Fri, Jul 04, 2003 - Page 9 News List

Global warming: Prehistory may soon repeat itself

A mass extinction marked the end of the Permian period some 251 million years ago. Our world is only 6? away from a similar catastrophe

By George Monbiot  /  THE GUARDIAN , LONDON

It is old news, I admit. Two hundred and fifty-one million years old, to be precise. But the story of what happened then, which has now been told for the first time, demands our urgent attention. Its implications are more profound than anything taking place in Iraq, or Washington, or even (and I am sorry to burst your bubble) Wimbledon.

Unless we understand what happened, and act upon that intelligence, prehistory may very soon repeat itself, not as tragedy, but as catastrophe.

The events that brought the Permian period (between 286 million and 251 million years ago) to an end could not be clearly determined until the mapping of the key geological sequences had been completed. Until recently, paleontologists had assumed that the changes that took place then were gradual and piecemeal. But three years ago a precise date for the end of the period was established, which enabled geologists to draw direct comparisons between the rocks laid down at that time in different parts of the world.

Having done so, they made a shattering discovery. In China, South Africa, Australia, Greenland, Russia and Svalbard, the rocks record an almost identical sequence of events, taking place not gradually, but relatively instantaneously. They show that a cataclysm caused by natural processes almost brought life on earth to an end. They also suggest that a set of human activities that threatens to replicate those processes could exert the same effect, within the lifetimes of some of those who are on earth today.

As the professor of paleontology Michael Benton records in his new book, When Life Nearly Died, the marine sediments deposited at the end of the Permian period record two sudden changes. The first is that the red or green or gray rock laid down in the presence of oxygen is suddenly replaced by black muds of the kind deposited when oxygen is absent. At the same time, an instant shift in the ratio of the isotopes (alternative forms) of carbon within the rocks suggests a spectacular change in the concentration of atmospheric gases.

On land, another dramatic transition has been dated to precisely the same time. In Russia and South Africa, gently deposited mudstones and limestones suddenly give way to massive dumps of pebbles and boulders. But the geological changes are minor in comparison with what happened to the animals and plants.

The Permian was one of the most biologically diverse periods in the earth's history. Herbivorous reptiles the size of rhinos were hunted through forests of tree ferns and flowering trees by saber-toothed predators. At sea, massive coral reefs accumulated, among which lived great sharks, fish of all kinds and hundreds of species of shelly creatures.

Then suddenly there is almost nothing. The fossil record very nearly stops dead. The reefs die instantly, and do not reappear on earth for 10 million years. All the large and medium-sized sharks disappear, most of the shelly species, and even the great majority of the toughest and most numerous organisms in the sea, the plankton.

Among many classes of marine animals, the only survivors were those adapted to the near-absence of oxygen.

On land, the shift was even more severe. Plant life was almost eliminated from the earth's surface. The four-footed animals, the category to which humans belong, were nearly exterminated: so far only two fossil reptile species have been found anywhere on earth that survived the end of the Permian. The world's surface came to be dominated by just one of these, an animal a bit like a pig. It became ubiquitous because nothing else was left to compete with it or to prey upon it.

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