When the Black Death swept through Europe in 1347, it was one of the deadliest disease outbreaks in human history, eventually killing between a third and half of Europeans. Prior work by investigators has traced the cause to plague-carrying fleas borne by rats that jumped ship in trading ports. In addition, historical researchers believe that famine in northern Europe before the plague came ashore may have weakened the population there and set the stage for its devastation. Now, new research using a unique combination of ice-core data and written historical records indicates that the cool, wet weather blamed for the...