Comment on egusphere-2023-1004

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> An alternative measure of the ocean&rsquo;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&rsquo;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&rsquo;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.