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SYDNEY: It's normal for a species to go extinct, and an average rate of one a year is the natural background rate. But over the past 4.5 billion years, there have been times when extinctions occurred at 100 to 1,000 times faster - with the largest event wiping out 95 % of all species. Somewhere between five and 20 such mass extinctions have occurred. Here are 10 possible causes for future extinction events.

IMPACT FROM SPACE
An asteroid is the leading contender for the cause of the most famous mass extinction, the K-T event, which killed off most dinosaurs 65 million years ago. Mexico's Chicxulub Crater, now buried under the Yucatan Peninsula, may once have been 280 kilometres across and 56 kilometres deep. That gives a sense of scale of the impact, which wiped out perhaps 50% of all species. The force of the explosion would have killed off everything within 500 km, but smoke and ash would have clogged the planet's atmosphere, cutting out sunlight and blocking photosynthesis for many months, causing the collapse of food chains.

When will this happen again? Palaeontologist David Raup estimated that extinction-sized strikes happen, on average, once every 100 million years.

SUPERVOLCANO
Supervolcanoes are giant eruptions of lava that can last a million years and cover a million km2 of the surface. Supervolcanoes ooze lava from long fractures in the Earth's crust and give off trillions of tonnes of carbon dioxide, sulphur dioxide, fluorine and chlorine. This pollution on a would cause erratic fluctuations in temperature and acid rain ona global scale. Supervolcanoes have been cited as possible causes for mass extinction events at the end of the Permian, Triassic, and Cretaceous periods.

When will this happen again? With an average of one every eight to ten million years, we're not due another for several million, says Paul Wignall of the University of Leeds. In any case, distortion of the Earth's crust should give 10,000 years notice, he says.

MAGNETIC FIELD REVERSAL
A supervolcano could also cause the Earth's magnetic field to reverse, so that the magnetic South Pole becomes the North Pole, and vice versa, something that has happened numerous times throughout history. During the flipping process, Earth's magnetic field is weakened, exposing the planet to cosmic rays. As well as causing mutations and cancers, this could increase clud cover, cooling the climate. Supervolcano activity may have that caused the break-up of the supercontinent Pangea and flipped the field repeatedly during the late Permian, triggering the first phase of the Great Dying.

When will this happen again? Andrew Biggins, of the University of Utrecht in the Netherlands, says reversals have happened around once every 400,000 years in recent history. The last was 800,000 years ago, so by that count we're long overdue.

GAMMA-RAY BURST
Gamma-ray bursts are the most violent and energetic explosions in the universe, caused when a star collapses to form a black hole (see "Death star" p70). They're short, but deadly - if the Earth were caught in just a 10-second burst from a black hole in the Milky Way, half the ozone layer could be destroyed. That would lead to increased UV radiation, causing cancer and killing off much surface life. A longer burst, or one from a nearby star, would fry species directly, burning their skin and dicing their DNA. Experts speculate that the end-Ordovician event, which destroyed more than 50 per cent of species, may have been caused by gamma rays.

When will this happen again? NASA predicts that bursts close enough to damage Earth happen around once every billion years, and the last was 500 million years ago.