Plains CO2 Reduction (PCOR) Partnership

What Is CO2?

Carbon dioxide (chemical name CO2) is a clear gas composed of one atom of carbon (C) and two atoms of oxygen (O2). CO2 is just one of many chemical forms of carbon on the Earth. The near-surface environment of the Earth contains approximately 121,000,000 GtC (gigatons of carbon); a gigaton is equivalent to a billion metric tons; the number means "121 million gigatons or 121 million billion metric tons" of carbon).1,2  When a ton of carbon combines with oxygen, it makes nearly four tons of CO2 gas.3

CO2 occurs naturally in small amounts (about 0.04%) in the Earth's atmosphere.4 The volume of CO2 in the atmosphere is equivalent to one person in a crowd of 2500 people.

Under normal conditions, CO2 is a gas. At temperatures below -78°C (-109°F), CO2 condenses into a white solid called dry ice. When warmed, dry ice vaporizes directly from a solid to CO2 gas in a process called sublimation. Liquid CO2 can be formed under pressure (pressures above 5.1 atmospheres, roughly the pressure at 165 feet of depth in the ocean).

CO2 is produced naturally by processes deep in the earth. This CO2 can be released at the surface by volcanoes or might be trapped in natural underground geologic CO2 deposits similar to the underground deposits of oil and natural gas.

Every day, millions of tons of CO2 are injected into underground geologic zones to help produce oil in a well-known industry practice called "CO2 flooding." In the high-temperature and pressure conditions of the oil-bearing geologic zones (below depths of around 800 meters or 2600 feet), CO2 will exist in a dense gas phase that acts like a liquid. This is called "supercritical" CO2.

In the high-temperature and pressure conditions of the oil-bearing geologic zones and many natural CO2 deposits (below depths of around 800 meters or 2600 feet), CO2 will exist in a dense gas phase. This type of CO2 is called "supercritical" CO2.

CO2 is essential to plant life and is a key part of the global carbon cycle. In nature, plants take in CO2, exhale the oxygen, and use the carbon to live and grow. When the plant dies or burns, the carbon recombines with oxygen in the atmosphere, and CO2 is formed again.

As a major greenhouse gas, CO2 helps create and maintain the natural greenhouse effect that keeps our planet hospitable to life.

CO2 is a minor part of the air we breathe in and is also a by-product of our body’s metabolism. The air we breathe in contains about three parts nitrogen, one part oxygen, a small amount of argon, and a very small amount of CO2. The air we exhale is a mixture that contains 100 times more CO2 than the air we took in (0.04% CO2 in the air we inhale and 4.0% CO2 in the air we exhale).5

Although plants take in CO2, break down the CO2 into carbon and oxygen, release the oxygen to the atmosphere, and retain the carbon to live and grow, humans and animals cannot extract the O2 from CO2 when they breathe. In high concentrations, CO2 displaces oxygen and, in large doses, can be an asphyxiant to humans and animals. CO2 is denser than air and can collect in open pits and other low areas, especially if ventilation is inadequate. Health and safety standards are available for CO2.6

We use CO2 to make the bubbles in soft drinks, and CO2 (as dry ice) is used to keep things cold. CO2 was used to power soda fountains, a new type of store where you could get one of those new-fangled carbonated beverages. Most carbonated beverages have CO2 added during bottling, but there are many examples of water from mineral springs that are naturally carbonated. Naturally carbonated waters have historically been highly sought after for their supposed curative properties because the naturally carbonated waters are high in mineral content. Commercially sold waters with natural carbonation include Apollinaris, Badoit, Gerolsteiner, Wattwiller, Ferrarelle, Borsec, and Perrier. CO2 is also used in fire extinguishers (CO2 displaces the oxygen the fire needs to burn).

CO2 formed by human action is called anthropogenic CO2. Plowing the land exposes the carbon in the soil to the oxygen in the air and makes anthropogenic CO2. When limestone is heated to make lime for cement, the carbon in the limestone combines with oxygen in the air to make anthropogenic CO2. Burning fossil fuels for energy combines the carbon in the fuel with oxygen and releases anthropogenic CO2. This anthropogenic CO2 adds additional carbon to the global carbon cycle.

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  1. Image of global carbon cycle from NOAA (National Oceanographic and Atmospheric Administration) August 2005.
  2. GtC = gigatons of carbon; 1 gigaton equals 1 billion or 1,000,000,000 metric tons (a metric ton is 1000 kilograms); 1 metric ton = 2204.6 pounds (an English system ton is 2000 pounds).
  3. Based on the ratio of the weights of the carbon and oxygen, 1 ton of carbon would combine with 2.667 tons of oxygen to form 3.667 tons of CO2.
  5. Google Answers, “What quantity of carbon dioxide per day is contained in the exhaled respiration of an average adult human being?” (accessed on Feburary 12, 2009).
  6. For American Safety Regulations, visit or (accessed August 2006); for Canadian Safety Regulations, visit (accessed August 2006).