Caldeira Lab Research:Paleoclimate and geochemical cycles/Ocean acidification and ocean carbon cycle

Seawater pH and atmospheric carbon dioxide

Ken Caldeira & Robert Berner

In a previous paper, it was postulated that even though the middle Eocene maintained a mean temperature 5 degrees higher than today, it had similar CO2 concentration. This can further be used to suggest that CO2 had little effect on temperature at that time period. However, this conclusion was reached through an inaccurate method used to determine CO2 concentration from ocean pH. When a more accurate model is used, pH at the time corresponds well to large amounts of carbon dioxide in the atmosphere.

Caldeira, K., and Berner, R., Seawater pH and atmospheric carbon dioxide (Technical comment), Science 286, 2043a–2043a, 1999.

CO2 concentration as a function of pH: when determined assuming a constant total dissolved inorganic concentration, high atmospheric CO2 concentration does not correspond to pH levels during the middle Eocene. However, this assumption is fairly inaccurate, and when concentration is determined assuming constant carbonate-ion concentration instead, Eocene pH corresponds very well to high atmospheric CO2 levels.


Pearson and Palmer (1) concluded that even though middle Eocene (43 Ma) global mean temperature was perhaps 5°C warmer than today, “atmospheric pCO2 was probably similar to modern concentrations or slightly higher.” They suggest “either that Earth’s climate is very sensitive to small changes in pCO2, or that the global cooling since the Eocene was not driven primarily by changes in pCO2. . . .” This result has led some (2) to suggest that “the whole carbon dioxide paradigm is crumbling.” Pearson and Palmer used boron isotopes to infer paleo-ocean pH, and then used this paleo-pH proxy to estimate paleo atmospheric pCO2 under the assumption of constant concentration of total dissolved inorganic carbon in seawater. However, carbon is continuously supplied to the atmosphere and ocean by degassing from metamorphism and magmatism and by the weathering of carbonate minerals and organic carbon, and is continuously consumed by the production of carbonate and organic carbon sediments (3). Hence, the total dissolved inorganic carbon load of the ocean could be expected to vary over time.