At https://popular-archaeology.com/article/unusual-climate-during-roman-tim… …. University of Helsinki seem to be catching up with Mike Baillie and David Keys on the severity of the weather around 536, and 541-5AD. Like both Keys and Baillie they point towards a sudden dip in global temperatures which they attribute to volcanic activity somewhere on the globe. In other words, the downturn, they6 claim, was due to an extended period of reduced sun light. Food production was hit (arable farms as well as animal husbandry) which rely on solar energy. Plots require warmth to grow. They claim the downturn in climate affected the northern hemisphere as a whole.
An annual growth tree ring calendar based on 'treeline pines' in Finland is now complimentary to other dendrochronologies in Europe and elsewhere, covering a period of around 7600 years. Not only did they date low insolation in 536 and 541-4 but the Justinian Plague, they say, broke out in 542 (adding to the distress). They also say it killed off half the population of Rome's eastern empire – but see also www.nature.com/articles/S41598-018-19760-w