If you were one of the 10 million air travellers shaking your fist at the departures board in April 2010, you will appreciate the fact that – even in this tectonically peaceful realm – we ignore volcanic threats at our peril. Despite being nothing to write home about in terms of size, the eruption from Iceland’s Eyjafjallajökull volcano brought air travel chaos to the UK and mainland Europe when its ash cloud grounded an astonishing 107,000 flights in the biggest air traffic shutdown since the second world war. The eight days of mayhem brought airline CEOs to the point of apoplexy and, once the ash had settled, the air travel business was left with a €1.3bn bill. The threat posed by Icelandic eruptions has since been recognised and added, retrospectively, to the UK’s National Risk Register, in the hope that next time we will be better prepared. But what about volcanic explosions further afield? These, it appears, are still regarded as posing no threat to our country, and so can be safely ignored. But turn the clock back 200 years and there is at least one event that suggests we ought to think twice.  In April 1815, the biggest known eruption of the historical period blew apart the Tambora volcano, on the Indonesian island of Sumbawa, 12,000km from the UK. What happened next testifies to the enormous reach of the biggest volcanic blasts.  The Tambora volcano had shown no signs of life for 1,000 years; a single eruption in the previous five millennia provided the only indication that magma was still churning far beneath. It is very likely that the residents of the island considered the volcano extinct, and possible even that they did not know the impressive 4,300m (14,107ft) mountain – at the time, probably the highest in the East Indies – was a volcano at all. This all changed, however, with the rumblings and earthquakes of 1812, a full three years before the climactic blast. Over time, the seismic shocks were superseded by steam blasts and small ash explosions, engendering increasing trepidation on the island and signalling that something bigger might be imminent. It was. On 5 April 1815, a titanic explosion hurled a cloud of ash to a height of more than 30km. Violent, but short-lived, the blast lasted just two hours, after which the volcano returned to a state of brooding menace. According to the lieutenant governor, Thomas Stamford (later Sir Stamford) Bingley Raffles, to whom volcanologists are indebted for his accounts of the eruption, the detonation was so loud that it was mistaken across Java for cannon fire, causing consternation among the British troops, which had ousted the Dutch and French forces just a few years earlier.  But the blast was small beer in comparison with what followed. After five days of relative calm, the climactic phase of the eruption began with a colossal explosion that launched a towering column of ash to the edge of space. For four or five days, utter blackness reigned across the island as the hurricane blasts of hot ash and scalding gas – known as pyroclastic flows – scoured the flanks of the volcano of everything and everyone, and drifts of ash metres thick entombed what few signs of life remained. When the explosions ceased and the darkness finally lifted, the view revealed was a vision of Tolkien’s Mordor; a grey landscape within which nothing lived or moved. The top 500m of the volcano was gone, blasted into smithereens, and replaced by a 6km-wide maw from which steam spiralled skywards. Communities on the flanks of the volcano had vanished, along with the lives of around 12,000 men, women and children. These, perhaps, were the lucky ones, as a further 60,000 survivors of the eruption succumbed slowly and agonisingly to famine or disease.  But the consequences were not confined to this Indonesian backwater. The explosion was heard 2,600km away in Sumatra, while giant rafts of floating pumice – some kilometres in length – clogged shipping routes for years. The 50 cubic kilometres or so of ash ejected over the course of the eruption returned to earth in the following days and weeks, leaving a thick covering as far away as Borneo, 500km to the north. In addition to the ash, an estimated 200 million tonnes of microscopic sulphur particles pumped into the stratosphere, spread outwards from Sumbawa to form a giant aerosol veil that enclosed the planet and acted as a block to incoming sunlight.  The consequences for the developed societies of the northern hemisphere were dire. A dry, sulphurous, fog draped itself across the landscape of eastern North America, causing temperatures to plunge and bringing unprecedented summer cold. In New York State, snow fell in June, while the bitter cold and killing frosts wiped out crops and halved the length of the growing season across much of the region. On the other side of the Atlantic, Europe saw summer temperatures down by 2C compared to the average for the decade; the unseasonal cold accompanied by incessant rains and – into the following winter – by unusually powerful storms. Analysis of climate records reveals that 1816, the so-called “year without a summer”, was the second coldest in the northern hemisphere of the past six centuries.  The alleged cultural implications of this “volcano weather” for Europe are somewhat whimsical. The brilliant, gas-charged, sunsets have been declared by some to have provided the inspiration for some of JMW Turner’s more flamboyant skies. In a similar vein, the damp and gloom of the 1816 summer has been charged with setting the scene for both Lord Byron’s grim vision Darkness, and Mary Shelley’s gothic novel Frankenstein. For the less well-to-do of Europe, however, the Tambora eruption brought nothing less than hunger, disease and death. Widespread harvest failure resulted in the most serious famine for more than a hundred years, doubling the price of grain and spawning bread riots and widespreadcivil unrest. Such was the degree of breakdown of food supply that economic historian John Post has called the episode “the last, great subsistence crisis in the western world”. Malnourished and weakened, the starving succumbed rapidly to disease, with typhus in particular rife. Many tens of thousands are thought to have died across the continent, including more than 40,000 in Ireland alone.  How would we fare if faced with a Tambora-sized eruption today? Is it even something we could feasibly prepare for? Received wisdom has it that globalisation would make it easier to cope. Should the European harvest fail, so the thinking goes, we can always buy our food from elsewhere. The very interconnectedness of world markets may, however, make things worse – the collapse of food production across Europe, parts of North America and perhaps elsewhere, could result in global shortages which in turn would drive a dramatic rise in the cost of food commodities. At the same time, the intense worldwide competition for food supplies, scarce as a consequence of the harvest failures, could drastically reduce the range of products available in the UK, interfere with supply and distribution, and bring about a collapse of the supermarkets’ ultra-sensitive, time-critical, stock-control systems, leaving their shelves increasingly depleted. While the less well-off could be priced out of purchasing even staple foodstuffs, panic buying by those who can afford it could quickly empty the stores.  Related: Ashes to ashes: dramatic volcano sites from around the world – in pictures  On top of this, harvest disruption in response to volcano weather might extend far beyond Europe, and might – in ensuing decades – be exacerbated by the consequence of rampant climate change. In spite of our modern farming methods and distribution systems, the ramifications could be far more severe than we expect. It is also worth considering that while the Tambora blast was approximately 1,000 times bigger than the 2010 Icelandic eruption, it was a minor hiccup in comparison with the greatest volcanic explosions of history. The Toba eruption that excavated the world’s largest volcanic crater in Sumatra, around 74,000 years ago, for example, injected hundreds of times more sulphur gases into the stratosphere than Tambora. The severe “volcanic winter” that followed probably lasted for several years and saw a third or more of the Earth covered with snow and ice and the wholesale dieback of vegetation.  So, if a Tambora-scale scenario would be bad news, far worse could be lying in wait. While we can’t stop the next Tambora, nor handle its potential impacts on the climate and the harvest, we can ensure that contingency plans are in place to keep everyone adequately fed until the sulphur veil dissipates and temperatures return to normal. In the UK at present, contingency food supplies probably amount to little more than a few weeks’ worth. Some serious policy changes are needed if a future volcanic blast is not to bring about another subsistence crisis.  Hazarding a guess about when and where the next Tambora will explode is far from an exact science. Eruptions on such a scale seem to happen, on average, a few times every millennium and one estimate holds that there is a 1 in 10 chance of a comparable event in the next 50 years. The Earth does not, however, operate to a timetable so such an eruption is equally likely to occur in any single year. There is even a chance that climate change may have a hand to play. Looking back at previous episodes of dramatic climate warming provides us with plenty of robust evidence for a vigorous volcanic response, most notably as our world heated up rapidly at the end of the last Ice Age. The reaction is most pronounced at ice-covered volcanoes, where melting reduces the weight acting on the volcanoes beneath, facilitating eruptions and even promoting the production of more magma. Coastal volcanoes may also be brought to eruption as the increased load of water, due to climbing sea levels, bends the crust around the margins of the oceans, squeezing magma upwards like toothpaste out of a tube.  No volcano erupts without warning signs, caused by rising magma triggering earthquake swarms and inflating the ground surface. The problem is that out of our world’s 1,300 or more active and potentially active volcanoes, we monitor only a few hundred. The Tambora eruption reinforces the unofficial volcanological axiom: the longer the wait, the bigger the bang. That rule of thumb is borne out by the fact that fully half of the biggest eruptions since 1800 originated at volcanoes that had previously been dormant throughout history. What we should be keeping a special watch on then, in order to prepare ourselves for the next arrival of Vulcan’s shock troops, are those seemingly innocuous volcanoes that have kept their heads down for centuries or even millennia. While there are too many candidates to keep a serious eye on, the numbers can be narrowed down by focusing on those that have been recently “restless”; perhaps best regarded as the volcanologists’ term for “bubbling under”. Beyond that, though, it’s anyone’s guess.  Bill McGuire is professor emeritus of geophysical and climate hazards at UCL. His latest book is Waking the Giant: how a Changing Climate Triggers Earthquakes, Tsunamis and Volcanoes, £11.99. Click here to buy it for £9.59  1 Laguna del Maule (Chile)Currently inflating at the astonishing rate of 25cm a year, above a growing body of magma just 5km beneath the surface.  2 Uturuncu (Bolivia) A 70km-wide bulge that has been growing since the early 1990s could culminate in a gigantic eruption.  3 Alban Hills (Italy) Just 20km south-east of Rome, this huge volcano has started to become restless following more than 30,000 quiet years.  4 Campi Flegrei (Italy) The archetypal “restless volcano” on the edge of Naples has not erupted since 1538, but has shown worrying signs, on and off, since the 1970s.  5 Yellowstone (Wyoming, US) No eruption for around 70,000 years, but congenitally restless.  6 Mount Fuji (Japan) Quiet since 1707, but scientists recently warned that the volcano was in a “critical state” with a “high potential for eruption”.  7 Mammoth Mountain (California, USA)In September 2014, up to 300 small earthquakes a day shook this part of the Long Valley supervolcano.  8 Askja (Iceland) Swarms of small earthquakes and a crater-lake that was ice-free last winter hint at magma on the move for the first time since a major blast in 1875.  9 Mount Paektu (North Korea-China) In 940 it hosted one of the greatest eruptions of the past 10,000 years; signs of unrest are again evident.  10 Cumbre Vieja (La Palma, Canaries) A collapse of the unstable west flank could spawn a North Atlantic mega-tsunami.
The largest eruption ever recorded, in Indonesia 200 years ago, wreaked havoc across the world. When a volcanic event on that scale happens again – and it will – we should be prepared