New analysis of ancient ice cores suggests a pair of volcanic eruptions in the 6th century explains a period of societal upheaval in the Northern Hemisphere.

Between A.D. 536 and 537, the historians of late antiquity wrote about the arrival of a "mystery cloud" that cast a shadow on the Mediterranean for several years. Another dimming event is described around A.D. 540.

Researchers have previously discovered tree rings that suggest the Northern Hemisphere suffered stunted growth during the period. And now, scientists have direct evidence of the volcanic activity that likely caused the dimming of the sun.

Scientists with the Helmholtz Centre for Ocean Research located traces of volcanic sulfur in ice cores from Greenland and Antarctica.

Using ice core data and historical descriptions of the dimming, researchers developed a model predicting the general geographical origins of the volcanic eruptions and the development of large aerosol clouds.

The simulation predicted the formation of sulfur clouds to most directly effect the Northern Hemisphere for several years. During the dimming events of 536 and 540, temperatures dropped an average of 2 degrees Celsius.

Researchers believe their findings fit neatly with archaeological evidence of societal turmoil in Scandinavia. It's likely depressed temperatures and a lack of sun resulted in crop failures and famine -- stressors that may have ignited societal conflict.

"Each one of the eruptions of 536/540 would have strongly impacted societies, and it happened twice within four years," Kirstin Kruger, a researcher at the University of Oslo, said in a news release.

The researchers published their findings in the journal Climatic Change.

The study's authors say further analysis is needed to determine which exact volcanoes are to blame.

"Several candidates are being discussed, including volcanoes in Central America, Indonesia and North America," said Matthew Toohey, a researcher with the Helmholtz Centre. "Future studies will be necessary to show the exact source of the aerosol clouds of 536/540."