In the years 535 CE and 536 CE, several remarkable aberrations in world climate took place. The Byzantine historian Procopius recorded of 536 CE, "during this year a most dread portent took place. For the sun gave forth its light without brightness ... and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear.". Tree ring analysis by dendrochronologist Mike Baillie, Queen's University, Belfast, shows abnormally little growth in Irish oak in 536 CE and another sharp drop in 542 CE, after a partial recovery. Similar patterns are recorded in tree rings from Sweden and Finland, in California's Sierra Nevada and in rings from Chilean Alerce trees.

Further phenomena reported by a number of independent contemporary sources:

• low temperatures, even snow during the summer
• dark clouds, only few hours of sunlight during the day
• floods in formerly dry regions
• crop failures

It has been conjectured that these changes were due to ashes or dust thrown into the air after the impact of a comet or meteorite, or after the eruption of a volcano (a phenomenon known as "volcanic winter").

A similar, lesser episode of climatic aberration was also observed in 1816 CE, popularly known as the Year Without A Summer, which has been connected to the explosion of the volcano Tambora in Sumbawa, Indonesia.

In a 1999 book, David Keys, supported by work of the American volcanologist Ken Wohletz, suggested that the Indonesian volcano Krakatoa exploded at the time and caused the changes. He further speculated that the climate changes may have contributed to various developments, such as the emergence of bubonic plague (the Plague of Justinian), the migration of Mongolian tribes towards the West, the end of the Persian empire, the rise of Islam and the end of various civilizations in Central and South America. PBS based a documentary, Catastrophe!, on Keys and Wohletz' ideas. These ideas are not widely accepted at this point.

Further Reading

• David Keys: Catastrophe: A Quest for the Origins of the Modern World, Ballantine Books, New York, 1999
• Ken Wohletz: Were the Dark Ages Triggered by Volcano-Related Climate Changes in the 6th Century?
• Joel D. Gunn (ed.): The Years without Summer: Tracing A.D. 536 and its Aftermath. ISBN 1841710741
• Simon Winchester: Krakatoa: The Day the World Exploded, August 27, 1883. ISBN 0066212855, 2003