not be easily absorbed by the oceans. Revelle was one of a long line of scientists who first considered human-induced global warming and climate change, including Eunice Foote, Joseph Fourier, John Tyndall, Svante Arrhenius, Thomas Chrowder Chamberlin, Guy Stewart Callendar, Norman A Phillips, Maurice Ewing, William Donn, and Gilbert Plass.

Oceanic carbon cycle Processes that exchange carbon between various pools within the ocean and the atmosphere, Earth interior, and the seafloor.

The oceanic carbon cycle is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains both inorganic carbon and organic carbon. Part of the marine carbon cycle transforms carbon between non-living and living matter.

Estuarine acidification happens when the pH balance of water in coastal marine ecosystems, specifically those of estuaries, decreases. Water, generally considered neutral on the pH scale, normally perfectly balanced between alkalinity and acidity.While ocean acidification occurs due to the ongoing decrease in the pH of the Earth's oceans, caused by the absorption of carbon dioxide (CO2) from the atmosphere, pH change in estuaries is more complicated than in the open ocean due to direct impacts from land run-off, human impact, and coastal current dynamics. In the ocean, wave and wind movement allows carbon dioxide (CO2) to mixes with water (H2O) forming carbonic acid (H2CO3). Through wave motion this chemical bond is mixed up, allowing for the further break of the bond, eventually becoming carbonate (CO3) which is basic and helps form shells for ocean creatures, and two hydron molecules. This creates the potential for acidic threat since hydron ions readily bond with any Lewis Structure to form an acidic bond. This is referred to as an oxidation-reduction reaction.

Freshwater acidification

Freshwater becomes acidic when acid inputs surpass the quantity of bases produced in the reservoir through weathering of rocks, or by the reduction of acid anions, like sulfate and nitrate within the lake. The main reason for Freshwater acidification is atmospheric depositions and soil leaching of SOx and NOx. In an acid-sensitive ecosystem, which includes slow-weathering bedrock and depleted base cation pools, SOx and NOx from runoff will be accompanied by acidifying hydrogen ions and inorganic aluminum, which can be toxic to marine organisms. Acid rain is also a contributor to freshwater acidification, however acid rain is formed when SOx and NOx react with water, oxygen and oxidants within the clouds. In addition to SOx and NOx, the buffering capacity of soils and bedrocks within the freshwater ecosystem can contribute to the acidity of the water. Each freshwater reservoir has a capacity to buffer acids. However, with excess input of acids into the reservoir, the buffering capacity will essentially “run out” and the water will eventually become more acidic. Increase in atmospheric CO2 affects freshwater acidity very similarly to the way rising CO2 affects ocean ecosystems. However, because of the various carbon fluxes in freshwater ecosystems, it is difficult to quantify the effects of anthropogenic CO2. Finally, rising freshwater acidification is harmful to various aquatic organisms.

Ocean acidification in the Great Barrier Reef Threat to the reef which reduces the viability and strength of reef-building corals

Ocean acidification threatens the Great Barrier Reef by reducing the viability and strength of coral reefs. The Great Barrier Reef, considered one of the seven natural wonders of the world and a biodiversity hotspot, is located in Australia. Similar to other coral reefs, it is experiencing degradation due to ocean acidification. Ocean acidification results from a rise in atmospheric carbon dioxide, which is taken up by the ocean. This process can increase sea surface temperature, decrease aragonite, and lower the pH of the ocean.

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project is a large scale National Science Foundation funded research project based at Princeton University that started in September 2014. The project aims to increase the understanding of the Southern Ocean and the role it plays in factors such as climate, as well as educate new scientists with oceanic observation.

Ocean acidification in the Arctic Ocean

The Arctic ocean has experienced drastic change over the years due to global warming. It has been known that the Arctic ocean acidity levels have been increasing and at twice the rate compared to the Pacific and Atlantic oceans. The loss of sea ice has been connected to a decrease in pH levels in the ocean water. Sea ice has experienced an extreme reduction over the past 30 years, forming a minimum area of 2.9×106 km2 at the end of the boreal summer of 2007, 47%, less than in 1980. Sea ice limits the air-sea gas exchange with carbon dioxide. With less water completely exposed to the atmosphere, the levels of carbon dioxide gas in the water remain low. The Arctic ocean should have low carbon dioxide levels due to intense cooling, run off of fresh water and photosynthesis from marine organisms. However, the decrease of sea ice over the years due to global warming has limited freshwater runoff and has exposed a higher percentage of the ocean surface to the atmosphere. The increase of carbon dioxide in the water decreases the pH of the ocean causing ocean acidification. The decrease in sea ice has also allowed more Pacific water to flow into in the Arctic ocean during the winter, this is called Pacific winter water. The Pacific water flows into the Arctic ocean carrying additional amounts of carbon dioxide by being exposed to the atmosphere and absorbing carbon dioxide from decaying organic matter and from sediments.

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Further reading