As part of the carbon cycle known as photosynthesis, plants, algae, and cyanobacteria absorb carbon dioxide, sunlight, and water to produce carbohydrate energy for themselves and oxygen as a waste product. By contrast, during respiration they emit carbon dioxide, as do all other living things that depend either directly or indirectly on plants for food. Carbon dioxide is also generated as a by-product of combustion; emitted from volcanoes, hot springs, and geysers; and freed from carbonate rocks by dissolution.
As of October 2010[update], carbon dioxide in the Earth’s atmosphere is at a concentration of 388 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 standard atmospheres (520 kPa). At 1 atmosphere (near mean sea level pressure), the gas deposits directly to a solid at temperatures below âˆ’78 Â°C (âˆ’108 Â°F; 195.1 K) and the solid sublimes directly to a gas above âˆ’78 Â°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.
CO2 + H2O H2CO3
CO2 is toxic in higher concentrations: 1% (10,000 ppm) will make some people feel drowsy. Concentrations of 7% to 10% cause dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour.