It's the subject of a terrifying new TV drama. But could the supervolcano nightmare come true?

Deep below Yellowstone National Park in Wyoming lies a slumbering giant that could bring the world to an end when it finally awakes. Fact or fiction? Science Editor Steve Connor reports

When the supervolcano that lies beneath Yellowstone National Park in Wyoming finally awakes from its 640,000-year slumber, it will spew out enough ash and magma to change the world as we know it. This is the prediction of scientists who have calculated that the global risk posed by a supervolcanic eruption somewhere in the world is between five and ten times greater than the probability of being struck by a giant asteroid.

When the supervolcano that lies beneath Yellowstone National Park in Wyoming finally awakes from its 640,000-year slumber, it will spew out enough ash and magma to change the world as we know it. This is the prediction of scientists who have calculated that the global risk posed by a supervolcanic eruption somewhere in the world is between five and ten times greater than the probability of being struck by a giant asteroid.

But it is the huge lake of molten magma lying dormant under the lush landscape of Yellowstone that is causing the greatest concern to vulcanologists studying the special threat posed by supervolcanoes.

Earth scientists commissioned by the Geological Society of London have calculated that there may be several super-eruptions big enough to cause a global disaster every 100,000 years - whereas an asteroid larger than 1km (0.62 miles) in diameter would be expected to hit the Earth once in about 600,000 years.

Supervolcanoes may not look much - most do not even have the traditional cone of a Vesuvius or a Mount St Helens - but their potential for destruction is many times greater than a traditional volcanic eruption.

A super-eruption at Yellowstone would be far more devastating for the world than the eruptions at Tambora in 1815, Krakatoa in 1883 and Pinatubo in 1991 which all caused global climate disturbances for several years after the event. Super-eruptions are hundreds of times larger than the

biggest volcanic explosions of recorded history and their effects on the global climate are much more severe, said Professor Stephen Self, a vulcanologist at the Open University.

"An area the size of North America can be devastated and pronounced deterioration of global climate would be expected for a few years following the eruption," Professor Self explained. "They could result in the devastation of world agriculture, severe disruption of food supplies and mass starvation. These effects could be sufficiently severe to threaten the fabric of civilisation."

A two-part drama-documentary - Supervolcano - to be transmitted this Sunday and Monday on BBC1 spells out what could happen if the supervolcano under Yellowstone National Park should erupt in the near future. The programme makers worked closely with volcano specialists, including scientists at the US Geological Survey, who are closely monitoring Yellowstone, to depict the most realistic scenario leading up to and in the immediate aftermath of a massive eruption.

It shows what would happen if some 2,000 million tons of sulphuric acid were ejected into the atmosphere to block out sunlight over much of the planet causing global temperatures to plummet by between 10C and 20C.

It also describes the chaos and panic caused by the dumping of billions of tons of volcanic ash over huge swaths of North America. Scientists calculate that it would be equivalent to covering an area the size of Britain in four metres of ash.

Ailsa Orr, the series producer, said the film-makers consulted the US Federal Emergency Management Agency (Fema), which handled the aftermath of the 11 September terror attack on New York and Washington, and found there was little planning for such a natural disaster.

"Fema had no contingency plans for a disaster on this scale. The largest disaster they ever had to deal with was 9/11 and that stretched their resources to the limit," Ms Orr said.

"Our scenario would affect an area 10 million times greater than 9/11 did. Fema were extremely interested in working with us to come up with a theoretical plan as to how they might deal with it. They gave us data on how many people would be affected by the eruption in the US."

Satellite images show that the mouth or caldera of the Yellowstone supervolcano is 85km (53 miles) long and 45km (28 miles) wide - which amounts to an area big enough to swallow Tokyo, the largest city in the world.

Five miles underneath the surface of Yellowstone sits the volcanic chamber itself which is estimated to hold 25,000 cubic kilometres of molten rock or magma. Seismologists and vulcanologists working for the Yellowstone Volcano Observatory routinely monitor the regular swellings and subsidences of the land as it responds to shifting underground lake of molten rock below.

Ms Orr said that the makers of the drama-documentary liaised closely with the scientists at the observatory as well as other specialists and consultants. "We started by examining data from the first super-eruption of Yellowstone which happened 2.1 million years ago. We also looked at the evidence of the last supervolcanic eruption on the planet which happened at Toba in Indonesia 74,000 years ago," she added.

Some scientists believe that the Toba eruption, which caused global climatic disturbances, may have even caused a genetic "bottleneck" in human genetic diversity following a dramatic decline in the global population. If the Yellowstone supervolcano were to erupt in a similar fashion the ash that it would spew out would cover three-quarters of North America in a layer deep enough to kill crops and other plants.

Few people would survive in the zone immediately around the eruption as the volcanic gases and choking sulphur dioxide would burn the lungs of anyone caught in the open air. Those sheltering in their homes would not be safe because layers of heavy volcanic ash would eventually cause their roofs to collapse.

The supervolcanic eruption of the Toba volcano in Sumatra ejected about 300 times more volcanic ash than the eruption of Tambora in Indonesia in 1815 - which caused a "year without a summer" in 1816 and prompted Lord Byron to write his poem "Darkness".

A report on supervolcanoes compiled by the Geological Society states: "It is easy to imagine that an eruption on the scale of Toba would have devastating global effects. A layer of ash estimated at 15 cm thick fell over the entire Indian subcontinent with similar amounts over much of south-east Asia. Most recently, the Toba ash has been found in the South China Sea, implying that several centimetres also covered southern China.

"Just one centimetre of ash is enough to devastate agricultural activity ... Many millions of lives throughout most of Asia would be threatened if Toba erupted today," it says.

Ms Orr said the University of Utah and the UK Met Office had helped to compile a map of the fallout that might result from the eruption of ash from the Yellowstone supervolcano.

"From this, we created an ash projection map which took into account wind direction and time of year of our eruption. Every time we refined our storyline we would send it back to them for approval so they were closely involved," she said.

But it is the emission of sulphuric acid into the atmosphere that would create the greatest long-term problems for countries further afield, as the biggest volcanic eruptions of the past 200 years have shown, warns Professor Steve Sparks of Bristol University, a consultant to the programme. "They caused major climatic anomalies in the two or three years after the eruption by creating a cloud of sulphuric acid droplets in the upper atmosphere. These droplets reflect and absorb sunlight, and absorb heat from the Earth - warming the upper atmosphere and cooling the lower atmosphere," Professor Sparks said.

"The global climate system is disturbed, resulting in pronounced, anomalous warming and cooling of different parts of the Earth at different times."

If enough sulphuric acid were released - and Yellowstone could emit 2,000 million tons - then what could take place would be the equivalent of a "nuclear winter", when the dust and debris from the fallout of a nuclear war block out sunlight for several years causing worldwide famines.

The Max Planck Institute in Hamburg helped the makers of Supervolcano to model the spread of sulphuric acid around the world.

"We're talking about catastrophic amounts of sulphuric acid circling the world within just a few weeks. It forms a veil that blocks out sunlight, causing temperatures to plummet," Ms Orr said.

"The Met Office models predicted a drop of about 15C across Europe and 20C in the southern hemisphere, the monsoon would stop, crops would fail and somewhere in the region of one billion people would die through climate change and starvation," she added.

Supervolcano depicts the Yellowstone caldera erupting over several days, progressively "unzipping" the build-up of underground pressure in a series of eruptions around the rim of the crater rather than releasing everything all at once in one giant eruption.

Ms Orr said: "The first thing we had to get right was to understand the dynamics of a super-volcanic eruption - how it would unfold, what it would look like. It's very difficult to know for sure because nobody has ever seen a super-eruption happen but we consulted with a lot of scientists and the consensus of opinion was that a super-eruption is not just one big massive eruption but a series of separate eruptions around the rim of the caldera.

"Only towards the end of the eruption process do they all converge into one. Once this scenario had been signed off by the scientists, we got a storyboard artist to visualise it so everyone was clear on what we had to create in the film."

Nobody knows whether a supervolcanic eruption at Yellowstone is imminent. The programme-makers say at the start of their film that they have not made fiction, and they have made a true story - it's just that it hasn't happened yet.

The Yellowstone supervolcano is know to have erupted three times in the past 2.1 million years at a regularity of about 600,000 years. The last one happened 640,000 years ago.

Yet vulcanologists such as Professor Sparks point out that this does not mean that another eruption is overdue. "It doesn't work like that. We just don't know when the next eruption will occur," he said.

Neither do scientists know how much warning the world will be given. "Frankly we don't really know, that's the real problem," Professor Sparks said.

But what we do know is that we are ill-prepared for such an event if it should take place in the near future. "You can't stop it. One could have to start to think about the strategies for dealing with consequences and to be frank, that's not been thought through at all," Professor Sparks said.

One thing remains certain in this uncertain world of low-risk, high-impact disasters. If the Yellowstone supervolcano should ever blow, our world will never be the same again, and might not even survive in its present form.

HOW THE RISKS MEASURE UP

According to scientists, the risk of a super-eruption somewhere in the world is five to 10 times greater than that of the world being hit by an asteroid. How does that compare with the other dangers we face daily?

Lightning

The chances of being killed by being struck by lightning is thought to be about one in 10 million. In the UK, an estimated five people, out of a population of about 50 million, are killed by lightning each year.

Air crash

The chances of being involved in an aircraft accident are about one in 11 million, while the chances of being killed in a car accident is one in 8,000.

Train crash

The risk of dying in an accident on the railway is one in 500,000.

Nuclear accident

The risk of an individual dying from radiation from a nuclear power station is one in 10 million.

Dangerous jobs

The occupational risk of being killed in deep-sea fishing is one in 750. With coal mining, it is one in 7,500. In construction, this increases to one in 10,000, and for the service industries, it is one in 150,000.

Playing football

The risk of dying while playing a game of football is one in 25,000.

Asteroid collision

Some scientists believe that an asteroid spotted in January 2004 had a one-in-four chance of hitting the planet within 36 hours. Researchers contemplated a call to President George Bush before new data finally showed there was no danger. The bookmakers William Hill, meanwhile, said the odds of the asteroid hitting Earth on March 2014 and wiping life off the planet was 909,000 to one.

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