Although the eruption of a supervolcano would have a catastrophic effect on global climate, they are not capable of wiping out the entire human race. What's more, survivors would very likely include residents of the UK, according to Dr Morgan Jones of the National Oceanography Centre in Southampton.
24 September 2009
Alongside global warming, a more dramatic threat to global climate is on the horizon. The eruption of a supervolcano - somewhat arbitrarily defined as an eruption whose volume exceeds 1000 km
2 – has not occured in recorded history, but there are six known ‘supervolcanoes’ on Earth, three of them in the United States. When compared to volcanic eruptions which have occurred in recent memory, these are on a colossal scale. The eruption of Mount St Helens in 1980, for example, measured a meagre 1.2 km
2.
Ejecting material into the stratosphere at speeds of 100 – 500 metres per second, such large eruptions even have the potential to go supersonic. Historical examples include an eruption at Lake Toba approximately 74,000 years ago, which some scientists believe brought about a ‘Millenial Ice Age’ which killed around 60 to 75% of the human population.
The good news, Dr Morgan Jones told the ESTA conference last week, is that super eruptions ‘cannot cause global mass extinctions, purely because they happen too often’. Furthermore, the UK may be ‘perfectly placed to survive one of these catastrophic events’. Although the ash erupting from a volcano reflects sunlight, bringing about the well known ‘volcanic winter’ phenomenon, latent heat preserved by the oceans during summer months is transferred to the atmosphere by increased winds. This carries warmer air onto the land, producing a ‘winter warming’ effect on the western side of the northern hemisphere continents, which could mean that the UK experiences a warming effect whilst other places undergo cooling.
Supervolcanoes are not to be underestimated, however. Jones and his colleagues used computer modelling, as well as satellite images from more recent eruptions, to map the potential effects of a super eruption. If the ash cloud from a volcanic eruption is large enough, it is affected by the Earth’s rotation as it begins to spread out, producing a clockwise ‘spin’ in the Northern hemisphere, and the opposite in the Southern hemisphere. Ash clouds from a super eruption can be in the magnitude of 3000 km in diameter or more, making this effect even more pronounced. Ash is transported across both hemispheres and can spread out worldwide. Deposits have been collected from the eruption at Lake Toba covering an area including “most of the Indian Ocean, a lot of Indonesia, all of Siam and the Indian subcontinent”.
What effect could such a large eruption have on the global climate?
“It’s a little bit of a silly question, because over geological time volcanoes made the atmosphere and oceans” says Jones. That means that the major products of an eruption – nitrogen, carbon dioxide and water – have very little effect, being dwarfed by the amount which is already in the oceans and atmospheres. Other products, though less significant in terms of volume, have a much greater impact.
“The most important from a climatic point of view is the injection of sulphur into the atmosphere”. Sulphur reacts with water to make sulphate acid, an aerosol whose particles are perfectly sized to scatter sunlight. In the stratosphere, it scatters radiation from the sun, reducing the amount of sunlight which reaches the earth. This results in what has become known as a ‘volcanic winter’. With a large eruption the aerosols can remain in the atmosphere for one to three years. Within a matter of weeks, they can spread around the globe, reaching the poles in a matter of years. The resulting lack of sunlight has a catastrophic effect on biota on land and in the seas, bringing about famines and disrupting weather patterns.
Jones also spoke about some of the less well known effects of large volcanic eruptions. Sulphate aerosols, for example, absorb infra red light, which might shed some light on the origins of the phrase 'once in a blue moon'. They have also inspired artists from various disciplines. The eruption of Krakatoa in 1883 has been held responsible for the red tinge of the sky in Edvard Munch's
The Scream, and many of Turner's paintings reflect a skyline affected by volcanic activity.
Most famous, however, is the response to the eruption of Mount Tambora in 1815, linked to a period known as 'The year without a summer'. In Switzerland, a group of English holiday makers were forced to stay indoors due to the cold and dreary weather, and told ghost stories to pass the time. Mary Shelley's
Frankenstein was the result, as was Lord Byron's poem,
The Darkness, which gives some idea of what a super eruption might leave behind it:
"I had a dream, which was not all a dream.
The bright sun was extinguish'd, and the stars
Did wander darkling in the eternal space,
Rayless, and pathless, and the icy earth
Swung blind and blackening in the moonless air;
Morn came and went--and came, and brought no day,
And men forgot their passions in the dread
Of this their desolation; and all hearts
Were chill'd into a selfish prayer for light:...."