What caused the previous mass extinctions?
We know about the previous five mass extinctions from the fossil record. Studies of the number of marine families over the past 600 million years show five points in time when large numbers of species disappeared abruptly. The last was 65 million years ago, in which the dinosaurs became extinct, and is thought to have resulted from an asteroid hitting the Earth and causing a dramatic change in the climate. In fact all previous mass extinctions are thought to have been caused by some large-scale geo-physical process, such as supervolcanic eruptions or the sudden release of vast quantities of greenhouse gases from the seabed. The biggest mass extinction of all occurred some 251 million year ago, when about 90 per cent of marine life and 70 per cent of land species vanished. One theory is that something happened to the Earth’s atmosphere at the end of the Permian period, robbing it of vital oxygen. It has happened before, so why worry? There are several reasons we should be concerned. The
