Well, this is ominous. Researchers at the Swiss Federal Institute for Forest, Snow and Landscape Research have discovered a link between changes in climate and the fall of the Roman Empire. Using tree rings from 9,000 samples, the Institute reconstructed temperature and rainfall in Western Europe over the last 2,500 years, and found that from AD 250 to 550, the climate shifted from decade to decade between dry and cool, and warm and wet. The changes were too sudden and short-lived for people to adapt, causing agricultural problems and mass migration. By AD 500, the Roman Empire fell. The Black Death also coincided with a shift to a wetter climate after a period of stability. "Relatively modest changes in European climate in the past have had profound implications for society," says Michael Mann of Penn State University.