http://www.sciencemag.org/news/2018/11/top-stories-earth-s-darkest-year-errors-ocean-study-and-young-crater-under-greenland-s

Why 536 was ‘the worst year to be alive’

After analyzing volcanic glass particles in ice from a Swiss glacier, a team of researchers has identified why some medieval historians say 536 was the worst year to be alive. Early that year, a cataclysmic volcano in Iceland spewed ash across the Northern Hemisphere, creating a fog that plunged Europe, the Middle East, and parts of Asia into darkness—day and night—for 18 months. Summer temperatures dropped 1.5°C to 2.5°C, initiating the coldest decade in the past 2300 years.