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 (GtC means "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 carbon dioxide can be formed under pressure (pressures above 5.1 atmospheres, roughly the pressure at 165 feet of depth in the ocean).

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.

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 is 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 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.

Want to learn more? Try www.en.wikipedia.org/wiki/Carbon_dioxide.

References:

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.
4. nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html.
5. www.sirinet.net/~jgjohnso/respiratory.html (accessed August 2006).
6. For American Safety Regulations, visit www.osha.gov/dts/sltc/methods/inorganic/id172/id172.html or www.osha.gov/dts/chemicalsampling/data/CH_225400.html (accessed August 2006); for Canadian Safety Regulations, visit www.ccohs.ca/oshanswers/chemicals/chem_profiles/carbon_dioxide/basic_cd.html (accessed August 2006).