Linking climate variability to human history
In a new study published this week in the journal Science, an international research team of archaeologists and climatologists has, for the first time ever, reconstructed Europe’s annual summer climate over the past 2500 years and compared summer climate variations with conspicuous episodes in human history. Their study provides new evidence that agrarian wealth and overall economic growth may be related to climate change.
The researchers detected Europe’s summer precipitation and temperature back to 500 B.C., extending the record 1,500 years further than previous studies into the past. Their results are based on measurements of annual tree-rings from thousands of archaeological and living tree samples from Germany, France and Austria.
The climate information stored in these trees allows comparison of natural precipitation and temperature fluctuations with the development of European societies. European summer climate during the Roman Era about 2000 years ago was relatively wet and warm and characterized by less variability. Increased climate variability from around 250-600 A.D. coincided with the demise of the Western Roman Empire and the exceptional turmoil of the Migration Period.
The new study also revealed that unfavorable climate may have played a role in the underlying health conditions that contributed to the devastating economic crisis that arose in connection with the Black Death plague pandemic in the 14th century. More recently, temperature minima in the early 17th and 19th centuries coincided with large-scale settlement abandonment during the Thirty Years’ War and the modern mass migrations from Europe to America.
The lead author of the study, Ulf Büntgen from the Swiss Federal Research Institute WSL, pointed out “that according to our results past hydroclimatic variations may have exceeded the magnitude and duration of variations seen in modern times. The situation is different for temperature though, as the recent warming in the late 20th and early 21st century appears unprecedented with respect to the past 2,500 years.”
The authors note that, although such comparative studies cannot be used to indicate a direct causal relationship between climate variability and human history, their detailed palaeoclimatic history lends new credence to the idea that climate can have significant influence on human society. Sounding a cautionary note, the researchers suggest that projected global climate change may affect human societies more than is currently expected, and that the complex causal links between past climate changes and human responses urgently require more investigation.
Harvard University’s Michael McCormick, Goelet Professor of Medieval History and Chair of the Standing Committee on Archaeology, used the wealth of published medieval eyewitnesses available in Widener Library to verify the record of precipitation established from the analysis of growth patterns of 7000 trees by the European scientific team. Climate scientists deduce ancient climate patterns by comparing modern natural climate records or “proxy data” such as tree rings and ice cores with the instrumental records of temperature, precipitation, etc. that exist for the last century or so. They then use the signals of earlier proxy data to project those climate conditions into the past. However, the impact on the environment of human-induced changes such as the Industrial Revolution is so great that some have doubted whether the climate mechanisms observed over the last century or so applied in the more remote past.
McCormick tested the team’s new data on droughts and heavy rains against the historical records from the Middle Ages. From 1013 to 1504 A.D., McCormick was able to identify 88 eyewitness accounts bearing on 32 of the extreme precipitation years deduced from the tree rings, and composed in the areas where the trees were growing. For 30 of those years, the eyewitnesses confirmed the trees’ signal.
The database of eyewitness accounts will soon be made available for consultation on the Internet on the free online Digital Atlas of Roman and Medieval Civilizations, created by McCormick, Guoping Huang, Kelly L. Gibson, and a group of Harvard undergraduate and graduate students: http://darmc.harvard.edu/ with the support of Harvard’s Center for Geographic Analysis.
The human confirmation strongly argues that the same climate mechanisms observed today applied in Europe in the Middle Ages, long before the Industrial Revolution began to transform the natural environment around 1800. The extraordinarily rich record of Europe’s deep historical and archaeological past is proving indispensable to scientific efforts to understand today’s global present, even as the new scientific insights are shedding unexpected light on the human past. McCormick’s contribution to the analysis of the new data was funded by a Distinguished Achievement Award from the Andrew W. Mellon Foundation.
Ulf Büntgen, Willy Tegel, Kurt Nicolussi, Michael McCormick, David Frank, Valerie Trouet, Jed O. Kaplan, Franz Herzig, Karl-Uwe Heussner, Heinz Wanner, Jürg Luterbacher and Jan Esper, “2500 Years of European Climate Variability and Human Susceptibility” Science, vol 331, published online 13 January 2011.