The doomsday

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What happened after an asteroid slammed into Earth? There is no shortage of frightening scenarios on how an impact would turn Earth's climate into a killer.

Cold, wet and windy

Within 45 minutes of the impact, a vapour-rich plume of debris would envelop the Earth. There would be enough dust - consisting of roughly equal parts of materials from the asteroid and the Earth's crust - to cause darkness around the world. Scientists have estimated that the Chicxulub impact would have injected 50,000 cubic kilometres of dust in the atmosphere, which settled to form a layer averaging 3 millimetres thick. Without sunlight, photosynthesis would stop. Food chains everywhere would collapse. The darkness would also produce extremely cold temperatures. Scientists call this condition 'impact winter'.

An impact winter can be compared with the scenarios of a 'nuclear winter'. According to these, the explosion of a large number of nuclear missiles would throw huge amounts of dust and smoke into the upper atmosphere, where it would stay for long periods. The result would be darkened skies and lower temperatures for months. In these scenarios, nuclear radiation plays a small part; much more important are the injection of dust and smoke into the upper atmosphere and the subsequent effects on climate.

After a detailed study of plant fossils from 65 million years ago in what is now Wyoming, the American botanist Jack Wolfe has concluded that there was definitely a sudden mass freezing. Fossils all show that the plants are shrivelled, suggesting frost damage. His studies of past climates have even led him to predict when the asteroid hit Earth - on a day in early June. 'Hogwash', says fellow botanist Leo Hickey of Wolfe's findings. Hickey's verdict is based on the study of thousands of leaves from rock layers before and after the dinosaurs' death.

Some experts say that the long winter did not kill the dinosaurs. The discovery of dinosaur fossils in Alaska and southeastern Australia (which was closer to the South Pole 65 million years ago) suggests that dinosaurs could survive many weeks of total darkness. If this is correct, it challenges the argument that darkness and cold caused by an asteroid impact wiped out the dinosaurs.

A massive impact could start earthquakes hundreds of times bigger than the largest one recorded. If the asteroid hit an ocean it would produce immense tsunamis, fast-moving waves higher than skyscrapers which retain their destructive energy while travelling enormous distances. These tsunamis would drown all land areas except mountain ranges. The poor dinosaurs never learned to swim.

The impact might produce winds reaching 1,080 kilometres per hour, according to the American climatolo-gist Kerry Emanuel. These winds could throw huge amounts of dust into the upper atmosphere, changing the climate and destroying the ozone layer. Like many other scenarios, Emanuel's is based on a computer model. Emanuel believes that the Chicxulub impact could easily have caused a catastrophic storm. The immense amount of energy released in the collision would have made the crater extremely hot. Sea water rushing back to cover the new crater would have been heated in turn and would have driven the formation of storms. Emanuel calls these storms 'hypercanes' because they would cause much more damage than normal hurricanes.

Hot, fiery and pungent

If the asteroid hit a limestone rock it would vaporise it. On heating, limestone produces carbon dioxide. As a result the atmosphere would be filled with massive quantities of this gas. The carbon dioxide would trap heat, creating a greenhouse effect with lethal temperatures. To test this theory, two American scientists, John O'Keefe and Thomas Ahrens, shot steel balls from a cannon into limestone rocks at 7,200 kilometres per hour and measured the amount of carbon dioxide released by the impact. They calculated that if a 10-kilometre asteroid were to slam into limestone, it would double the amount of carbon dioxide in the whole atmosphere overnight.

Testing samples of the K-T boundary clay from around the world, in 1985 the American chemist Wendy Wolbach and her colleagues found high levels of carbon, like the soot in the flame of a candle. She believes that globally 70 billion tonnes of soot - the ash of the dinosaurs' world - came from wildfires that started after the impact. The force of the impact created an enormous fireball that spread out, igniting forest fires from North America to Asia. The resulting winds dispersed the soot worldwide, which absorbed the sunlight and thus blocked plant photosynthesis.

Wildfires also created toxic gases such as carbon dioxide, carbon monoxide and methane, which harmed most land life. One estimate shows that the amount of carbon dioxide (10,000 billion tonnes), carbon monoxide

(100 billion tonnes) and methane (100 billion tonnes) released by wildfires was equivalent to 3,000 years of modern fossil-fuel burning. The fires died in the long winter after the firestorms.

Rocks around the Chicxulub crater contain large quantities of sulphur. This has led some scientists to theorise that the blast vaporised the sulphur and spewed more than 90 billion tonnes into the air, where it mixed with moisture to form tiny drops of sulphuric acid. These drops covered the planet like a blanket, blocking sunlight. The blanket remained for decades, pushing the temperatures to near freezing.

Post-impact environment

Scenarios prepared by the American scientists David Kring and Daniel Durda in 2003 show that the postimpact world 'looked, smelled and even sounded different'. Life's diversity saved it from complete extinction, but the new environment was less diverse. Within a year, ferns and algae recovered. After 50 years, shrubs took advantage of the vacant landscape and began to cover it. Trees also began to recover. Re-growth took at least 100 years. Some scientists argue that the process was, in fact, far slower, taking thousands of years; and it took millions of years for life in the oceans to return to normal. 'The impact opened ecological niches for mammalian evolution, which eventually led to the development of our own species', Kring and Durda write in Scientific American. 'In this sense, the Chicxulub crater is the crucible of human evolution.'

Those who do not believe in the idea of an asteroid causing the demise of the dinosaurs argue that the effects of the impact would be limited to a small region only, and could not cause worldwide devastation.

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