Claire Witham and Clive Oppenheimer, volcanologists at the University of Cambridge, have applied a more sophisticated statistical analysis to the records and discovered things were even worse than Grattan suspected, with around 30,000 extra deaths that year - an increase of almost 17 per cent. Yet there was no famine, plague or war to account for it.
The pair also found two distinct peaks in the death rate. The worst months were August and September, when the numbers were 40 per cent up on the 50-year averages. "That translates into around 11,500 extra deaths countrywide," says Witham. In Bedfordshire, September brought five times the average number of deaths. "It was the worst month in the whole of the 18th century." The second peak came the following January and February, with mortality up 23 per cent - an extra 8200 dead nationwide. The numbers dying remained abnormally high until May.
Things were at least as bad in France and the Low Countries, where documentary evidence suggests the haze stayed longest. Student volunteers have now scoured local records in 53 parishes in three regions of France, providing some idea of how bad things were. Burial records show that between August 1783 and May 1784 the number of deaths was 25 per cent higher than average. Between August and October 1783, it was 38 per cent up. If the same was happening across France, says Grattan, then more people died than in the deadly summer of 2003, when heat and smog killed 16,000.
Why did so many die? The unusual pattern of summer deaths in England and France suggests a common cause, which clearly implicates the volcano. Grattan blames a combination of volcanic pollution and extreme weather. If people could smell sulphur in the air, then the levels of pollution were high enough to cause illness.
But, as Witham and Oppenheimer point out, people didn't begin to die immediately as they do during the worst summer smogs today. Why not? "The effects were probably cumulative," says Grattan. The level of pollution was probably not so bad as in, say, a severe summer smog in traffic-choked Athens, but it lasted months. "People's bodies had to cope with the intense heat and bad air on top of that. Under this sort of stress even mild illness or overexertion would push the body too far and lead to heart failure," says Grattan. During the harvest, people started at dawn and worked hard all day. "Then they keeled over."
Grattan suspects most of those who died were very young or old. The London Bills of Mortality, weekly lists of burials that include the ages of the dead, suggest he could be right. "I think what we are seeing is the deaths of those who would normally have died in winter, killed off by colds or other illnesses," he says. "The 'winter cull' of the weak was simply brought forward by a few months." Winter then brought a second wave of deaths, killing those who had struggled through the summer but had nothing left to cope with one of the worst winters on record.
Today, Europe's population includes many more frail, elderly people and asthma sufferers, and many more would die if a Laki-style eruption happened now. Grattan estimates that the toll in England alone could hit 100,000. But, he warns, the atmosphere is now so polluted it wouldn't take an eruption on that scale to cause a major crisis. "People who don't live near volcanoes don't think an eruption will affect them. What happened in 1783 shows we should all be worried."
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