Carbon dioxide (CO2)

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Carbon Dioxide (CO2)

Carbon dioxide is a chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom. It is a gas at standard temperature and pressure and exists in Earth's atmosphere in this state. It is currently at a globally averaged concentration of approximately 383 ppm. by volume in the Earth's atmosphere, although this varies both by location and time. Carbon dioxide's chemical formula is CO2.

Carbon dioxide is produced by all animals, plants, fungi and microorganisms during respiration and is used by plants during photosynthesis to make sugars which may either be consumed again in respiration or used as the raw material for plant growth. It is, therefore, a major component of the carbon cycle. Carbon dioxide is generated as a byproduct of the combustion of fossil fuels or vegetable matter, among other chemical processes. Inorganic carbon dioxide is output by volcanoes and other geothermal processes such as hot springs.

Carbon dioxide is an important greenhouse gas because it transmits visible light but absorbs strongly in the infrared.

Carbon dioxide has no liquid state at pressures below 5.1 atm, but is a solid at temperatures below -78 °C. In its solid state, carbon dioxide is commonly called dry ice.

Plants remove carbon dioxide from the atmosphere by photosynthesis, also called carbon assimilation, which uses light energy to produce organic plant materials by combining carbon dioxide and water. Free oxygen is released as gas from the decomposition of water molecules, while the hydrogen is split into its protons and electrons and used to generate chemical energy via photophosphorylation. This energy is required for the fixation of carbon dioxide in the Calvin cycle to form sugars. These sugars can then be used for growth within the plant through respiration.

Even when vented, carbon dioxide must be introduced into greenhouses to maintain plant growth, as the concentration of carbon dioxide can fall during daylight hours to as low as 200 ppm. A similar event on a global scale occurred after the Permian period, when Earth life almost became extinct. A repeat of such conditions (a decrease of carbon dioxide concentration to about 60-65% of its current level) would kill 99% of existing plants and undoubtedly eliminate human civilization (carbon dioxide deficit was also the most likely reason for most big life "extinctions" on the planet and will also finally end the Earth life). Plants can potentially grow up to 50 percent faster in concentrations of 1,000 ppm CO2 when compared with ambient conditions.

Plants also emit CO2 during respiration, so it is only during growth stages that plants are net absorbers. For example a growing forest will absorb many tons of CO2 each year, however a mature forest will produce as much CO2 from respiration and decomposition of dead specimens (e.g. fallen branches) as used in biosynthesis in growing plants. Regardless of this, mature forests are still valuable carbon sinks, helping maintain balance in the Earth's atmosphere. Additionally, and crucially to life on earth, phytoplankton photosynthesis absorbs dissolved CO2 in the upper ocean and thereby promotes the absorption of CO2 from the atmosphere.