Carbon dioxide

Carbon dioxide
Carbon dioxide
Other names Carbonic acid gas,
Carbonic anhydride,
dry ice (solid)
Molecular formula CO2
Molar mass 44.0095(14) g/mol
Solid state Dry ice, carbonia
Appearance colorless gas
CAS number [124-38-9]
Density and phase 1600 kg/m³, solid
approx. 1.98 kg/m³, gas at STP
Solubility in water 1.45 kg/m³
Latent heat of
25.13 kJ/mol
Melting point −57 °C (216 K), pressurized
Boiling point −78 °C (195 K), sublimes
Acidity (pKa) 6.35 and 10.33
Viscosity 0.07 cP at −78 °C
Molecular shape linear
Crystal structure quartz-like
Dipole moment zero
MSDS External MSDS
Main hazards asphyxiant, irritant
NFPA 704

R-phrases R: As, Fb
S-phrases S9, S23, S36 (liquid)
RTECS number FF6400000
Supplementary data page
Structure & properties n, εr, etc.
Spectral data UV, IR, NMR, MS
Related compounds
Related oxides carbon monoxide
carbon suboxide
dicarbon monoxide
carbon trioxide
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

Carbon dioxide is a chemical compound composed of one carbon and two oxygen atoms. It is often referred to by its formula CO2. It is present in the Earth's atmosphere at a low concentration and acts as a greenhouse gas. In its solid state, it is called dry ice. It is a major component of the carbon cycle.




Atmospheric carbon dioxide derives from multiple natural sources including volcanic outgassing, the combustion of organic matter, and the respiration processes of living aerobic organisms; man-made sources of carbon dioxide come mainly from the burning of various fossil fuels for heating, power generation and transport use. It is also produced by various microorganisms from fermentation and cellular respiration. Plants convert carbon dioxide to oxygen during a process called photosynthesis, using both the carbon and part of the oxygen to construct carbohydrates. The resulting gas, oxygen, is released into the atmosphere by plants, which is subsequently used for respiration by heterotrophic organisms, forming a cycle.


Chemical and physical properties

Carbon dioxide is a colorless gas which, when inhaled at high concentrations (a dangerous activity because of the associated asphyxiation risk), produces 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. One may notice this sensation if one attempts to stifle a burp after drinking a carbonated beverage.

Its density at standard temperature and pressure is around 1.98 kg/m3, about 1.53 times that of air. The carbon dioxide molecule (O=C=O) contains two double bonds and has a linear shape. It has no electrical dipole. As it is fully oxidized, it is not very reactive and is quite non-flammable.

Carbon dioxide pressure-temperature phase diagram
Carbon dioxide pressure-temperature phase diagram

At temperatures below −78 °C, carbon dioxide changes directly from a gas to a white solid called dry ice through a process called deposition. Liquid carbon dioxide forms only at pressures above 5.1 atm; at atmospheric pressure, it passes directly between the solid phase and the gaseous phase in a process called sublimation.


Synthesis and chemistry

Carbon dioxide may be obtained from air distillation, however this yields only relatively small quantities of CO2. A large variety of chemical reactions yield carbon dioxide, such as the reaction between most acids and most metal carbonates. As an example, the reaction between sulphuric acid and Calcium carbonate (limestone, or chalk) is depicted below:

H2SO4 + CaCO3 → CaSO4 + "H2CO3"

The H2CO3 then decomposes to water and CO2. Such reactions are accompanied by foaming and/or bubbling. In industry such reactions are widespread, for it can be used to neutralise waste acid streams. The production of quicklime, CaO, a chemical that has widespread use, from limestone by heating at about 850 oC also produces CO2:

CaCO3 → CaO + CO2

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.

CH4 + 2 O2 → CO2 + 2 H2O

Carbon dioxide can be used in chemistry to create a carbonic acid from a Grignard reagent.

R-MgX + CO2 → R-COOH

Yeast produces Carbon dioxide and ethanol, also known as alcohol, in the production of wines, beers and other spirits:

Glucose → 2 CO2 + 2 C2H5OH

All heterotroph organisms, such as animals, produce CO2 when they burn carbohydrates, fatty acids and proteins; it is the prime energy source of such forms of life and the main metabolic pathway in animal bodies. The large amount of reactions involved are exceedingly complex and not described easily. Photoautotrophs (i.e. plants, cyanobacteria) utilize another modus operandi: They absorb the CO2 from the air, and, together with water, react it to form carbohydrates:

nCO2 + nH2O → (CH2O)n + nO2

Carbon dioxide is soluble in water, in which it spontaneously interconverts between CO2 and H2CO3 (carbonic acid). The relative concentrations of CO2, H2CO3, and the deprotonated forms HCO3- (bicarbonate) and CO32-(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.


Solid CO2


Dry ice

Dry ice is a genericized trademark for solid (frozen) carbon dioxide. The term was made in 1925 by Prest Air Devices, founded in Long Island City, New York in 1923.[1] The name refers to the fact that under normal atmospheric pressure, solid CO2 sublimates, or changes directly into a gas without passing through a wet liquid phase. As a general rule, dry ice will sublimate at a rate of five to ten pounds every 24 hours in a typical ice chest.

Dry ice is produced by compressing carbon dioxide gas to a liquid form, removing the heat produced by the compression (see Charles's law), and then letting the liquid carbon dioxide expand quickly. This expansion and the high-speed evaporation of carbon dioxide gas cools the remainder of the liquid down to the melting point, where some of the CO2 freezes into 'snow,' which is then compressed into pellets or blocks. The freezing point of CO2 is -109.3 °F or -78.5 °C.

Dry ice has many industrial uses, including

Dry ice used to cool drinks in Central Park.(New York City, New York, U.S.)
Dry ice used to cool drinks in Central Park.
(New York City, New York, U.S.)

Dry ice requires special precautions when handling. It is cold enough to cause frostbite injury if handled without gloves. It constantly produces carbon dioxide gas, so it cannot be stored in a light duty sealed container as the pressure buildup will quickly cause the container to explode (see dry ice bomb). The sublimated gas must be ventilated; otherwise, it may fill the enclosed space and create a suffocation hazard. Special care for ventilating vehicles is needed as well because of the small space. People who handle dry ice should also be aware that carbon dioxide is heavier than air and will sink to the floor. Some markets require those purchasing dry ice to be 18 years of age or older.


Solid amorphous CO2

In June 2006, an Italian-French science team announced the synthetic production of solid amorphous CO2 glass. This form of glass, called carbonia, was produced by supercooling heated CO2 at extreme pressure (40–48 GPa or about 400,000 atmospheres) in a diamond anvil. This discovery confirmed the theory that carbon dioxide could exist in a glass state similar to other members of its elemental family, like silicon (silica glass) and germanium. Unlike silica and germanium oxide glasses, however, carbonia glass is not stable at normal pressures and reverts back to gas when pressure is released.



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

Carbon dioxide content in fresh air varies and is between 0.03% (300 ppm) to 0.06% (600 ppm), depending on location and in exhaled air approximately 4.5%. When inhaled in high concentrations (greater than 5% by volume), it is immediately dangerous to the life and health of plants, humans and other animals. The current threshold limit value (TLV) or maximum level that is considered safe for healthy adults for an 8-hour work day is 0.5% (5000 ppm). The maximum safe level for infants, children, the elderly and individuals with cardio-pulmonary health issues would be significantly less.

These figures are valid for carbon dioxide supplied in "pure" form. In indoor spaces occupied by humans the carbon dioxide concentration will also reach a level higher than in pure outdoor air. Concentrations higher than 1000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO2 concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO2 itself. At 2000 ppm will the majority of occupants feel a significant degree of discomfort, and many will develop nausea and headache. The CO2 concentration between 300 and 2500 ppm is used as an indicator of indoor air quality in spaces polluted by human occupation.

Acute carbon dioxide toxicity is sometimes known as choke damp, an old mining industry term, and was the cause of death at Lake Nyos in Cameroon, where an upwelling of CO2-laden lake water in 1986 covered a wide area in a blanket of the gas, killing nearly 2000. The lowering of carbon dioxide in the atmosphere is largely due to absorption by plants, which convert it to sugars through photosynthesis. Phytoplankton photosynthesis absorbs dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere (Falkowski P. Scholes RJ. Boyle E. Canadell J. Canfield D. Elser J. Gruber N. Hibbard K. Hogberg P. Linder S. Mackenzie FT. Moore B 3rd. Pedersen T. Rosenthal Y. Seitzinger S. Smetacek V. Steffen W. The global carbon cycle: a test of our knowledge of earth as a system. [Review] [65 refs] [Journal Article. Review] Science. 290(5490):291-6, 2000.

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 oxygen in the blood due to increased carbon dioxide 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 CO2 or a lower pH will cause offloading of oxygen from hemoglobin. This is known as the Bohr Effect.

According to a study by the USDA,[3] an average person's respiration generates approximately 450 liters (roughly 900 grams) of carbon dioxide per day.

CO2 is carried in blood in three different ways. Most of it (about 80%–90%) is converted to bicarbonate ions HCO3 by the enzyme carbonic anhydrase in the red blood cells. 5%–10% is dissolved in the plasma and 5%–10% is bound to hemoglobin as carbamino compounds. The exact percentages vary depending whether it is arterial or venous blood.

The CO2 bound to hemoglobin does not bind to the same site as oxygen; rather it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO2 does decrease the amount of oxygen that is bound for a given partial pressure of oxygen.

Carbon dioxide may be one of the mediators of local autoregulation of blood supply. If it is high, the capillaries expand to allow a greater blood flow to that tissue.[citation needed]

Bicarbonate ions are crucial for regulating blood pH. As breathing rate influences the level of CO2 in blood, too slow or shallow breathing causes respiratory acidosis, while too rapid breathing, hyperventilation, leads to respiratory alkalosis.

It is interesting to note that although it is oxygen that the body requires for metabolism, it is not low oxygen levels that stimulate breathing, but is instead higher carbon dioxide levels. As a result, breathing low-pressure air or a gas mixture with no oxygen at all (e.g., pure nitrogen) leads to loss of consciousness without subjective breathing problems. This is especially perilous for high-altitude fighter pilots, and is also the reason why the instructions in commercial airplanes for case of loss of cabin pressure stress that one should apply the oxygen mask to oneself before helping others—otherwise one risks going unconscious without being aware of the imminent peril.

Plants remove carbon dioxide from the atmosphere by photosynthesis, which uses light energy to produce organic plant materials by combining carbon dioxide and water. This releases free oxygen gas. Sometimes carbon dioxide gas is pumped into greenhouses to promote plant growth. Plants can potentially grow up to twice as fast in conditions where extra CO2 is available, although there is no additional benefit at concentrations beyond 1200 ppm. Plants also emit CO2 during respiration, but on balance they are net sinks of CO2.

Carbon dioxide is a surrogate for 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 carbon dioxide must be reduced to below 600 ppm. NIOSH considers that indoor air concentrations of carbon dioxide that exceed 1000 ppm are a marker suggesting inadequate ventilation (1,000 ppm equals 0.1%). ASHRAE recommends that CO2 levels not exceed 1000 ppm inside a space. OSHA limits carbon dioxide concentration in the workplace to 0.5% for prolonged periods. The U.S. National Institute for Occupational Safety and Health limits brief exposures (up to ten minutes) to 3% and considers concentrations exceeding 4% as "immediately dangerous to life and health." People who breathe 5% carbon dioxide for more than half an hour show signs of acute hypercapnia, while breathing 7–10% carbon dioxide can produce unconsciousness in only a few minutes. Carbon dioxide, either as a gas or as dry ice, should be handled only in well-ventilated areas.


Concentrations of CO2 in atmosphere

Atmospheric CO2 concentrations measured at Mauna Loa Observatory.
Atmospheric CO2 concentrations measured at Mauna Loa Observatory.
It has been suggested that this section be split into a new article entitled Atmospheric carbon dioxide. (Discuss)

As of January 2007, the earth's atmospheric CO2 concentration is is about 0.0383% by volume (383 ppmv) or 0.0582% by weight.[4] This represents about 2.996×1012 tonnes, and is estimated to be 105 ppm (37.77%) above the pre-industrial average.[5]

Because of the greater land area, and therefore greater plant life, in the northern hemisphere as compared to the southern hemisphere, there is an annual fluctuation of up to 6 ppmv (± 3 ppmv), peaking in May and reaching a minimum in October at the end of the northern hemisphere growing season, when the quantity of biomass on the planet is greatest.[citation needed]

Despite its small concentration, CO2 is a very important component of Earth's atmosphere, because it absorbs infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode) and enhances the greenhouse effect.[citation needed] See also "Carbon dioxide equivalent".

The three vibrational modes of carbon dioxide: (a) symmetric, (b) asymmetric stretching; (c) bending. In (a), there is no change in dipole moment, thus interaction with photons is impossible, while in (b) and (c) there is optical activity.
The three vibrational modes of carbon dioxide: (a) symmetric, (b) asymmetric stretching; (c) bending. In (a), there is no change in dipole moment, thus interaction with photons is impossible, while in (b) and (c) there is optical activity.

The initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity; this was essential for a warm and stable climate conducive to life. Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year.[citation needed] Volcanic releases are about 1% of the amount which is released by human activities.[citation needed]

Global fossil carbon emissions 1800–2000.
Global fossil carbon emissions 1800–2000.

Since the start of the Industrial Revolution, the atmospheric CO2 concentration has increased by approximately 110 µL/L or about 40%, most of it released since 1945. Monthly measurements taken at Mauna Loa[6] since 1958 show an increase from 316 µL/L in that year to 376 µL/L in 2003, an overall increase of 60 µL/L during the 44-year history of the measurements. Burning fossil fuels such as coal and petroleum is the leading cause of increased man-made CO2; deforestation is the second major cause. Around 24,000 million tonnes of CO2 are released per year worldwide, equivalent to about 6500 million tonnes of carbon. (See List of countries by carbon dioxide emissions.)

Smoke and ozone pollution from Indonesian fires, 1997.
Smoke and ozone pollution from Indonesian fires, 1997.

In 1997, Indonesian peat fires may have released 13%–40% as much carbon as fossil fuel burning does.[7][8] Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks. Not all the emitted CO2 remains in the atmosphere; some is absorbed in the oceans or biosphere. The ratio of the emitted CO2 to the increase in atmospheric CO2 is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages but is typically 57% over longer (5 year) periods.

The Global Warming Theory (GWT) predicts that increased amounts of CO2 in the atmosphere tend to enhance the greenhouse effect and thus contribute to global warming. The effect of combustion-produced carbon dioxide on climate is called the Callendar effect.


Variation in the past

CO2 concentrations over the last 400,000 years
CO2 concentrations over the last 400,000 years

The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO2 levels were about 260–280µL/L immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years.

The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800,000 years before the present.[9] During this time, the atmospheric carbon dioxide concentration has varied between 180–210 µL/L during ice ages, increasing to 280–300 µL/L during warmer interglacials.[10] The data can be accessed at here.

Some studies have disputed the claim of stable CO2 levels during the present interglacial (the last 10 kyr). Based on an analysis of fossil leaves, Wagner et al.[11] argued that CO2 levels during the period 7–10 kyr ago were significantly higher (~300 µL/L) and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO2.[12] Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO2 values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g., Smith et al.[13]) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust levels in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between Antarctic and Greenland CO2 measurements.

Changes in carbon dioxide during the Phanerozoic (the last 542 million years). The recent period is located on the left-hand side of the plot, and it appears that much of the last 550 million years has experienced carbon dioxide concentrations significantly higher than the present day.
Changes in carbon dioxide during the Phanerozoic (the last 542 million years). The recent period is located on the left-hand side of the plot, and it appears that much of the last 550 million years has experienced carbon dioxide concentrations significantly higher than the present day.

On longer timescales, various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO2 concentrations (>3,000 µL/L) between 600 and 400 Myr BP and between 200 and 150 Myr BP.[14] On long timescales, atmospheric CO2 content is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and vulcanism. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO2. The rates of these processes are extremely slow; hence they are of limited relevance to the atmospheric CO2 response to emissions over the next hundred years. In more recent times, atmospheric CO2 concentration continued to fall after about 60 Myr BP, and there is geochemical evidence that concentrations were <300 µL/L by about 20 Myr BP. Low CO2 concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 Myr BP. Although contemporary CO2 concentrations were exceeded during earlier geological epochs, present carbon dioxide levels are likely higher now than at any time during the past 20 million years[15] and at the same time lower than at any time in history if we look at time scales longer than 50 million years. NOAA research estimates that 97% of atmospheric CO2 created each year is from natural sources and approximately 3% is from human activities.[16]


Capturing/Extracting CO2

Methods of CO2 extraction/separation include:

  1. Aqueous solutions
    • Amine extraction
    • High pH solutions
    For example, Carbon dioxide reacts with dissolved CaO, to form Calcite (CaCO3)[17]
  2. Adsorption
    • Molecular Sieves
    • Activated Carbon[18][19]
    • Metal-organic frameworks(MOF's)[20]
  3. Solid reactants
    • Serpentine, Olivine, Quicklime[21][22]
  4. Membrane gas separation[23][24]
  5. Regenerative Carbon Dioxide Removal System (RCRS)
    The RCRS on the space shuttle Orbiter uses a two-bed system that provides continuous removal of CO2 without expendable products. Regenerable systems allow a shuttle mission a longer stay in space without having to replenish its sorbent canisters. Older lithium hydroxide (LiOH)-based systems, which are non-regenerable, are being replaced by regenerable metal-oxide-based systems. A metal-oxide-based system primarily consists of a metal oxide sorbent canister and a regenerator assembly. This system works by removing carbon dioxide using a sorbent material and then regenerating the sorbent material. The metal-oxide sorbent is regenerated by pumping air heated to around 200 °C at 7.5 standard cubic feet per minute through its canister for 10 hours.[25]


Air-sea exchange of CO2
Air-sea exchange of CO2

The Earth's oceans contain a huge amount of carbon dioxide in the form of bicarbonate and carbonate ions—much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate:

CaCO3 + CO2 + H2O Ca2+ + 2 HCO3-

Reactions like this tend to buffer changes in atmospheric CO2. However, since it produces an acidic compound, the pH of sea water is thought to go down with increasing carbon dioxide levels. Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas, which can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as CO2. Over hundreds of millions of years this has produced huge quantities of carbonate rocks.

The vast majority of CO2 added to the atmosphere will eventually be absorbed by the oceans and become bicarbonate ion, but the process takes on the order of a hundred years because most seawater rarely comes near the surface.



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

The properties of carbon dioxide were studied more thoroughly in the 1750s by the Scottish physician Joseph Black. He found that limestone (calcium carbonate) could be heated or treated with acids to yield a gas he termed "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 it would, when bubbled through an aqueous solution of lime (calcium hydroxide), precipitate calcium carbonate, and used this phenomenon to illustrate that carbon dioxide is produced by animal respiration and microbial fermentation. In 1772, Joseph Priestley used carbon dioxide produced from the action of sulfuric acid on limestone to prepare soda water, the first known instance of an artificially carbonated drink.[26]

Carbon dioxide was first liquefied (at elevated pressures) in 1823 by Humphry Davy and Michael Faraday.[27] The earliest description of solid carbon dioxide was given by Charles Thilorier, who in 1834 opened a pressurized container of liquid carbon dioxide, only to find that the cooling produced by the rapid evaporation of the liquid yielded a "snow" of solid CO2.


See also



  1. HISTORY OF DRY ICE. Retrieved on 2006-04-19.
  2. First Trials of Schaefer-Langmuir Dry Ice Cloud Seeding Technique in Hawaii.
  3. Hannan, Jerry. YOUR ROLE IN THE "GREENHOUSE EFFECT". Retrieved on 2006-04-19.
  5. Shukman, David (14 March 2006). Sharp rise in CO2 levels recorded. BBC News. Retrieved on 2006-04-19.
  6. Keeling, C.D.; T.P. Whorf. Atmospheric carbon dioxide record from Mauna Loa. Retrieved on 2006-04-19.
  11. Wagner, Friederike, Bent Aaby and Henk Visscher (2002). "Rapid atmospheric O2 changes associated with the 8,200-years-B.P. cooling event". PNAS 99 (19): 12011–12014. DOI:10.1073/pnas.182420699.
  12. Indermühle, Andreas, Bernhard Stauffer, Thomas F. Stocker (1999). "Early Holocene Atmospheric CO2 Concentrations". Science 286 (5446): 1815. DOI:10.1126/science.286.5446.1815a. Retrieved on May 26.
  13. Smith, H.J., M Wahlen and D. Mastroianni (1997). "The CO2 concentration of air trapped in GISP2 ice from the Last Glacial Maximum-Holocene transition". Geophysical Research Letters 24 (1): 1–4.
  26. Priestley, Joseph (1772). "Observations on Different Kinds of Air". Philosophical Transactions 62: 147–264.
  27. Davy, Humphry (1823). "On the Application of Liquids Formed by the Condensation of Gases as Mechanical Agents". Philosophical Transactions 113: 199–205.
The references in this article would be clearer with a different and/or consistent style of citation, footnoting or external linking. Please see the relevant discussion on the talk page for further details.

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