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Ash
US television networks would probably bring the first news of a Yellowstone super-eruption to the UK. It probably wouldn't take long for the first physical signs to appear.

River of fire: Lava streams from the Hawaiian volcano Kilauea in 1959. Although it may look bad, geologists consider such eruptions to be 'quiet'. Explosive eruptions like supervolcanoes pose a much greater threat to humans.

The model shows that the fallout from a Yellowstone super-eruption could affect three quarters of the US. The greatest danger would be within 1,000 km of the blast where 90 per cent of people could be killed. Large numbers of people would die across the country – inhaled ash forms a cement-like mixture in human lungs. Even the US East Coast could be paralysed by 1cm of ash.

Many people think that lava flows are the most dangerous volcanic hazards, but ash is often the biggest killer. Because supervolcanoes are highly explosive, much of the magma doesn't get a chance to become lava. Instead it is blasted into countless airborne ash particles – tiny scorching particles of jagged rock.

Ash can:

• Kill and sicken humans and animals
• Reduce sunlight
• Trigger rainfall causing mudslides known as lahars
• Severely disrupt air, road and rail transport
• Crush buildings – 30 cm of dry ash is enough to collapse a roof
• Contaminate water supplies
• Kill crops and other vegetation
• Clog machinery such as air filters.

The worst of these effects would not be experienced in Europe where the ash covering would only amount to a dusting.

	Useful stuff: A Washington State resident sweeps ash from his roof after the eruption of Mount St Helens. Ash creates numerous hazards, but it also has positive uses. Fertilisers, soaps, household cleaners, metal polish and cement often contain volcanic ash.

Volcanoes can inject sulphur gas into the upper atmosphere, forming sulphuric acid aerosols that rapidly spread around the globe. Scientists believe sulphuric aerosols are the main cause of climatic cooling after an eruption.

Aerosols in the upper atmosphere would also scatter sunlight making the sky look like a cloudy winter morning all day long. The skies in Europe would appear red in the days after the eruption.

To predict how the climate may be affected, the BBC relied on historic data from the Toba supervolcano in Indonesia about 74,000 years ago and computer model forecasts commissioned from the UK Met Office and the Max Planck Institute in Hamburg.

Experts believe a Yellowstone eruption would inject 2,000 million tonnes of sulphur 40-50km above the Earth's surface. Once there it would take 2-3 weeks for the resulting sulphuric acid aerosols to cloak the globe – with devastating effects.

Global annual average temperatures would drop by up to 10 degrees, according to computer predictions. And the Northern Hemisphere could cool by up to 12 degrees. Experts say colder temperatures could last 6-10 years, gradually returning to normal.

Scientists predict that the Monsoon would fail as a result of even larger temperature changes in the Southern Hemisphere, causing mass starvation in the Asian countries that depend on these life-giving rains.

Temperatures in Europe could be at least 5 degrees cooler the summer after the eruption.

The actual effects of a Yellowstone super-eruption could be different depending on the size of eruption, the time of year and any number of other factors.

How likely is all of this?
An event as massive as a super-eruption would change the Earth and our society forever. It's difficult to predict the full devastation that would follow. We know there would be great loss of life and ill health, changes to our planet and major economic losses.

Scientists believe another super-eruption will happen someday. They can't completely discount a Yellowstone eruption in the near future, but the chances of this happening are extremely remote.

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