Volcanoes and climate: 200th anniversary of Mt Tambora explosion

Mount Tambora, a volcano on the Indonesian island of Sumbawa, was once similar in stature to Mont Blanc or Mount Rainier. But in April 1815 it blew its top off in spectacular fashion. On the 10th and 11th it sent molten rock more than 40 kilometres into the sky in the most powerful eruption of the past 500 years. The umbrella of ash spread out over a million square kilometres; in its shadow day was as night. Billions of tonnes of dust, gas, rock and ash scoured the mountain’s flanks in pyroclastic flows, hitting the surrounding sea hard enough to set off deadly tsunamis; the wave that hit eastern Java, 500 km away, two hours later was still two metres high when it did so. The dying mountain’s roar was heard 2,000 km away. Ships saw floating islands of pumice in the surrounding seas for years.

In his book “Eruptions that Shook the World”, Clive Oppenheimer, a volcanologist at Cambridge University, puts the number killed by the ash flows, the tsunamis and the starvation that followed them in Indonesia at 60,000-120,000. That alone would make Tambora’s eruption the deadliest on record. But the eruption did not restrict its impact to the areas pummelled by waves and smothered by ash.

The year after the eruption clothes froze to washing lines in the New England summer and glaciers surged down Alpine valleys at an alarming rate. Countless thousands starved in China’s Yunnan province and typhus spread across Europe. Grain was in such short supply in Britain that the Corn Laws were suspended and a poetic coterie succumbing to cabin fever on the shores of Lake Geneva dreamed up nightmares that would haunt the imagination for centuries to come. And no one knew that the common cause of all these things was a ruined mountain in a far-off sea.

While lesser eruptions since then have had measurable effects on the climate across the planet, none has been large enough to disrupt lives to anything like the same worldwide extent. It may be that no eruption ever does so again. But if that turns out to be the case, it will be because the human world has changed, not because volcanoes have. The future will undoubtedly see eruptions as large as Tambora, and a good bit larger still.

Mixed in with the 30 cubic kilometres or more of rock spewed out from Tambora’s crater were more than 50m tonnes of sulphur dioxide, a large fraction of which rose up with the ash cloud into the stratosphere. While most of the ash fell back quite quickly, the sulphur dioxide stayed up and spread both around the equator and towards the poles. Over the following months it oxidised to form sulphate ions, which developed into tiny particles that reflected away some of the light coming from the sun. Because less sunlight was reaching the surface, the Earth began to cool down.

The sulphate particles were small enough to stay aloft for many months, so the cooling continued into the following year. By the summer of 1816 the world was on average about 1ºC cooler than it had been the year before—an average which hides much larger regional effects. Because the continents are quicker to cool than the heat-storing seas are, land temperatures dropped almost twice as much as the global average.

This cooling dried the planet out. A cooler surface meant less evaporation, which meant less water vapour in the lower atmosphere and thus less rain. Rainfall over the planet as a whole was down by between 3.6% and 4% in 1816.

If such numbers seem suspiciously accurate, considering that most of the world of 1816 was devoid of thermometers and rain gauges, it is because they come from recent computer modelling of the climate that seeks to mimic the conditions Tambora created. Like all modelling results, such numbers need caveats. These results, though, and similar ones from other models, can be accorded the credence that comes from having been proved right in similar situations.

The 1991 eruption of Mount Pinatubo in the Philippines was about a sixth as large as Tambora’s in terms of the volume of lava, rock and ash, and about a third as large in terms of sulphur emissions. Satellites showed that in the summer of 1992 the sulphur it had spewed into the atmosphere was reducing the amount of sunlight getting to the Earth’s surface by well over three watts per square metre; for comparison, the warming effect of the 40% increase in the atmosphere’s carbon-dioxide level since the age of Tambora is just two watts per square metre.

With the energy absorbed by the Earth reduced, temperatures fell by around half a degree in the year after Pinatubo; rainfall dropped off significantly, too. Computer models run after the eruption but before these effects became visible captured the effects reasonably accurately (though they had a tendency to overestimate the cooling). This is one of the best reasons for thinking that such models capture the workings of the climate quite well.

The historical record largely bears out what the models suggest Tambora did. Across Europe the summer of 1816 was cold and wet, and the harvest terrible. The effects were most notable around the Alps; in Saint Gallen, in Switzerland, the price of grain more than quadrupled between 1815 and 1817. Starving migrants took to the roads in their hundreds of thousands; mortality rates climbed due to starvation and disease. Death also stalked Yunnan, where Tambora’s cooling shut down the monsoon and cold days in summer killed the rice harvest for three years running.

Monsoons, which are driven by the difference in temperature between hot land and cooler sea, are particularly vulnerable to the excessive cooling of the land that volcanoes bring. Their weakening can have effects on more than crops. In his excellent account of the global impacts of the 1815 eruption, “Tambora”, Gillen D’Arcy Wood of the University of Illinois draws on the writings of James Jameson, a doctor in Calcutta, who held the lack of fresh water which followed the failure of the 1816 monsoon responsible for the cholera epidemic that swept through Bengal the following year…

…Alan Robock, an expert on links between volcanoes and climate at Rutgers University, notes a particularly intriguing initial condition that could have influenced the world’s response to Tambora. There had been another large eruption—larger than Pinatubo—just six years before. No one knows where this 1809 eruption was, but its signature can clearly be seen in the Greenland and Antarctic ice sheets. The sulphur put into the stratosphere by volcanoes shows up quite clearly in the year-by-year records of what was going on in the atmosphere that climate scientists extract from polar ice cores. These records make it possible to give dates to large eruptions in the past even if no one recorded the event at the time.

The ice cores show that the 1809 eruption was easily large enough to have had effects on the climate, and there is some evidence of cooling in subsequent years. In Jane Austen’s “Emma”, which according to Euan Nisbet, a geologist at Royal Holloway, London, seems to follow the weather of 1814, spring is remarkably late, with apple trees blossoming in the middle of June. Pre-cooling along these lines might have made some of the subsequent effects of Tambora more marked, while possibly lessening others. Some researchers believe that a number of eruptions close together might be able to trigger a climate downturn that lasts considerably longer than the few years models normally predict; a set of eruptions in the late 13th century, this idea suggests, may have been part of the reason for the subsequent global cooling known as the “little ice age”.

If the prior state of the climate system constrains an eruption’s effects, so does that of the human world. The damage done to Europe by the preceding quarter-century of revolutionary and Napoleonic war could have left it particularly vulnerable to 1816’s “year without a summer”. The situation in Yunnan would hardly have been as dire had the population not been hugely expanded by the Qing dynasty’s encouragement of new settlers.

Similarly uncaptured in models, but even more fascinating to speculate about, are the after-effects of the Tambora downturn. In America, the spike in grain prices caused by Europe’s hunger drove a wave of farmers across the Appalachians to where the Ohio Valley was enjoying far more clement weather, with barges taking exports for Europe down the Mississippi in ever larger amounts. The collapse in the grain price when Europe’s harvest recovered contributed to the American economy’s first major depression.

The historian John Post, in a study of Tambora’s effects published in 1977, “The Last Great Subsistence Crisis in the Western World”, held that the volcano reshaped European politics. The disorder that sprang up in the bad weather from 1816 to 1818, and its subsequent repression, created a climate for authoritarian rule that held sway until the middle of the century. Mr D’Arcy Wood points out that it was in the aftermath of the Tambora famines that farmers in Yunnan started to plant opium poppies, the value of which as a cash crop offered some insurance against future failures of the grain harvest.

On top of such structural shifts, there are the personal stories. If Shelley, Byron and their romantic entourage had not been cooped up in a Swiss villa by incessant rain, would they have amused themselves by writing horror stories for each other—including John Polidori’s “The Vampyre”, the first novel to deal with seductive bloodsucking aristocrats, and Mary Shelley’s “Frankenstein”, which has shaped fears of scientific innovation from that day to this? If the summer frosts of “Eighteen-hundred-and-froze-to-death” had not driven Joseph Smith, a farmer, from Norwich, Vermont to Palmyra, New York, a place of vigorous religious enthusiasms, would his son Joseph junior still have been able to find the golden tablets to which the angel Moroni led him a few years later, or would the history of Mormonism have been very different?…