Roman emperors not only had war, jealousy, and political infighting to worry about—there was also the rain. A new study linking climate data to Roman history finds that lower average rainfall boosted the odds of an emperor being murdered during the empire's roughly 500-year run, from 27 BCE to 476 CE, the Smithsonian reports. The reasoning: Low rainfall in what is now France and Germany (then the Roman frontier, where many troops were stationed) would cause low harvests and starve soldiers, who became more likely to mutiny. "That mutiny, in turn, would collapse support for the emperor and make him more prone to assassination," study co-author Cornelius Christian, an assistant economics professor in Canada, tells Live Science.
Relying on a 2011 study of oak-tree rings, researchers found that a 20% drop in annual rainfall created a 0.11 standard deviation increase in the chance a Roman emperor being murdered the following year, the Economist reports. Those most in danger lived during the Gordian dynasty from 235 CE to 285 CE, when 14 emperors were killed. "We're not trying to claim that rainfall is the only explanation for all these things," says Christian. "But usually there is a drought preceding the assassination of the emperor." Their study is part of a growing field linking historical events to climate data, like a Nature study last year about how volcanic activity may have created drier conditions and crippled Egypt's Ptolemaic dynasty. (In other assassination news, Robert F. Kennedy Jr said Sirhan Sirhan didn't kill his father.)