The impact of rising atmospheric CO2 levels and resulting ocean acidification on the physical (solubility) ocean pump of CO2
Abstract. An alternative measure of the ocean’s carbonate buffer system efficiency to absorb CO2 from the atmosphere is proposed. Instead of the Revelle factor R = (∆CO2/CO2)/(∆DIC/DIC) = (DIC/CO2)/ (∆DIC/∆CO2) the sensitivity S = (∆DIC/∆CO2) is preferable because it gives directly the change ∆DIC of the concentration of DIC in the seawater caused by the change ∆CO2 of carbon dioxide in the atmosphere. To this end the DIC concentration of seawater at temperature T in equilibrium with a defined CO2 level in the surrounding atmosphere is calculated by use of the geochemical program PHREEQC. From the function DIC(CO2,T) one obtains by differentiation the sensitivity S = dDIC/dCO2 = ∆DIC/∆CO2 and also the Revelle factor R. Using S as the change of the ocean’s buffer capacity reveals a better insight of its future evolution than using the Revelle factor R.
One finds that the buffer capacity S has declined by about 30 % from 1945 to present and that its future decline from 400 to 600 ppm will be a further 30 %. By calculating the uptake of CO2 of his equilibrium pump an upper value of 1.3 Gigatons/year is obtained, small in comparison to the 10 Gigatons/year absorbed by the ocean at present. The Revelle factor R at present is calculated R = 13 and rises to 18 at a CO2 level of 800 ppm. This increase of R has been interpreted as indication of the collapse of the solubility pump. S and R, however, are defined from equilibrium chemistry and are a measure of the CO2 absorbed by the ocean’s upper mixed layer by increase of the CO2 level in the atmosphere without regarding its sinking into the deep-ocean by the thermohaline circulation. The difference ∆DIC between the actual value and the value at 280 ppm is transported into the deep-ocean by the global meridional conveyor belt. ∆DIC increases with increasing CO2 level. At 280 ppm the system ocean-atmosphere is in equilibrium and the sink is zero. At 400 ppm a value of about 1.9 Gtons/year is estimated that increases to 3.9 Gtons/year at 600 ppm and to 5 Gtons/year at 800 ppm. At present CO2 level increase of 2 ppm/year 10 Gtons/year are absorbed by the ocean. The solubility pump contributes 3.2 Gtons/year: 1.3 Gtons/year by equilibrium absorption into the mixed layer and 1.9 Gtons/yeat by thermohaline circulation. At 600 ppm the total sink is 4.6 Gtons/year and at 800 ppm 5.5 Gtons/year. To conclude, the solubility pump is not endangered by ocean acidification. In contrast, it increases with increasing CO2 level of the atmosphere to yield significant contribution.
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