Surface ocean acidification with increasing atmospheric carbon dioxide and its effects on coral calcification

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Global warming related with excess emission of carbon dioxide is a burning issue for the world's environments nowadays. Due to fossil fuel burning, atmospheric carbon dioxide is predicted to be double (560 ppm) of pre-industrial period in 2065 and 700 ppm in 2100. Such an increase is reducing surface oceanic pH and carbonate ion concentrations, and thus the degree of calcium carbonate (aragonite crystal) saturation (Ω=[Ca^<2+>][CO_3^<2->]/K), where K is the stoichiometric solubility product for aragonite. Since [Ca^<2+>] is constant in seawater, changes in seawater [CO_3^<2->] could proportionally make changes in seawater Ω. Corals use CO_3^<2-> from seawater to precipitate CaCO_3 (calcification) in their skeletons as the reaction, Ca^<2+>+2HCO_3^-=CaCO_3+H_2O+CO_2. Changes in seawater [CO_3^<2->] possibly affects coral calcification rate. We evaluate the effects of seawater Ω on coral calcification associated with elevated atmospheric carbon dioxide. From the culture experiment data, … More we could conclude that coral calcification is possibly controlled by an inorganic precipitation law (R=k(Ω-1)^n), where R is the precipitation rate, k is the rate constant and n is the empirical-reaction order. In order to figure out the changes in calcification rate with increasing carbon dioxide in future, the rates are normalized (G in %) to the rate at Ω of 4.6, the value for Ω during the pre-industrial period (CO_2=280 ppm). When we use the first-order model (n=1), the model gives a relationship as follows : G=(Ω-1)100/(4.6-1)=27.8(Ω-1), for coral calcification. If atmospheric CO_2 increases from ～280 ppm in 1800 to ～560 ppm in 2065 and to ～700 ppm in 2100, seawater Ω is predicted to decrease from 4.6 to 2.9 and 2.5, respectively. Using the model equation, coral calcification is predicted to decline ～47% and ～58% by 2065 and 2100, respectively. We suggest that coral calcification is closely coupled to CO_2 in seawater and seawater Ω, and declined coral calcification is predicted near future when atmospheric CO_2 further increases. Less