© The Art Archive / Querini Stampalia Foundation Venice / Gianni Dagli OrtiPart of a lagoon which froze over in 1708, Venice, Italy, by Gabriele Bella (1733-99)

People across Europe awoke on 6 January 1709 to find the temperature had plummeted. A three-week freeze was followed by a brief thaw - and then the mercury plunged again and stayed there. From Scandinavia in the north to Italy in the south, and from Czechoslovakia in the east to the west coast of France, everything turned to ice. The sea froze. Lakes and rivers froze, and the soil froze to a depth of a metre or more. Livestock died from cold in their barns, chicken's combs froze and fell off, trees exploded and travellers froze to death on the roads. It was the coldest winter in 500 years.

In England, they called the winter of 1709 the Great Frost. In France it entered legend as Le Grand Hiver, three months of deadly cold that ushered in a year of famine and food riots. In Scandinavia the Baltic froze so thoroughly that people could walk across the ice as late as April. In Switzerland hungry wolves crept into villages. Venetians skidded across their frozen lagoon, while off Italy's west coast, sailors aboard English men-of-war died from the cold. "I believe the Frost was greater (if not more universal also) than any other within the Memory of Man," wrote William Derham, one of England's most meticulous meteorological observers. He was right. Three hundred years on, it holds the record as the coldest European winter of the past half-millennium.

Derham was the Rector of Upminster, a short ride north-east of London. He had been checking his thermometer and barometer three times a day since 1697. Similarly dedicated observers scattered across Europe did much the same and their records tally remarkably closely. On the night of 5 January, the temperature fell dramatically and kept on falling. On 10 January, Derham logged -12 °C, the lowest temperature he had ever measured. In France, the temperature dipped lower still. In Paris, it sank to -15 °C on 14 January and stayed there for 11 days. After a brief thaw at the end of that month the cold returned with a vengeance and stayed until mid-March.

Later that year, Derham wrote a detailed account of the freeze and the destruction it caused for the Royal Society's Transactions. Fish froze in the rivers, game lay down in the fields and died, and small birds perished by the million. The loss of tender herbs and exotic fruit trees was no surprise, but even hardy native oaks and ash trees succumbed. The loss of the wheat crop was "a general calamity". England's troubles were trifling, however, compared to the suffering across the English Channel.

In France, the freeze gripped the whole country as far as the Mediterranean. Even the king and his courtiers at the sumptuous Palace of Versailles struggled to keep warm. The Duchess of Orleans wrote to her aunt in Germany: "I am sitting by a roaring fire, have a screen before the door, which is closed, so that I can sit here with a sable fur piece around my neck and my feet in a bearskin sack and I am still shivering with cold and can barely hold the pen. Never in my life have I seen a winter such as this one."

In more humble homes, people went to bed and woke to find their nightcaps frozen to the bed-head. Bread froze so hard it took an axe to cut it. According to a canon from Beaune in Burgundy, "travellers died in the countryside, livestock in the stables, wild animals in the woods; nearly all the birds died, wine froze in barrels and public fires were lit to warm the poor". From all over the country came reports of people found frozen to death. And with roads and rivers blocked by snow and ice, it was impossible to transport food to the cities. Paris waited three months for fresh supplies.

There was worse to come. Everywhere, fruit, nut and olive trees died. The winter wheat crop was destroyed. When spring finally arrived, the cold was replaced by worsening food shortages. In Paris, many survived only because the authorities, fearing an uprising, forced the rich to provide soup kitchens. With no grain to make bread, some country people made "flour" by grinding ferns, bulking out their loaves with nettles and thistles. By the summer, there were reports of starving people in the fields "eating grass like sheep". Before the year was out more than a million had died from cold or starvation.

The fact that so many people left accounts of the freeze suggests the winter of 1708/1709 was unusually bad, but just how extraordinary was it?

In 2004, Jürg Luterbacher, a climatologist at the University of Bern in Switzerland, produced a month-by-month reconstruction of Europe's climate since 1500, using a combination of direct measurements, proxy indicators of temperature such as tree rings and ice cores, and data gleaned from historical documents (Science, vol 303, p 1499). The winter of 1708-1709 was the coldest. Across large parts of Europe the temperature was as much as 7 °C below the average for 20th-century Europe.

Why it was quite so cold is harder to explain. The Little Ice Age was at its climax and Europe was experiencing climatically turbulent times: the 1690s saw a string of cold summers and failed harvests, while the summer of 1707 was so hot people died from heat exhaustion. Overall, the climate was colder, with the sun's output at its lowest for millennia. There were some spectacular volcanic eruptions in 1707 and 1708, including Mount Fuji in Japan and Santorini and Vesuvius in Europe. These would have sent dust high into the atmosphere, forming a veil over Europe. Such dust veils normally lead to cooler summers and sometimes warmer winters, but climatologists think that during this persistent cold phase, dust may have depressed both summer and winter temperatures.

None of these things accounts for the extremity of that particular winter, however. "Something unusual seems to have been happening," says Dennis Wheeler, a climatologist at the University of Sunderland, UK. As part of the European Union's Millennium Project, which aims to reconstruct the past 1000 years of Europe's climate, Wheeler is extracting data from Royal Navy logbooks, which provide daily observations of wind and weather. "With daily data you can produce very reliable monthly averages but you can also see what happened from one day to the next," says Wheeler. He and his colleagues have now compiled a database of daily observations stretching back to 1685 from the English Channel area. "This is a key climatic zone. The weather there reflects wider conditions across the Atlantic, which is where in normal circumstances much European weather originates."

The most immediate cause of cold winters in Europe is usually an icy wind from Siberia. "What you would expect would be long runs of easterly winds with a well-developed anticyclone over Scandinavia sucking in cold air from Siberia," says Wheeler. Instead, his data show a predominance of southerly and westerly winds - which would normally bring warm air to Europe. "There were only occasional northerlies and easterlies and those were never for more than a few days," says Wheeler. Another odd finding was that January was unusually stormy. Winter storms tend to bring milder, if wilder, weather to Europe. "This combination of cold, storms and westerlies suggests some other mechanism was responsible for that winter."

There may be no easy explanation for the Great Frost of 1709, but unexpected weather patterns revealed by Wheeler's data underline why climate reconstructions are so important. "We need to explain the natural variation in climate over past centuries so that we can tease apart all those factors that contribute to climate change. But before we can do that we need to nail down those changes in detail," says Wheeler. "Climate doesn't behave consistently and warmer and colder, drier and wetter periods can't always be explained by the same mechanisms." In the two decades after that terrible winter, the climate warmed very rapidly. "Some people point to that and say today's warming is nothing new. But they are not comparable. The factors causing warming then were quite different from those operating now."