In The Earth's Atmosphere
Carbon dioxide in earth's atmosphere is considered a trace gas currently occurring at an average concentration of about 390 parts per million by volume (or 591 parts per million by mass). The total mass of atmospheric carbon dioxide is 3.16×1015 kg (about 3,000 gigatonnes). Its concentration varies seasonally (see graph at right) and also considerably on a regional basis, especially near the ground. In urban areas concentrations are generally higher and indoors they can reach 10 times background levels. Carbon dioxide is a greenhouse gas.
As of November 2011, carbon dioxide in the Earth's atmosphere is at a concentration of approximately 390 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 slowly re-emitting the infrared at the same wavelength as what was absorbed.
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.
Five hundred million years ago carbon dioxide was 20 times more prevalent than today, decreasing to 4–5 times during the Jurassic period and then slowly declining with a particularly swift reduction occurring 49 million years ago. Human activities such as the combustion of fossil fuels and deforestation have caused the atmospheric concentration of carbon dioxide to increase by about 35% since the beginning of the age of industrialization.
Up to 40% of the gas emitted by some volcanoes during subaerial eruptions is carbon dioxide. It is estimated that volcanoes release about 130–230 million tonnes (145–255 million tons) of CO2 into the atmosphere each year. Carbon dioxide is also produced by hot springs such as those at the Bossoleto site near Rapolano Terme in Tuscany, Italy. Here, in a bowl-shaped depression of about 100 m diameter, local concentrations of CO2 rise to above 75% overnight, sufficient to kill insects and small animals, but it warms rapidly when sunlit and the gas is dispersed by convection during the day. Locally high concentrations of CO2, produced by disturbance of deep lake water saturated with CO2 are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986. Emissions of CO2 by human activities are estimated to be 135 times greater than the quantity emitted by volcanoes.
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