"periods of unusually *cold* weather in China during the last millennium preceded 12 of the 15 major bouts of warfare"

World News Australia - Fall of Roman Empire linked to climate

"Relatively modest changes in European climate in the past have had profound implications for society," says Michael Mann of Penn State University in University Park, Pennsylvania.

Other studies have shown how war and climate are often intimately linked. For example, periods of unusually cold weather in China during the last millennium preceded 12 of the 15 major bouts of warfare.