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.