The mysterious global cooling of 536 AD

Interesting, detailed read.

Something traumatic appears to have affected the world in around the year 536 CE. The five reports that survive for this "536 event" say nothing of an eruption. They merely describe in vague terms a sort of unusual sun dimming or atmospheric veiling. The Roman statesman Cassiodorus, for example, describes a dim moon, and a sun that lost its "wonted light" and appeared "bluish," as if in "transitory eclipse throughout the whole year."

Mediterranean texts describe the 536 event as 12 or perhaps 18 months long, but Baillie surveyed trees from Ireland, Germany, Scandinavia and the U.S.A. that clearly show that the event lasted for roughly a decade. Tree rings also demonstrate that the 536 event was not a Byzantine oddity. Rather, it was vast: hemispheric or even global. Trees also reveal not one steady stretch of poor growth but a marked departure from normal growing conditions, with acute troughs and peaks. Multiple tree ring temperature reconstructions have found several of the coldest growing seasons (typically June-August) of the last two (or, in some cases, seven-and-a-half) thousand years fall within the sixth-century downturn.

The "Baillie bump," the forward-pushing of Larsen's eruptions (and now most first millennium eruptions detected in ice), placed major volcanism at each of the cooling episodes identified in tree ring data. Michael Sigl and a team of scientists recently included these results within an important synthesis of glacial volcanic eruption chronologies. It is still not clear which volcanoes erupted in 535/536 and 539/540 CE, but a cluster of volcanoes seem to have caused the downturn.

It is more difficult now to diminish the downturn or doubt that it triggered a marked, though temporary, demographic contraction in many regions of the world through its effects on plants. However, minimalist readings remain popular... A reluctance to engage with the palaeoclimate sciences and a willingness to write nature out of history have allowed historians to dismiss the significance of the 536 event for contemporary peoples.

Recently, more scientifically-minded historians, such as Michael McCormick, have offered more appropriate (if maximalist-leaning) narratives, in which cooling had moderate implications for sixth-century peoples. A vast, near-unparalleled environmental event need not have cataclysmic consequences to warrant study. Histories of resilience and adaptation to sudden and dramatic climate change should be as important and intriguing as histories of failure and collapse. This is clear in new work on the effects of the downturn, from the Yucatán to Fennoscandia, which emphasizes coping strategies and a certain hardiness in those that lived beneath the veils.

It should be emphasized that large eruptions do not simply chill the world. The effects on weather and climate are non-uniform. For instance, major near-equatorial volcanism is known to cause winter warming in North America, Europe, and Russia, but winter cooling in Western and Eastern Asia. Extratropical Northern Hemispheric volcanism cools hot and cold seasons alike. Seasonality matters too. That high latitude eruptions seem to be more impactful if they occur in summer could indicate that the 535/536 eruption happened in that season.

A few contemporary reports of despair and devastation seem hyperbolic. Did Italian mothers really eat their daughters? Did three quarters of the population north of the Yellow river really die off? Yet neither they, nor less-sensational descriptions, should be written off as lacking any grounding in the immediate post-eruption reality. Most sixth-century societies were able to absorb one bad year, but very few were able to absorb two or three. Back-to-back(-to-back) years of poor growing conditions, caused by a sharp cooling of average temperatures, were certain to take a toll.

http://www.historicalclimatology.com/blog/something-cooled-the-world-in-the-sixth-century-what-was-it