As the evidence accumulates, it now seems that climate change was the commonest cause of mass extinction in the Earth’s prehistory.
In the media – if not scientific literature – global catastrophes have long been associated with asteroid strikes. But as the dating of rocks has improved – the links have vanished. Even the famous meteorite impact at Chicxulub in Mexico – widely blamed for the destruction of the dinosaurs – was out of sync by more than 100,000 years.
The story that emerges repeatedly from the fossil record is mass extinction caused by three deadly impacts – occurring simultaneously: global warming – the acidification of the oceans – and the loss of oxygen from seawater. All these effects are caused by large amounts of carbon dioxide entering the atmosphere. When seawater absorbs CO2 – its acidity increases. As temperatures rise – circulation in the oceans stalls – preventing oxygen from reaching the depths.
The great outgassings of the past were caused by volcanic activity that were orders of magnitude greater than the eruptions we sometimes witness today. The dinosaurs appear to have been wiped out by the formation of the Deccan Traps in India: an outpouring on such a scale that one river of lava flowed for 1,500km. But that event was dwarfed by a far greater one – 190m years earlier – that wiped out 96% of marine life as well as most of the species on land.
What was the cause? It now appears that it might have been the burning of fossil fuel. Before I explain this extraordinary contention, it’s worth taking a moment to consider what mass extinction means. This catastrophe – at the end of the Permian period about 252m years ago – wiped out not just species within the world’s ecosystems but the ecosystems themselves. Forests and coral reefs vanished from the fossil record for some 10 million years. When eventually they were reconstituted – it was with a different collection of species which evolved to fill the ecological vacuum. Much of the world’s surface was reduced to bare rubble. Were such an extinction to take place today – it would be likely to eliminate almost all the living systems that sustain us. When plants are stripped from the land – the soil soon follows.
The latest research into the catastrophe at the end of the Permian is summarised in two articles by the geologist John Mason on the Skeptical Science site. The strongest clues all seem to point to the same conclusion: that the extinctions were triggered by the eruption of an igneous belt even bigger than the Deccan plateau: the Siberian Traps. As well as CO2 – the volcanoes there produced sulphur dioxide – chlorides – and fluorides – causing acid rain – and the depletion of ozone.