Ocean Acidification during the Paleocene-Eocene Thermal Maximum: Lessons for the Future

One of the underappreciated consequences of unabated anthropogenic carbon emissions is ocean acidification. Over the last century the ocean has absorbed more than 30% of the roughly 380 Pg C that has been emitted to the atmosphere. Because the ocean is stratified and vertical mixing is relatively slow (~1 k.y.), most of the absorbed CO₂ has accumulated in the thin surface layer. As a consequence, both the pH and carbonate saturation state of sea water are already decreasing. With continued emissions over the next several centuries at business as usual rates, the pH of the surface ocean is projected to decrease by 0.7 pH units, an amount large enough to warrant concern for impacts on the oceans calcifying biota.

This change in ocean chemistry appears to be unprecedented in Earth history. The closest analog is the massive release of carbon that caused the Paleocene-Eocene Thermal Maximum (~55.5 Mya), a time of extreme global warming. Various geochemical records suggest the ocean may have absorbed as much as 6000 Pg C during this event. Although the total mass of carbon released during the PETM is similar to the projected anthropogenic mass (~4500 Pg C), the flux was spread over a much longer period (5 to 10 k.y.). As a result, because of mixing with the deep ocean where chemical dissolution of seafloor carbonates prevents large swings in pH, the surface ocean saturation state did not change appreciably. Nevertheless, the changes in carbonate chemistry were long lasting (>100 k.y.) and likely contributed to the demise of pelagic protists, and possibly corals. To avoid substantial changes in ocean pH, it will be necessary for society to substantially slow the rate of carbon emissions, either by reducing fossil fuel consumption or by sequestration.