<P> Not only do increasing carbon dioxide concentrations lead to increases in global surface temperature, but increasing global temperatures also cause increasing concentrations of carbon dioxide . This produces a positive feedback for changes induced by other processes such as orbital cycles . Five hundred million years ago the carbon dioxide concentration was 20 times greater 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 . </P> <P> Local concentrations of carbon dioxide can reach high values near strong sources, especially those that are isolated by surrounding terrain . At the Bossoleto hot spring near Rapolano Terme in Tuscany, Italy, situated in a bowl - shaped depression about 100 m (330 ft) in diameter, concentrations of CO rise to above 75% overnight, sufficient to kill insects and small animals . After sunrise the gas is dispersed by convection . High concentrations of CO produced by disturbance of deep lake water saturated with CO are thought to have caused 37 fatalities at Lake Monoun, Cameroon in 1984 and 1700 casualties at Lake Nyos, Cameroon in 1986 . </P> <P> Carbon dioxide dissolves in the ocean to form carbonic acid (H CO), bicarbonate (HCO) and carbonate (CO). There is about fifty times as much carbon dissolved in the oceans as exists in the atmosphere . The oceans act as an enormous carbon sink, and have taken up about a third of CO emitted by human activity . </P> <P> As the concentration of carbon dioxide increases in the atmosphere, the increased uptake of carbon dioxide into the oceans is causing a measurable decrease in the pH of the oceans, which is referred to as ocean acidification . This reduction in pH affects biological systems in the oceans, primarily oceanic calcifying organisms . These effects span the food chain from autotrophs to heterotrophs and include organisms such as coccolithophores, corals, foraminifera, echinoderms, crustaceans and mollusks . Under normal conditions, calcium carbonate is stable in surface waters since the carbonate ion is at supersaturating concentrations . However, as ocean pH falls, so does the concentration of this ion, and when carbonate becomes undersaturated, structures made of calcium carbonate are vulnerable to dissolution . Corals, coccolithophore algae, coralline algae, foraminifera, shellfish and pteropods experience reduced calcification or enhanced dissolution when exposed to elevated CO 2 . </P>

Where does the carbon dioxide gas come from