Ann Gibbons, writing for Science.org:
Ask medieval historian Michael McCormick what year was the worst to be alive, and he’s got an answer: “536.”
A mysterious fog plunged Europe, the Middle East, and parts of Asia into darkness, day and night—for 18 months… initiating the coldest decade in the past 2300 years. Snow fell that summer in China; crops failed; people starved.
Now, an ultraprecise analysis of ice from a Swiss glacier by a team led by McCormick and glaciologist Paul Mayewski… reported that a cataclysmic volcanic eruption in Iceland spewed ash across the Northern Hemisphere early in 536. Two other massive eruptions followed, in 540 and 547.
The team deciphered this record using a new ultra–high-resolution method, in which a laser carves 120-micron slivers of ice, representing just a few days or weeks of snowfall, along the length of the core… The approach enabled the team to pinpoint storms, volcanic eruptions, and lead pollution down to the month or even less, going back 2000 years
120 microns is roughly the diameter of a single grain of table salt.