Magazine article Geographical

The End of the World as We Know It: As Scientists Learn More about Earth's Previous Five Mass Extinctions and the Geological Events That Led to Them They Are Beginning to Detect Alarming Parallels with the Effects of Global Warming Today and Are Warning That History Could Be about to Repeat Itself

Magazine article Geographical

The End of the World as We Know It: As Scientists Learn More about Earth's Previous Five Mass Extinctions and the Geological Events That Led to Them They Are Beginning to Detect Alarming Parallels with the Effects of Global Warming Today and Are Warning That History Could Be about to Repeat Itself

Article excerpt

In the glorious 3.5-billion-year history of life on Earth there hides a dark fact of the four billion species that are thought to have evolved, 99 per cent have become extinct.

On this planet, extinction is the norm. Species of plant and animal nave some and gone as the climate and environment nave changed. Some thrived n cold, others only when it was hot: some preferred dry conditions, others needed high humidity to survive. According to their preferences, speccies come and go as their fortunes wax and wane. inextricably linked to the fortunes and conditions of our shared planet

In particular, during five periods in the past 500 million Years, the steady rate of extinction has been smashed to pieces. Something, no-one knows for sure what, turned the Earth into exactly the wrong planet for life at several points and, in the process, most of the world's species disappeared. During each so-called mass extinction, more than three quarters of the existing species died off in the blink of an eye, geologically speaKing

The big one the one that palaeontoiogists call the Great Dying, was the Permian-Triassic event. Starting around 250 million years ago, it was the Earth's worst Known mass-extinction event, killing 95 per cent of species living at the time. 84 per cent of marine genera and an estimated 70 per cent of and species. A third of the world's insects perished. marking the only mass-extinction of this order of life. On land. the mamma-like reptiles were wiped out and the vertebrates took tens of millions of years to bounce back. What caused this event is disputed--some argue that it was a comet or asteroid impact. although no tell-tale crater has ever been found.

ATMOSPHERIC IMBALANCE

Sometimes. the Earth itself becomes a bad place to live. For example the world's plants and animals need oxygen to survive, and if, for some reason, the amount of oxygen n the air or sea dropped, life would clearly become difficult. Even if the levels dropped a small amount, billions of individual living things would die straight away and billions more that depended on those in interconnected ecosystems, would eventually die off.

The worrying thing is that. in the Bast, levels of oxygen have fluctuated enough to cause major problems. There was a marked period of deficiency in oceanic oxygen during the Cretaceous Period. for example, which was the heyday of the dinosaurs. Probab y the result of an increase in the activity of undersea volcanism that deficiency led to mass extinctions of life As scientists learn more about such events and the geological changes that led up to them, they see alarming Parallels with the warming world of today: history could be readying to repeat itself

"Episodes of anoxia. Known as oceanic anoxic events (OAEs), nave occurred periodically during Earth's history, but none was more severe than that which occurred 93 [million years] ago, during the Cretaceous period,' wrote Timothy J Bralower of the Department of Geosciences at Pennsylvania State University in a 2008 article for Nature. 'This OAE caused the extinction of Barge cRams Known as inoceramids and tiny protists called Foraminifera that lived on the sea floor. Profound changes r ocean circulation also led to the production and preservation of enormous quantities of marine organic material that was subsequently transformed into oil during its burial.'

In the same issue of Nature, scientists also speculated on the mechanism by which the volcanic activity of the time might nave caused this strange anoxic environment. 'One possibility is that the volcanism seeded me upper ocean with metal micronutrients, increasing phytoplankton production which in turn led to increased oxygen use during the decay of organic matter. Another, not mutually exclusive, possibility is that a consequence of the global warming stemming from volcanically produced C[O.sub.2] was a more stratified ocean, in which oxygen delivery to deep waters became restricted. …

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