What is carbon dioxide?
Carbon dioxide (chemical name CO2) is a clear gas composed of one atom of carbon (C) and two atoms of oxygen (O). Carbon dioxide is one of many chemical forms of carbon on the Earth. It does not burn, and in standard temperature and pressure conditions it is stable, inert, and non-toxic. Carbon dioxide occurs naturally in small amounts (about 0.04%) in the Earth's atmosphere. The volume of CO2 in the atmosphere is equivalent to one individual in a crowd of 2,500.
Carbon dioxide is produced naturally by processes deep within the Earth. This CO2 can be released at the surface by volcanoes or might be trapped in natural underground geologic CO2 deposits, similar to underground deposits of oil and natural gas. As a major greenhouse gas, CO2 helps create and maintain the natural greenhouse effect that keeps Earth hospitable to life. Without the greenhouse effect, Earth’s temperature would only reach 0° F (-18° C).
Humans breathe it
Carbon dioxide is a minor part of the air humans breathe in and is also a byproduct of our body’s metabolism. Despite the minor amount of CO2 in the air, it is essential to plant life and is a key part of the global carbon cycle. Plants take in CO2 and emit oxygen (O2), and use the carbon to live and grow. When the plant dies or is burned, the carbon recombines with O2 in the air, and CO2 is formed again, completing the cycle. Within the global carbon cycle, there are natural sources that add CO2 into the atmosphere, and there are natural sinks that react with or absorb CO2, removing it from the atmosphere. Natural CO2 sinks include plant photosynthesis and ocean absorption. Natural CO2 sources include plant/animal decomposition, ocean release, animal/plant respiration, forest fires, and volcanoes.
Plants need it
Plants take in CO2, break down the CO2 into carbon and oxygen, release the oxygen to the atmosphere, and then retain the carbon to live and grow. However, humans and animals cannot extract the O2 from CO2 when they breathe. In high concentrations, CO2 displaces O2 and, in large doses, can impact humans and animals. Even though CO2 is easily dispersed by moving air, since it is denser than air, it can collect in open pits and other low areas if ventilation is inadequate.
Too much of a good thing
Carbon dioxide generated by human activity is called anthropogenic CO2. The plowing of land exposes carbon within the soil to the O2 in the air and forms anthropogenic CO2. When limestone is heated to make lime for cement, the carbon in the limestone combines with O2 in the air to make anthropogenic CO2. An even larger source of anthropogenic CO2 is from burning fossil fuels for energy, which combines the fuel’s carbon with O2, forming CO2.
Every day, CO2 is injected into underground geologic zones to help produce oil in a well-known industry practice called "CO2 flooding." This is an example of an anthropogenic CO2 sink, as the CO2 is isolated underground. Other anthropogenic sinks include geologic storage sites and landfills.
Carbon dioxide can be produced as a commercial product, which requires that it be purified from a high-volume, CO2-rich gas stream. Carbon dioxide is produced this way for several practical applications, such as:
- Chemical production
- Soft drink carbonation
- Food preservation
- Refrigeration (dry-ice)
- Fire extinguishers (CO2 displaces the O2 that fuels fire)
- Water treatment
Misconception: The presence of CO2 in the atmosphere is bad, is anthropogenic, and only comes from the burning of fossil fuels.
Reality: Carbon dioxide is derived from both natural and anthropogenic sources and is essential to plant life.