The world's sixth mass extinction may be upon us as a direct result of human depredations, writes Dick Ahlstrom, Science Editor.
It will start with massive volcanic eruptions followed soon after by runaway global warming. Plant life will rapidly die away, overwhelmed by climatic change. With it will go the complex web of organisms that populate the land and the sea, leaving a near-dead planet with only the toughest organisms left struggling to survive.
This scenario sounds like the kick-off to a predictable Hollywood disaster flick with the main characters left either to reverse the disaster or learning to live with the changed world. It would be easy to dismiss as junk were it not for the fact that life on Earth stood on this same precipice some 251 million years ago.
There is incontrovertible evidence in the geological record that the Earth went through a stupendous and unprecedented collapse in the number of life forms it supported. This great mass extinction on what is known as the Permian-Triassic boundary wiped out between 90 and 95 per cent of all the species that then populated the Earth.
In hot dispute however, are the reasons behind this dramatic collapse. Many scientists believe that only the impact of a massive asteroid could have caused such dramatic loss of biological diversity. There is a growing body of scientific opinion however, that suggests that a sequence of much smaller but cumulative events on the Earth's surface killed off so much life, leaving a 100 million year gap before diversity returned again to its levels of 251 million years ago.
The arguments for and against these two positions are laid out in a new book entitled, When Life Nearly Died, the Greatest Mass Extinction of All Time, by Michael J. Benton. He speaks with some authority on the matter as professor of vertebrate palaeontology and head of the Department of Earth Sciences at the University of Bristol.
He emerges on the side of the stepwise destruction of diversity rather than the deus ex machina simplicity of a giant impactor. While the latter is handy for explaining the extremely rapid loss of life forms, compressed into anything from 10,000 years to 600,000 years, the geological evidence increasingly points to a sequence of events that put paid to the Permian era long before the age of the dinosaurs.
Most disconcertingly, he doesn't allow this information to remain a scientific curiosity delivered to entertain and inform. Rather, he lets us know that we may already have embarked on the world's sixth great mass extinction, one caused by our own hand. This book, he points out, shows us what could befall us and how life around us could collapse, giving way to an uncertain future.
The arguments he presents are based on the very latest scientific findings, some as recent as 2001. Many are so new that some evidence is only just emerging to confirm their truth. And the jury remains out on many of the findings as scientists for and against the two main competing theories struggle with the information being dug out of ancient rock formations around the world.
Benton doesn't completely dismiss the possibility of a massive asteroid collision 251 million years ago. After all, we have been there before.
Everyone now accepts the very clear evidence that a major impact caused the death of the dinosaurs 65 million years ago, in the process wiping out 50 per cent of the world's then life forms.
Similar compelling evidence has not yet been delivered to confirm an impact scenario for the largest extinction of all time, 251 million years ago, Benton argues. While some groups say they have found the characteristic signs of an impactor, there is much more evidence for a slower yet just as deadly collapse of life taking place over thousands of years, he believes.
The findings suggest that there were a sequence of events that took place both before and after that line in the sand 251 million years ago. The chain started with massive volcanic activity in eastern Russia in what is known as the Siberian Traps.
The geological record shows the Traps released vast quantities of basalt lava, perhaps two to three million cubic kilometres of material. The flows blanketed almost four million square kilometres, greater than the landmass of the current EU, with a lava crust up to three kilometres thick in places.
The lava flows themselves would have caused complete destruction of anything in their path, but something more profound had been initiated, according to Benton's arguments. These flows also belched out millions of tonnes of gases including sulphur dioxide, carbon dioxide and chlorine, the effects of which would have a much greater reach.
The sulphur dioxide and dust in the atmosphere would have caused a dramatic but short-lived drop in temperatures, enough to frostbite plant life for thousands of square kilometres beyond the lava flows. But this cold snap would quickly have given way to a sudden rise in temperatures caused by carbon dioxide-driven global warming.
There is evidence pointing towards a six degree Centigrade rise in average temperatures around the time of the Trap eruptions, says Benton. This would have put both plant life and the animals that depended on it under pressure right around the world in the coming few thousand years.
The acid rain caused by the sulphur and chlorine in the atmosphere would have accelerated destruction of plants, with the loss of cover allowing significant soil erosion. Millions of tonnes of rotting plant material and soils would have been flushed down rivers and into the seas to corrupt the ocean environments.
There was a sequence of eruptions from the Siberian Traps over several thousand years, causing pulses of destruction, the theory holds. The greatest indignity, however, would have come as the warming process continued. The world's life went out not with a bang, nor a whimper but a burp.
Benton describes how a "methane burp" finished off most species by sparking runaway global warming. In the 1970s scientists discovered "gas hydrates" in the cold deep waters from the tropics to the poles. Water molecules under pressure can form hydrates, crystalline structures that trap large amounts of methane and lesser volumes of other gases including carbon dioxide.
If warmed, the hydrates release these gases, which expand 160-fold with a million litres of hydrates discharging 160 million litres of methane gas.
Large amounts of this powerful greenhouse gas were likely released as the climate warmed, fuelling a runaway warming process that overwhelmed the Earth's normal control systems.
This "triple whammy" of cold snaps, acid rain and global warming served to kill life on land and in the seas, slashing diversity back to catastrophically low levels. But the most disturbing thing is it could happen again.
Human activity on Earth already causes the extinction of an estimated 70 to 150 plants, animals and insects each day. We may already have triggered a downward decline in diversity through our actions, says Benton.
"Of course some life will survive human depredations. It may be cockroaches or rats, but to claim that humans cannot drive all life to extinction is hardly cause for congratulations," he writes.
And what if a rise in temperature, caused by our discharges of carbon dioxide, causes enough warming to unlock large volumes of gas hydrates, unleashing runaway global warming? Would we as a species be counted among the lucky ones to survive the ecological pressures or perish like the top-of-food-chain animals have in previous mass extinctions? There are lessons to be learned in Benton's book but it may already be too late to do anything about them.
When Life Nearly Died, The Greatest Mass Extinction of All Time by Michael J. Benton, Thames & Hudson, 336pp, 46 illustrations, £16.95 in UK