STOP worrying about Earth-bound asteroids 鈥� save your sleepless nights for massive volcanic eruptions. They pose twice as much of a threat to civilisation as impacts from outer space.
Cosmic bodies with a diameter of more than 1 kilometre hit the Earth once every 100,000 years or so. On today鈥檚 crowded planet, the dust storms and wildfires created by such impacts would wipe out over a billion people. But super-eruptions capable of wreaking this kind of havoc happen around once every 50,000 years, says earth scientist Michael Rampino of New York University.
And while governments are waking up to the asteroid threat and considering defence plans, no one even knows how to predict the next super-eruption. Rampino鈥檚 warnings are based on investigations of the geological remnants left by previous super-eruptions. Volcanoes in Yellowstone Park and Long Valley in California have erupted three times in the past 2 million years, each time coating the whole of the US with ash. But the biggest and most recent super-eruption happened at Toba, on the island of Sumatra, 73,000 years ago.
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Toba blasted a crater 100 kilometres long, spewing dust and ash 40 kilometres into the air, and releasing 3 billion tonnes of sulphur dioxide 鈥� equivalent to a hundred Pinatubos erupting at once. Sulphur dioxide in the atmosphere is converted into aerosols of sulphuric acid, which wash out in rain over several years. In cores taken from Greenland鈥檚 ice sheet, the largest peak in sulphuric acid coincides with the Toba eruption and lasts for six years. That shows Toba鈥檚 dense volcanic cloud spread right around the globe, says Rampino. He also suspects that Toba鈥檚 super-eruption was responsible for the population crash 70,000 years ago, when the number of people fell to no more than 10,000.
The big problem for survivors of the immediate fallout from a super-eruption would be the catastrophic drop in global temperatures, caused by ash and aerosols blocking the Sun. Eruptions at Krakatoa in 1883 and Pinatubo in 1991 each pushed temperatures down by several tenths of a degree for a few years. By Rampino鈥檚 calculations, another Toba would drive them down by 3 to 5 掳C for a decade or more, and trigger regional cooling of 15 掳C.
鈥淭hat鈥檚 going to kill off most of the above ground vegetation in Africa,鈥� he says. Globally, the amount of vegetation would be cut by 25 per cent. 鈥淚t鈥檚 chilling,鈥� agrees palaeontologist Simon Conway Morris of Cambridge University. 鈥淎ll we can do is keep our fingers crossed and hope that by the [next super-eruption] we have a robust social system based on justice and fairness, and the beginnings of an interstellar colonisation programme.鈥�
Super-eruptions are also bad news for SETI. The search for alien intelligence relies on the idea that distant civilisations would survive long enough to develop the technology needed to communicate across the vast reaches of space. Volcanoes are a source of essential gases such as carbon dioxide and water vapour, so inhabited planets are likely to be volcanically active, and therefore susceptible to the occasional civilisation-busting super-eruption.
