By Paul Rincon
BBC News Online science staff
Volcanic eruptions in Iceland probably caused an unusual rise in deaths in England during the summer of 1783.
The eruptions released a volcanic pollutant cloud over Europe. Image: Claire Witham
UK experts suggest a cloud of volcanic gases and particles sweeping south from the Laki Craters event of that year may have killed more than 10,000 people.
The team combed climate data, burial records and contemporary accounts that reported a "volcanic haze" and health problems in the English population.
The University of Cambridge study is carried in the Bulletin of Volcanology.
The eruptions at the Laki Craters began on 8 June, 1783, and continued for eight months.
An estimated 122 megatonnes of sulphur dioxide was released, along with smaller amounts of other gases, from explosive fissures and vents and from lava flows.
In Iceland alone, some 9,000 people - about a quarter of the population - were killed. But the massive discharge from beneath the Earth also fumigated many parts of Europe with volcanic gases and airborne particles.
Claire Witham and Clive Oppenheimer looked at the burial records for 404 church parishes in 39 English counties.
They discovered there were two peaks in mortality during the Laki Craters event.
The first occurred between August and September 1783, the second between January and February 1784. In both cases, the worst affected region was the east of England.
Central England temperature data shows the summer of 1783 was particularly hot and that the first months of 1784 were amongst the coldest on record. The researchers hypothesised that some part of the mortality peaks could be attributed to these temperature extremes.
To find out how much, the researchers calculated the expected changes in mortality per degree of temperature change on a monthly scale.
They found that the winter peak in mortality could have been caused by the severe cold of January 1784. But the extreme July temperature could explain only about 30% of the observed variation in deaths, after normal trends in mortality had been accounted for.
There were an estimated 11,500 extra deaths during this late summer mortality peak in England.
"Something seems to have been going on in England and Europe during the more vigorous phases of the eruption," Ms Witham told BBC News Online.
Contemporary reports from across Europe mention the periodical presence of an atmospheric haze in summer and autumn 1783, linked by several lines of evidence to the pollutant cloud produced by the Laki eruptions.
One such account from Lincoln published in Gentleman's Magazine, July 1783, reads: "A thick, hot vapour had for several days before filled up the valley... so that both the Sun and Moon appeared like heated brick-bars."
In Iceland, the effects of the eruptions killed about a quarter of the population. Image: USGS
There are also reports of headaches and respiratory discomfort suffered by people during the period spanned by the Laki event.
The authors believe these health problems, and the unusual late summer mortality peak, could have been caused by volcanic gases and aerosols (fine, airborne particles) transported in the haze.
A high-pressure weather system over Western Europe during June and July 1783 could have brought these emissions down into the lower atmosphere and trapped them there, the researchers argue.
"During the 1952 smog in London, increased sulphur from coal burning was trapped by a similar temperature inversion effect," Ms Witham explained.
"Then, you saw a large increase in deaths because the aerosol was composed of very fine particles which can cause respiratory and cardiovascular problems."
Dr Steve Blake, a volcanologist at the Open University in Milton Keynes, told BBC News Online: "Both [temperature and emissions] must have had some influence. But off the top of my head, I'm not sure whether you can unpick those and say which one is critical."
But he agreed the pollutant cloud produced by the Laki eruptions could very well have caused environmental damage and ill-health in many parts of Europe.
Other researchers have also argued that the extremely hot summer of 1783 and the particularly cold winter of 1783/4 could have been caused by the volcanic eruptions at Laki.
The products of volcanism are known to have effects on climate. But the Cambridge researchers suggest other, unrelated, factors could equally have been to blame for these temperature extremes.
The Icelandic death toll was due mainly to a famine that took hold after most of the island's sheep were killed by eating grass contaminated with fluorine from the eruptions.