When empires rise and fall and plagues sweep over the land, people have traditionally cursed the stars. But perhaps they should blame the weather. A new analysis of European tree-ring samples suggests that mild summers may have been the key to the rise of the Roman Empire—and that prolonged droughts, cold snaps, and other climate changes might have played a part in historical upheavals, from the barbarian invasions that brought about Rome's collapse to the Black Death that wiped out much of medieval Europe.
"Looking back on 2500 years, there are examples where climate change impacted human history," says the study's lead author, Ulf Büntgen, a paleoclimatologist at the Swiss Federal Research Institute for Forest, Snow and Landscape in Zurich. "This kind of information is not only relevant for ancient agrarian societies, it might also impact modern societies."
The study, published online today in Science, examined nearly 9000 pieces of wood, mostly collected over the past 30 years by archaeologists who use tree rings to establish the age of ancient sites or structures, a technique known as dendrochronology. With tree rings taken from living trees as a baseline, dendrochronologists work their way back in time, comparing overlapping samples to edge ever further into the past.
The researchers worked out climate information the same way. First, they compared weather records collected over the past 200 years with samples from living trees to see how temperature and moisture affected tree-ring growth. Then Büntgen and his co-authors looked at timbers from historic buildings, wood preserved in rivers or bogs, and samples from archaeological sites to push the record further back. The study used 7284 oak samples from France and Germany to see how moisture showed up in tree rings and nearly 1500 different stone pine and larch samples from high altitudes in Austria to establish a separate temperature record.
The result was a continuous—and precisely dated—record of weather in France and Germany going back 2500 years. "We were aware of these super-big data sets, and we brought them together and analyzed them in a new way to get the climate signal," Büntgen says. "If you have enough wood, the dating is secure. You just need a lot of material and a lot of rings."
When Büntgen showed the data to historians and archaeologists, they pointed out remarkable consistencies with what we know of past societies. At times of social stability and prosperity, like the rise of the Roman Empire between 300 B.C.E. and 200 C.E., Europe experienced warm, wet summers ideal for agriculture. Similar conditions accompanied the peak years of medieval Europe between 1000 C.E. and 1200 C.E.
The study also showed that climate and catastrophe often line up. In the 3rd century C.E., for example, extended droughts matched the timing of barbarian invasions and political turmoil. Around 1300 C.E., on the other hand, a cold snap combined with wetter summers coincides with widespread famines and plague that wiped out nearly half of Europe's population by 1347.
"It's a phenomenal data set with some eye-opening conclusions," says University of Arkansas, Fayetteville, geoscientist David Stahle, who was not involved with the study. "The provocative outcome is that harsh climate conditions happen to be associated with upheavals in society, like the Black Death."
By counting wood samples, the analysis also created a rough measure of human activity. In eras of prosperity, more trees were cut down for building and fuel, yielding more samples in the archaeological record. At other times, like the years after the Black Death and the so-called Migration Period between 300 C.E. and 600 C.E. when the Roman Empire was overwhelmed by tribes pushing in from the east, the number of wood samples dwindles to nearly nothing. "It's an interesting proxy of demographic trends and really the most provocative part of the study," says Stahle.