Past constraints on the vulnerability of marine calcifiers to massive carbon dioxide release

Increasing concentrations of carbon dioxide in sea water are driving a progressive acidification of the ocean(1). Although the associated changes in the carbonate chemistry of surface and deep waters may adversely affect marine calcifying organisms(2-4), current experiments do not always produce consistent results for a given species(5). Ocean sediments record past biological responses to transient greenhouse warming and ocean acidification. During the Palaeocene-Eocene thermal maximum, for example, the biodiversity of benthic calcifying organisms decreased markedly(6,7), whereas extinctions of surface dwellers were very limited(8,9). Here we use the Earth system model GENIE-1 to simulate and compare directly past and present environmental changes in the marine realm. In our simulation of future ocean conditions, we find an undersaturation with respect to carbonate in the deep ocean that exceeds that experienced during the Palaeocene-Eocene thermal maximum and could endanger calcifying organisms. Furthermore, our simulations show higher rates of environmental change at the surface for the future than the Palaeocene-Eocene thermal maximum, which could potentially challenge the ability of plankton to adapt.