What caused a 10-year winter starting in 536?


A winter that lasts years isn't just a problem in Game of Thrones. Roughly 1500 years ago, our world was turned upsidown by a winter that witnesses say "never ended." Now there is scientific evidence that there really was a decade of winter.
Scholars writing in Europe and Asia at the time reported that the year 536 and the years following were bitterly cold. They described conditions that reminded them of an eclipse, and claim that the sun remained "small," with ice frosting up crops even in summer. That year and the decade following were also times of great famine, plague and war — possibly connected to the devastating harvests that left many people hungry, angry, and wandering in search of more fertile lands.
Over at New Scientist, Colin Barras has a terrific article about the scientific quest to discover whether these reports have any basis in reality. For years, scientists have studied tree rings and ice cores, looking for clues that could reveal whether the weather change was caused by a supervolcano (which have been known to cool the planet considerably).
Some promising evidence suggests there may have been a supereruption in El Salvador, which could help explain why Maya settlements nearby mysteriously stopped producing written records for a few years. But that wouldn't explain why the planet remained cold for many years. Usually a supervolcano only affects the weather for a year at most.