By analyzing 1,500-year-old tree-rings, scientists have found that temperatures dropped an average of three degrees celsius during the 6th century.
Lead researcher Ulf Büntgen calls it: “The most dramatic cooling in the Northern Hemisphere in the past 2,000 years.”
Disease and famine spread across the world, killing millions of people over the ensuing centuries and contributing to the collapse of the Roman Empire.
Meanwhile, the cooling brought more rain to the Arabian Peninsula. The ensuing increase in vegetation allowed the Arab Empire to bloom, affording it a larger army.
The researchers believe this cooling, which they dubbed the Late Antique Little Ice Age, was triggered by three volcanic eruptions in 536, 540 and 547CE.
I would take it one step further, of course, and propose that those eruptions were triggered by the sunspot cycle.
Thanks to Vance for this link