What if one of the largest volcanic eruptions in recent history happened today? A new study suggests that a blast akin to one that devastated Iceland in the 1780s would waft noxious gases southeastward and kill tens of thousands of people in Europe. And in a modern world that is intimately connected by air traffic and international trade, economic activity across much of Europe, including the production and import of food, could plummet.
From June of 1783 until February of 1784, the Laki volcano  in south-central Iceland erupted. Although the event didn't produce large amounts of volcanic ash, it did spew an estimated 122 million metric tons of sulfur dioxide gas into the sky—a volume slightly higher than human industrial activity today produces in the course of a year, says Anja Schmidt, an atmospheric scientist at the University of Leeds in the United Kingdom.
Historical records suggest that in the 2 years after the Laki eruption, approximately 10,000 Icelanders died—about one-fifth of the population—along with nearly three-quarters of the island's livestock. Parish records in England reveal that in the summer of 1783, when the event began, death rates were between 10% and 20% above normal. The Netherlands, Sweden, and Italy reported episodes of decreased visibility, respiratory difficulties, and increased mortality associated with the eruption. According to one study, an estimated 23,000 people died from exposure to the volcanic aerosols in Britain alone. But elsewhere in Europe, it's difficult to separate deaths triggered by the air pollution from those caused by starvation or disease, which were prominent causes of death at the time.
To assess how such an eruption might affect the densely populated Europe of today, Schmidt and her colleagues plugged a few numbers into a computer simulation. They used weather models to estimate where sulfur dioxide emissions from an 8-month-long eruption that commenced in June would end up. They also estimated the resulting increases in the concentrations of airborne particles smaller than 2.5 micrometers across, the size of aerosols that are most easily drawn into human lungs and that cause cardiopulmonary distress. Then, they used modern medical data to estimate how many people those aerosols would kill.
In the first 3 months after the hypothetical eruption began, the average aerosol concentration over Europe would increase by 120%, the team reports online today in the Proceedings of the National Academy of Sciences. The number of days during the eruption in which aerosol concentrations exceed air-quality standards would rise to 74, when a normal period that length typically includes only 38. Not surprisingly, the air would become thickest with dangerous particles in areas downwind of the eruption, such as Iceland and northwestern Europe, where aerosol concentrations would more than triple. But aerosol concentrations in southern Europe would also increase dramatically, rising by 60%.
In the year after the hypothetical eruption commences, the increased air pollution swept from Iceland to Europe would cause massive amounts of heart and lung disease, killing an estimated 142,000 people . Fewer than half that number of Europeans die from seasonal flu each year.
At least four Laki-sized eruptions have occurred in Iceland in the past 1150 years, Schmidt and her colleagues say. So the new figures are cause for concern.
The team "has done a good job of showing where volcanic aerosols would end up, and the human health response to such aerosols is well understood," says Brian Toon, an atmospheric scientist at the University of Colorado, Boulder. "This is all very solid science."
Icelandic volcanoes shut down European air traffic  for more than a week in April 2010 and for several days in May of this year. But those eruptions are tiny compared with a Laki-sized eruption, which could ground airplanes for 6 months or more, says Alan Robock, an atmospheric scientist at Rutgers University in New Brunswick, New Jersey. Such an event would have a huge impact on crop yields and, by affecting shipping and air traffic, would also affect Europeans' ability to import food, he notes. It could even have a dramatic effect on daily life, he says. "If there are sulfur dioxide clouds over Europe, people with respiratory problems can't do much about it except stay indoors."
*This article has been corrected. In the first paragraph and in the image caption, compass directions were originally misstated and should have read southeastward and southeastern, respectively.