Introduction
Amidst the uncertainties associated with the environmental effects of accumulating fossil-fuel CO2 is one certainty: as atmospheric pCO2 increases, the pH and saturation state of surface seawater with respect to aragonite and calicte are declining, and will continue to do so as long as we continue to pump CO2 into the atmosphere. The observed trends in ocean chemistry can be quite confidently projected into the future [1] using models of various levels of complexity. Moreover, Earth history provides case studies of sudden addition of CO2 to the atmosphere and their environmental consequences that confirm these projections and reveal the inner workings of the carbon cycle as it responds to perturbation.
Déjà vu
The best studied carbon perturbation in Earth history is the Paleocene-Eocene Thermal Maximum (PETM). Indeed, among the paleoclimate community the PETM has become the poster-child for the dramatic consequences of ocean acidification. The onset of the PETM is rapid, within a few thousand years, and its effects sustained for nearly 200 k.y. [2]. The carbonate compensation depth (CCD) of the ocean shoaled markedly, but there is scant evidence for surfacewater acidification. Simulations of the event using the intermediate complexity model Genie (http://www.genie.ac.uk/) demonstrate the critical importance of rate of addition: 6800 Pg CO2 added to the atmosphere over 10,000 years leads to substantial seafloor carbonate dissolution, but surface waters remain highly supersaturated. Shortening the emission time to 1,000 years drives surface water calcite saturation states in the tropics from 4-7 to ~2-3 and in higher latitudes to below 1.5, in some places becoming undersaturated. This faster rate of CO2 emission thus has more severe consequences for surface-dwelling calcifying organisms. However, at 6.8 Pg per year it has already been exceeded by current fossil fuel use.
Kump, L. R. and Ridgwell, A., 2009. Ocean acidification: been there, done that. Awards Ceremony Speeches and Abstracts of the 19th Annual V.M. Goldschmidt Conference, V.M. Goldschmidt Conference. Geochimica et Cosmochimica Acta 73(13)-1: A706. Supplement.
