Definition for word: Carbon Dioxide (C02)

An odorless, non-poisonous gas that is a normal part of Earth's atmosphere. Carbon dioxide is a product of fossil-fuel combustion as well as other processes. It is considered a greenhouse gas because it traps heat (infrared energy) radiated by the Earth into the atmosphere and thereby has the potential to cause global warming. The global warming potential (GWP) of other greenhouse gases is measured in relation to that of carbon dioxide, which has an assigned value of one.

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Carbon dioxide (chemical formula CO2) 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. CO2 is a trace gas comprising 0.039% of the atmosphere.

Carbon dioxide is used by plants during photosynthesis to make sugars, which may either be consumed in respiration or used as the raw material to produce other organic compounds needed for plant growth and development. It is emitted during respiration by plants, and by all animals, fungi and microorganisms that depend either directly or indirectly on plants for food. It is thus a major component of the carbon cycle. Carbon dioxide is generated as a by-product of the combustion of fossil fuels or the burning of vegetable matter, among other chemical processes. Amounts of carbon dioxide are emitted from volcanoes and other geothermal processes such as hot springs and geysers and by the dissolution of carbonates in crustal rocks.

As of April 2010[update], carbon dioxide in the Earth's atmosphere is at a concentration of 391 ppm by volume. Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by seasonal plant growth in the Northern Hemisphere. Concentrations of carbon dioxide fall during the northern spring and summer as plants consume the gas, and rise during the northern autumn and winter as plants go dormant, die and decay. Taking all this into account, the concentration of CO2 grew by about 2 ppm in 2009. Carbon dioxide is a greenhouse gas as it transmits visible light but absorbs strongly in the infrared and near-infrared.

Before the advent of human-caused release of carbon dioxide to the atmosphere, concentrations tended to increase with increasing global temperatures, acting as a positive feedback for changes induced by other processes such as orbital cycles. There is a seasonal cycle in CO2 concentration associated primarily with the Northern Hemisphere growing season.

Carbon dioxide has no liquid state at pressures below 5.1 atmospheres. At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below −78 °C (−108.4 °F; 195.1 K) and the solid sublimes directly to a gas above −78 °C. In its solid state, carbon dioxide is commonly called dry ice.

CO2 is an acidic oxide: an aqueous solution turns litmus from blue to pink. It is the anhydride of carbonic acid, an acid which is unstable in aqueous solution, from which it cannot be concentrated. In organisms carbonic acid production is catalysed by the enzyme, carbonic anhydrase.

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