Climate's influence on European history

(About a comprehensive archive of climate data in Europe for the past 2,500 years established from tree-rings.) - Der Spiegel, January 13, 2011.
Translated by Ami de Grazia

We have all heard of the story of Hannibal crossing the Alps - but is it accurate? In 218 BC the Carthaginian general crossed the highest mountain range of Europe with 37 elephants, thousands of horsemen and ten of thousands of foot soldiers to fight Rome. All the elephants survived this ordeal. Is this possible?

Now more of the story is coming to light. A new study, published in Science magazine, provides year by year an accurate history of the climate in Europe for the past 2,500 years. As it appears from the study, in the summer of 218 BC, the weather was particularly warm. The story of Hannibal's crossing of the Alps gains in credibility.

Other events too can be checked against the study and acquire from it new arguments to consolidate or invalidate them: why did famines, wanderings of peoples, epidemics and wars occur? Often, drastic changes in the weather and in the climate may have effected equally dramatic changes in history, say historians.

Researchers Ulf Büntgen from the Swiss Environment Research Institut WSL in Bern and Jan Esper from University of Mainz have sorted the information contained in almost 9,000 pieces of wood - and constituted a unique archive of climate. The tree-rings inform us about the weather of the past: every year trees add a new ring, the breadth of which gives precious information about temperature and precipitation - depending on the location where the tree grew.

The most important results of the study are:

* Historical epochs fit into climatic cycles: the flourishing of the Roman Empire and of the German empire coincided with warm periods; bad times like invasions, the Plague and the Thirty Years war happened in times of bad climatic conditions.
*Central Europe experienced in Roman times and in the high Middle Ages warm periods like those of today. Yet the summer of 2003 remains remarkable: it was the hottest Summer in the Alpine region in 2,500 years.
* The amount of precipitation in Central Europe varied considerably more from one year to the other in Antiquity and in the Middle Ages than it does today, moreover, extremes were more pronounced.

"The precise coordination between climate and history is left for historians to establish", says Ulf Büntgen. Yet the study shows remarkable parallels between weather and history. And everything that has happened in Germany and Europe during these past 2,500 years can be confronted with the data.

It was a new start after the cold: when in the middle of the first millennium BC Europe emerged from the final throes of the last ice Age, the yearly average temperatures in Europe lay one to two degrees Celsius lower than today.

When in 300 BC the weather slowly improved and rains became more abundant, the Roman Empire came to flourish. Climate helped the ascendancy of Rome. The yield of crops improved, mines could be opened. When the Alps could be crossed all year, Northern Europe became accessible and was absorbed.

From this period alone Büntgen and Esper and their teams have analyzed some 550 wood samples. From the breadth of rings in oak they read the amount of precipitation in the Spring and in June, from the rings in larks and pines, the summer temperatures. They cannot provide information about the weather in other seasons, as trees grow only in summer.

Each ring can be matched exactly with a particular year. For researcher have now at their disposal series of dated tree-rings of the past millennia. Büntgen et al. have fitted their own samples into these series.

Tree trunks for the history of precipitations were lifted mostly in Germany and Eastern France, for instance in river beds and in archaeological digs. For the archives of temperature, only the trees at the edges of forests could be taken into consideration for their growth is dependent on temperature. All the other trees are more dependent on precipitation.

The researchers used only samples from trees from the Alpine region but their data is valid also for large parts of Central Europe, France, Italy and Balkans - as is shown by comparative temperature measurements in the XX century.

The data show from the fourth century AD on, a grave deterioration of the climate: central and southern Europe became cold and dry. Historian speak of the "climatic pessimum of the peoples wanderings i.e; the Barbarian Invasions." They know of course that it was triggered first of all by the wanderings of the Huns, which set Germans, Goths and other peoples on their way. Yet it is established that climatically caused failures of crops, famines and epidemics made the displacements more urgent, also for the Huns.

Temperatures continued to fall, and precipitations continued to diminish. Erosion of the top soil was the consequence, fields yielded ever less. The rains came back during the fourth century but the weather remained cold and glaciers grew in size.

The worst crisis was experienced in Europe in the years 536 to 546, when summer temperatures plummeted to record lows. "Our data shows for these times an extraordinary depression lasting a decade," according to Büntgen. Recently, geologists have suggested that its cause may have been the impact of a meteorite off the coast of Australia.

In the sixth century the crisis went on, the population of Europe "sank to an all time low, never to be reached again," says historian Wolfgang Behringer from University of Saarbrücken, Saarland. Archaeologists have found in Europe a great number of abandoned settlements. Pollen analysis shows a strong retreating of agriculture, forests advanced.

These were freezing times, as the new climate data shows. The consequences were terrible: in the year of famine 784 one third of the population of Europe may have died. "It was a rather cool summer," according to Büntgen's diagnosis. "With the climatic downturn in Europe, not only crops but cattle perished," according to historian Behringer. Every crop failure caused a famine. To the cold, was added humidity in the ninth century: endless rains prepared the ground for epidemics, such as leprosy.

It was the time of the wolves. Hunger brought them to Central Europe, for in their homeland Russia the climate had also considerably deteriorated. The beasts circled the villages. "The fight against the marauding animals was carried on with all possible weapons, traps, hunting, poison," says Behringer. Charlemagne ordered the creation of units of wolf-hunters in every county. In the year of famine 843 a wolf broke up Sunday mass service in the town of Senonnais in France. Büntgen confirms: "843 was colder than the years preceding or following."

In the middle of the tenth century the climate took a turn for the better, the climate optimum of the Middle-Ages settled in. The new data shows that the temperatures in Europe climbed to equality with those which were seen again only in the twentieth century. The tree line in the Alps was in many places even higher than today and wine was cultivated farther up North than at the beginning of the 21 century. The time of discoveries began: the Vikings sailed over Greenland to America.

Agriculture recovered, famine became rarer. In 150 years, the population in Europe increased by one third. Under the Hohenstaufen emperors the German empire reached its peak: Frederic II. resided in Sicily. At his court, philosophers, scientists and artists mingled; thought and speech became freer. From Arabia too came scientists, who had preserved and developed precious knowledge from antiquity. Architecture changed: gothic cathedral were furnished with immense windows to take advantage of the sunlight.

Some historical records need to be reconsidered, in light of the new data. In Nuremberg in 1022 one burger claims that "from the terrible heat people collapse and die of thirst in the streets." Yet, the summer of 1022 was not particularly hot, says Büntgen. Exaggeration? Or was the brutal heat of such short duration that it did not register in the tree rings? Other events find explanation and corroboration: in 1135, for instance, there was very little rain, confirming reports that the Danube ran almost dry. The people of Regensburg, Germany, took advantage of this to build the great Stone Bridge, still today the symbol of their city.

Other indications are also verified: on September 9, 1302, vineyards froze in Alsace and after a very cold winter, peasants in Germany found on May 2, 1303 that all their seed stocks had frozen. They did not know yet how bad things would become.

The new climate data are the impassive records of a gigantic catastrophe which broke over Europe. They show in the 14th century the occurrence of many cold summers. In 1314, diluvian rains and a harsh winter came on top of it.

Behind the data, cruel occurrences appear: it began with the loss of crops due to the weather. From 1315 to 1335 the "Great Hunger" decimated populations. In 1315 already, horses and dogs were eaten. 1346 and 1347 were especially cold, wine froze, grain rotted. The weakened population had diminished resistance against epidemics: probably from China, the "Black Death" arrived. Between 1346 and 1352, half the population of Europe died.

South of the Alps, temperatures sank less steeply. It may have been one reason why the Renaissance (the "Rebirth") could blossom there. The philosophers of antiquity came again to honor, banking developed and the bourgeoisie could compete with nobility with newly found self-assurance.

Renaissance had it not easy to cross the Alps. In the North the dark might of belief still held sway. The Church blamed witches for bad crops and illnesses and had women burned at the stake in great numbers. In 1524 peasants revolted against nobility.

It became ever colder. The little ice age had begun. Around the end of the 17th century, Europe suffered grave famines. In 1709 weather precipitated one of the worst catastrophes: in the "cruel coldwave of 1709" rivers froze over even in Portugal, palm trees in Southern Europe were covered in snow. Rivers carried masses of frozen fish, cattle froze to death in the stables, dead deer lay in the fields and birds are said to have fallen as frozen clumps from the sky. In the summer of 1710, men were seen "grazing" in the fields "like sheep," say chronicles.

The Enlightenment was accompanied by a warming of the climate. "Famines were now seen as the result of mismanagement," says Behringer. Peasants took to rotating crops, irrigation was improved, better roads and dykes were built, moors dried up. The agrarian revolution saw to it that famines became more rare."

These improvements did not help against the famines of the mid-19th century (Irish famine) caused by a short recession in the climate.

For a long time, experts have been in disagreement about the future consequences of the climate change: will changes bring new catastrophes, or is a warming for the good? "Rapid climate changes have often grave, negative effects of societies," says Ulf Büntgen. The new data will give historians abundant material to discover and study such connections.