By Paul Rincon
BBC News science reporter
Geologists have called for a taskforce to be set up to consider emergency management in the event of a massive volcanic eruption, or super-eruption.
The recommendation comes in a report timed to coincide with a BBC TV drama that depicts a fictional super-eruption at Yellowstone Park in Wyoming, US.
Experts say such an event would have a colossal impact on a global scale.
A super-eruption is also five to 10 times more likely to happen than an asteroid impact, the report claims.
The authors want to highlight the issue, which they feel is being ignored by governments. They emphasise that while catastrophic eruptions of this kind are rare in terms of a human lifetime, they are surprisingly common on a geological scale.
The effects, say the authors, "could be sufficiently severe to threaten the fabric of civilisation" - putting events such as the Asian tsunami into the shade.
The fallout from a super-eruption could cause a "volcanic winter", devastating global agriculture and causing mass starvation.
It would have a similar effect to a 1.5km-diameter space rock striking Earth, they claim.
But while impacts of this type are estimated to occur once every 400-500,000 years, the frequency of equivalent super-eruptions is about once every 100,000 years.
"These are minimum estimates. Super-eruptions could be even more frequent; we just don't know," said Professor Stephen Self, a geologist at the Open University in Milton Keynes and a member of the working group that produced the report.
The Mount Pinatubo eruption was the biggest recorded in photos
"We still have a lot of unassessed regions of the world. The US is the place where we see the largest number of super-eruptions. But that may be because more work has been done there."
One past super-eruption struck at Toba in Sumatra 74,000 years ago and is thought by some to have driven the human race to the edge of extinction. Signs from DNA suggest human numbers could have dropped to about 10,000, probably as a result of the effects of climate change.
The TV drama, called Supervolcano, sticks closely to scientific understanding of these events.
The plot revolves around a series of violent eruptions at Yellowstone in Wyoming that send thousands of cubic kilometres of rock, gas and ash spiralling up in cloud that rains down over three-quarters of the United States.
Highways become blocked with cars as millions flee the unfolding disaster, and as the chain of eruptions unzips Yellowstone's volcanic crater, hundreds of thousands are killed as the ash swamps whole towns and cities.
America's food-producing regions are devastated, communications are knocked out and planes are forced out of the sky.
Yellowstone is the largest volcanic system in North America
Sulphuric acid droplets form in the atmosphere, blocking out sunlight, and causing global temperatures to plummet.
Professor Stephen Sparks, of Bristol University, an author on the new report, said civil contingency plans would need to be similar to those for a nuclear war.
"You would need contingencies for food and shelter. But you would need to put a serious amount of resources into any effort to cope with an event on this scale, so it poses a dilemma," he said.
The volcanic winter resulting from a super-eruption could last several years or decades, depending on the scale of an eruption, and according to recent computer models, could cause cooling on a global scale of 5-10C.
Ailsa Orr, producer of Supervolcano, said that when the programme team presented the scenario to the US Federal Emergency Management Agency (Fema), the agency admitted it had given little thought to such an event happening on American soil.
"We don't want to be sensationalist about this, but it's going to happen. We just can't say exactly when," said Professor Self.
"But we have just had a natural disaster affecting hundreds of thousands of people. Now is the time to be thinking about this."
Yellowstone is the largest volcanic system in North America. The area's cauldrons of bubbling mud and roaring geysers attract nearly three million visitors each year.
It was an obvious choice for the programme makers as the site of their super-eruption because of its location on a highly populated continent and because it has already had three of these events, which have occurred roughly 600,000 years apart from each other.
The crater from the last super-eruption, 640,000 years ago, is large enough to fit Tokyo - the world's biggest city - inside it.
The report, released by The Geological Society in the UK, identifies at least 31 sites where super-eruptions have occurred in the past. They include Lake Taupo in New Zealand and the Phlegrean Fields near Naples, Italy.
The drama Supervolcano is broadcast in two parts, on BBC One on Sunday 13 March and Monday 14 March. Both transmissions are at 2100 GMT. Two science documentaries called Supervolcano: The Truth About Yellowstone are broadcast after the drama, on BBC Two. Again, these air on Sunday and Monday but at the later time of 2200 GMT