Some context
In the West, the century marks the end of Classical Antiquity and the beginning of the Middle Ages. The collapse of the Western Roman Empire late in the previous century left Europe fractured into many small Germanic kingdoms competing fiercely for land and wealth. From the upheaval the Franks rose to prominence and carved out a sizeable domain covering much of modern France and Germany. Meanwhile, the surviving Eastern Roman Empire began to expand under Emperor Justinian, who recaptured North Africa from the Vandals and attempted fully to recover Italy as well, in the hope of reinstating Roman control over the lands once ruled by the Western Roman Empire.
In its second Golden Age, the Sassanid Empire reached the peak of its power under Khosrau I in the 6th century. The classical Gupta Empire of Northern India, largely overrun by the Huna, ended in the mid-6th century. In Japan, the Kofun period gave way to the Asuka period. After being divided for more than 150 years among the Southern and Northern Dynasties, China was reunited under the Sui Dynasty toward the end of the 6th century. The Three Kingdoms of Korea persisted throughout the century. The Göktürks became a major power in Central Asia after defeating the Rouran.
In the Americas, Teotihuacan began to decline in the 6th century after having reached its zenith between AD 150 and 450. Classic Period of the Maya civilization in Central America.
This is the Justinian Age, basically in which he tried to conquer all former Roman territories after its collapse, but only partially succeeds. One can say that he would've succeeded if some things didn't happen.
1.
The Justinian Pandemic disproportionally affecting populous and urban areas, partially weakening the Byzantians but also weakening the Sassanids to the east (than Persian dynasty), which would weaken Persia, weaken the Zoroastrians, while a few decades later the prophet Mohamed would be born.
2. a - A series of volcanic eruptions and abrupt cooling would take place. The first of these would be in 535/536, and
would lead to a volcanic winter during 536 especially hitting the northern hemisphere very hard, the more northern the location the more harder hit.
It is to be assumed that the myth of Ragnarok and especially Fimbulwinter dates from here, since evidence suggests that Scandinavia in particular was hit very hard. The culprit isn't clear but it is clear though that it would be Icelandic or Alaskan, possibly Kamchatkan/Aleutian. Most likely Icelandic due to proximity to Europe.
Suggested is that this kickstarted some migratory waves (this is the period of Late Antiquity Migration Period) where some people moved southwest towards Europe, including slavic people, but also Turkish people and also Hungarian people from the Urals, also Bulgarians would move west here. There was a lot of movement around this time...
possibly due to a lot of open empty space to be occupied since it is to be assumed quite a lot died due to the aforementioned pandemic and severe winter impacts. It seems likely that the events are underestimated, simply because of the consequences we observe.
This also predates the period of muslim growth, probably because the influence of the Persians and Sassanid Empire declined and allowed smaller kingdoms and regions deeper in the Arab Peninsula to thrive, also being less affected possibly by the pandemic (less urban development) and also being located in a warmer climate, however this much cold would have led to drought elsewhere too.
b - Around 540 another volcanic eruptions seems to have taken place, possibly being even more severe than the former one but with less regional impacts on Europe than the 535/536 but more globally seen. This is an eruption that happened close to the Equator, where exactly is not known and the culprit has not been found yet.
Ilopango in Nicaragua is a hypothesized eruption crater but seems to have taken place a few centuries earlier. Another possible eruption that took place but where the date is uncertain is
an earlier eruption of Krakatoa, likely far more explosive than the
more famous eruption from 1883.
c. This was followed by yet another eruption, less huge than the former two ones, or at least less impactful but one attributed to have prolonged the already cold episode, dated back to 547AD, likely occuring again in the Tropes.
This all seems to have kickstarted the
Late Antiquity Little Ice Age which seems to have a more abrupt start than the "modern Little Ice Age" or the more recent one.
The cooling period coincided with the Plague of Justinian, which began in 541, though the connection between the plague and the volcanoes still remains tenuous. The cooling period contributed to the migrations of the Lombards and the Slavs into Roman territory in Italy and the Balkans
According to research by a team from the Swiss Federal Research Institute at Birmensdorf, the fall in temperatures led to the Arabian Peninsula experiencing a dramatic increase in fertility. The boost of food supply contributed to the Arab expansion beyond the peninsula in the Islamic conquests. The cooling period also led to increased strain on the Eastern Roman Empire and the Sassanid Empire, which helped the Muslim conquest of the Levant, the Muslim conquest of Egypt and the Muslim conquest of Persia.
According to research done by Israeli scientists, in 540, the size of the population of the city of Elusa, in the Negev Desert, and the amount of garbage that it generated started to shrink greatly. Elusa housed tens of thousands of people during its height. The major decline took place around the mid-6th century, about a century before the Islamic conquest. One possible explanation for the crisis was the Late Antique Little Ice Age.
Some theorize that this is the period of time where the Roman Empire ceased to exist. Justinian I was on its way to restore to its former glory. He was one of the last emperors to truly speak Roman and Romans still taught thay were Romans, there was no "eastern roman" definition or "byzantine" way of thinking. They saw theirselves still as Romans. This is the moment were probably hopes of restoring the Roman Empire was lost, and that changed so much in history that basically this should be IMO seen as the start of the Middle Ages, not the actual fall of the Western Roman Empire. This is were the Empire lost its identity.
I believe this is even the moment in time were the nordic people had such a setback that they lost possible contact with the other European people, and that this is "fimbolwinter". I don't think we would have seen Vikings, because Vikings really became Vikings because Europeans were surprised to see something as Vikings, they were forgotten in history. While all of Europe became christianized in those 2.5 centuries, the nordic areas did not. They still had their originally german-nordic gods.
Obviously one way Christianity was able to expand was through Roman infrastructure and development which meant the parts that used to be Roman, it was easier to spread in "all roads lead to Rome", that's 1. But these were also periods of despair and I believe the 6th century was a period of time were people needed a belief that actually help them deal with misery back at the time. Christianity at the time could offer them that relief.
And also 476AD as the start date for the middle ages is an incredible western way of thinking, if that just affects that part of the world and even isn't really the end of the Roman Empire specifically. One could argue that 536AD is the better date for a start date of the Middle Ages period.
I think this part of history is underlooked.
