The relationship between global climate and marine biodiversity

Close examination of the geological record provides chilling evidence that Planet Earth has historically been vulnerable to climate-induced biodiversity loss on a huge scale. Prior to large extinction events in the late Permian/early Triassic, early Jurassic, middle Cretaceous, and late Paleocene, major volcanic events occurred and the carbon-isotope record confirms that during these periods atmospheric CO₂ concentrations rose considerably and stayed high for hundreds of thousands of years. Although the current rise in atmospheric CO₂ levels is not volcanically driven, the consequences could essentially be similar. The largest of all climatically driven extinction events occurred 251 Mya at the end of Permian period. During this event, 80% of all marine species and 49% of all marine families went extinct (Raup and Sepkoski 1982; Stanley and Yang 1994). It is suggested that the environmental changes associated with the late-Permian extinction event were triggered by huge quantities of volcanically produced CO₂ and methane that drove rapid global warming. As the oceans warmed, they carried less oxygen and these conditions encouraged the growth of bottom-dwelling anaerobic bacteria. These bacteria produced large quantities of hydrogen sulphide (H₂S), and eventually the chemocline (the division between oxygen-rich surface water and H₂ S-rich deep water) rose up into the photic zone allowing the proliferation of green and purple photosynthe- sizing sulphur bacteria while oxygen-breathing organisms suffocated. The H₂ S also diffused into the air where it killed animals and plants on land. Once into the upper atmosphere, the H₂S destroyed the ozone layer and the Sun's ultraviolet radiation caused even more extinctions (Kump et al. 2005).

Whilst it is difficult to imagine that such a fate could befall modern Earth, it is worth reflecting on the atmospheric levels of CO₂ that correspond to some of these large extinction events. At the start of the late-Permian event, the concentration was around 3000 parts per millon (ppm), almost an order of magnitude higher than today's value of 385 ppm. However, the smaller yet still significant extinctions which occurred at the end of the Paleocene and at the end of the Triassic began when atmospheric CO₂ levels were much lower, around 1000 ppm. If current increases in atmospheric CO₂ levels continue as projected, we could be looking at levels approaching 1000 ppm by the end of the next century (IPCC 2007).