Peter Brannen covers global warming’s threat to plankton and the species that depend on it:
It turns out that even in death, plankton can support life, by providing an emergency brake for a planet careening dangerously out of control. Like today, the surface waters of the [Palaeocene-Eocene Thermal Maximum (PETM)] absorbed gigatons of carbon dioxide. When the oceans turned over, these more acidic waters eventually reached the bottom, and the corrosive water dissolved the thick seams of dead plankton lying in repose on the sea floor. The result was something like an antacid settling a dyspeptic stomach. The dissolution of the carbonate shells acted as a buffer, balancing the ocean’s pH so that within 100,000 years, the oceans were once again saturated with calcium carbonate.
‘The turnover time of the ocean is about 1,000 years,’ [palaeoceanographer James] Zachos told me. ‘But most of the anthropogenic CO₂ is accumulating in the upper ocean within 100 years. The ocean can’t mix it fast enough into the deep sea.’ By 2050, Zachos expects the ocean’s pH to drop by the same amount as during the entire PETM.