Vast volcanic blasts that cause global devastation occur on average every 50,000 years — and, as the last one struck 74,000 years ago, at Toba, Indonesia, another may be overdue.

The scale of such a cataclysm would dwarf that of the recent Asian tsunami: the eruption could kill millions and the final death toll could reach a billion as dust thrown into the atmosphere triggers a natural “nuclear winter”.

Among natural disasters, only the impact of an asteroid a kilometre (0.6 miles) or more across would be comparable, a new report from the Geological Society has found. Asteroids big enough to cause global effects strike at intervals of 400,000 to 500,000 years. Super-eruptions happen about ten times more frequently.

The new report, presented to the Government’s Natural Hazard Working Group, was published as the BBC prepares to screen Supervolcano, a two-part factual drama that charts the effects of a super-eruption at Yellowstone National Park in the United States.

Yellowstone has produced three super-eruptions in the past — 2.1 million, 1.3 million and 640,000 years ago — the first being the second-largest known to science. A similar event today would lay waste to most of the continental US.

In the BBC film, which strictly follows scenarios presented by expert scientific advisers, the eruption causes more than 25 million deaths in the first week alone. Eighty per cent of the US is covered in volcanic ash and 20 per cent, including most of the rich agricultural lands of the Great Plains, becomes uninhabitable.

Emissions of ash and sulphur dioxide bring global temperatures down by between 5C and 15C (9F and 27F), leading to the failure of the Asian monsoon and millions more deaths from famine.

Steve Sparks, of the University of Bristol, a lead author of the report and an adviser to the BBC film-makers, said that governments needed to make contingency plans. “This is not just a scientific curiosity,” he said. “These events are rare on a human timescale, but in geological time they are common. The issues involved are similar to preparing for a nuclear war. Countries will have to make plans for food, shelters and evacuation. These sorts of events are extremely rare, and would require enormous investment of resources if we are to have any hope of coping.”

The dangers from a super-eruption are also greater than those from asteroids as there is no conceivable method of preventing the event.

“Humankind might develop the capability to deflect an asteroid, but we will never develop a way of averting a super-volcano,” Stephen Self, of the Open University, another of the report’s authors, said. “Damage-limitation is the only way forward.”

Super-eruptions are different in scale from ordinary eruptions: the first Yellowstone event ejected 2,500 times more gas and molten rock into the atmosphere than the Mount St Helens eruption of 1980.

None has taken place during recorded human history, though the Toba eruption may have come close to driving early Homo sapiens to extinction. The human population is thought to have dwindled to a few thousand soon after Toba, and some scientists think that climate change provoked by the volcano was responsible.

About 40 supervolcano sites are known, but most are extinct. Yellowstone is the site with the greatest lethal potential because of its position on a heavily populated continent.

Supervolcano will be shown on BBC1 at 9pm on Sunday, March 13 and Monday, March 14.

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