The sun turns black, earth sinks in the sea. Down from heaven, stars are whirled…
Thus sang the Seeress (the Völva) to Odin about the end of the old world in the Old Norse poem, Voluspá.  Of course it is incessantly debated exactly when the poem was (first) composed and how it was performed in the Old Norse Society. But whatever the answers to these questions are, there is no doubt that the poetic rendering of the cataclysmic event, which introduces the listener to the coming story of the apocalypse (Ragnarök) does uncannily remind us of the happenings in AD 536, when “it came about during this year that a most dreaded portent took place. For the sun gave forth its light like the moon without brightness during this whole year, and it seemed exceedingly like the sun in the eclipse, for the beams it shed were not clear nor such as it is accustomed to shed. And from the time, when this thing happened men were neither free from war nor pestilence nor any other thing leading to death (AD 536/37). 
A Cataclysmic Event
It is a well-known fact, that medieval chroniclers excelled in listing ominous portents and describe calamitous weather-conditions. Theirs was a world on the constant brink on catastrophes. Nevertheless the events, which took place in AD 536 – 550 (give and take a few years) were remarkable in view of the contemporary characterization, which was voiced in a series of chronicles and letters from the 6th century. It is not so much the fact that the sun dimmed. Nor that snow was severe and birds perished or the ocean was tumultuous with spray. Rather the remarkable feature was the prolonged character of these events. Of course the time-horizons given by the different chroniclers were never the same. Nor do the different writers seem to have experienced it in exactly the same way. However, the general feeling was that the events lasted for more than a year. In fact, we know now from scientific studies (dendrochronology and archaeology) that the events in 536 were nothing less than inaugurating a decade-long cold snap followed by reforestation, widespread hunger and plague – the so-called ‘migration period crisis’. This has especially been studied in a Northern European context by archaeologists.
Tree-rings or dendrochronology presented the first hard scientific witness to this. From Siberia in the East to Western America fossil tree-rings have shown that average summer-temperature dropped by 3 – 4 degrees between 536 –to 545 (550). However these studies have also shown that there probably were two dips – one in 536 and a second post 540. Although it cannot be precisely dated, paleobotanical evidence supports these findings, by demonstrating how reforestation took place on an unprecedented scale in the 6th century. It has been estimated that it took between five and eight generations to regain the former level of cultivation.
Archaeologists have complemented these findings with surveys of settlements in Sweden in the 6th century. In 2007 Hans Göthberg could demonstrate how the total number of occupied sites in Uppland fell by 75% (Twillight of the Gods p. 432). It is estimated from these and other surveys that the populations of Scandinavia were more or less halved. To what extent this massive crisis was the direct result of the deteriorating climate or more indirectly a result of the Justinian plague is probably no longer possible to decide.
A Complex Explanation
For a long time scientists have been puzzled by all this. On one hand a precisely datable climatic catastrophe obviously took place. On the other hand the exact nature was for a long time eluding the scientists, as studies of the ice-cores from Arctis and Antarctis did not yield any information about what took place. Was it a volcano or was it a intergalactic encounter with a comet or a steroid? New research seems to show that both events may have taken place inside the decade in question. Already in 2008 renewed analysis of the ice-cores showed a heightened level of sulphur signals. However, as they were found both in the Arctic and Antarctic ice-cores, it is believed that the volcanic eruption took place in the tropics. Careful dating of the eruption in El Salavdor of the Llopango Volcano have shown that a massive eruption took place in 535/36 and that it is estimated to have yielded 84 cubic kilometres of ash. Generally though volcanologists agree that ash-clouds from volcanoes disappear rather more quickly. It seems impossible that the devastating weather phenomena of three winters and two summers around 535 plus the following period of lower temperature can all be caused by this eruption.
Enter another team of researchers, who have analysed the ice-cores and made yet another find: a high concentration of metallic spherules consisting of nickel and tin. Both metals are abundant in extra-terrestrial debris and are moreover unlikely to reflect contamination from the harvesting of the ice-cores. Another remarkable find in the ice-cores are fossils of ancient marine microorganisms. How did they end up in the middle of the polar ice-shields? The scientists simply believe that they are the result of a collision of earth with some major fragments from Halley’s comet. Around the same time as the eruption in El Salvador took place, they may have slammed into the Gulf of Australia – or as has recently been claimed: the North Sea near Norway – and made a gargantuan splash of water, rocks and dust, which was sent orbiting in the stratosphere.
Archaeologists have long been convinced that the events in 535 did have devastating effects. First of all it has been argued that the mighty winter was later echoed in the myths conserved in the Norse literature from the beginning of the second millennium (the stories of the Fimbul winter ending in Ragnarök). Recently it has been argued that major cosmological changes can be dated precisely to this period, primarily the abandonment of the old sun-cult and the introduction of a new cult focusing on Odin. A third happening was the enormous amounts of gold sacrificed in the beginning of the 6th century, believed to be offered in order to placate the old Gods. Afterwards the economy was based on silver. In a Christian context it has recently been argued that the great ‘dust veil’ inspired the 6th century Christian cleric, Gildas, to write his history, De Excidio et Conquestu Britanniae, which recounts the late-Roman history of Britain, and which is the only substantial source for history of this period written by a near-contemporary individual living in Northern Europe. A third hypothesis is that the images of Grendel in Beowulf as well as The Celtic Myths have roots in the sightings of the Hailey Comet in the mid 530s.
To what extent other changes in the 6th century can be more or less directly derived from the cataclysmic events of 535+ is somewhat less obvious. However the social elite seems to have emerged strengthened from the crisis building new power-centres, introducing a new burial tradition (tumuli), creating a new form of central meeting places (mead-halls), and a new aesthetic (the animal style). All this was buttressed through the creation of ideas of “royal” descent and “sacred” lineages (the hall farm families) as well as a whole new form of religion (Odin-cult). Yet another part of this seems to have been an intensification of animal husbandry and a changed diet. It is probably totally uncalled for to claim that these complex socio-cultural, economic and cosmological shifts were univocally caused by the catastrophic events in 536 – 550. However, it seems reasonable to at least acknowledge that these new ways of life and worldviews were snowballed into existence by the catastrophes. (Just as dramatic changes of the same order were invoked in the 15th century, post-plague). Where the inspiration to the new life-style came from? Perhaps the Huns! But that is quite another story
The stanza (No 55) in Old Norse (which sounds much better): Sol ter sortna sigr fold imar hv erfa af hi mni heiðar stiornor From: Völuspá: The prophecy of the Seeress, verse 55  From: Procopius of Caesarea: History of the wars 4.14.5-6. In: Procopius. Transl. by H. B. Dewing. London 1914 – 40.
Akseli Gallen Kallela: Lemminkainens Mother 1897. Inspired by the Finnish Poem: Kalevala, which perhaps also contains reminiscences of the events in AD 536
READ MORE (a selection)
The contemporary sources:
The Mystery Cloud of 536 CE in the Meditterranean Sources. By Antti Arjava In: Dumbarton Oaks Papers 2005, Vol 59 pp. 73 -94
New research about the linkage between climate and socio-cultural change:
Climate and Causation in the Swedish Iron Age. By: Mats Widgren In: Geografisk Tidsskrift – Danish Journal of Geography. 2012 Vol. 2, pp. 126 – 134 Some Perspectives on Societal Impacts of Past Climatic Changes. By Henry Diaz and Valerie Trouet In: History Compass 2014, Vol. 12, No.2 pp. 160 – 177 An Iron-Age Schock Doctrine – Did the AD 536–7 event trigger large-scale social changes in the Mälaren Valley Area? By Daniel Löwenborg In: Journal of Archaeology and Ancient History 2012, No. 4: pp. 3 – 29
New research into the causes of the events AD 536
The Year of Darkness. By: Collin Barras In: New Scientist, 02624079, 1/18/2014, Vol. 221, Issue 2952 (with links to further literature) Why Halley’s Comet May Be Linked to Famine 1,500 Years Ago By Mike Wall In: Live Science 18.12. 2013. Did a conflagration of Comets contribute Dust to the Earth and cause Climate Downturns between 532 and 542 A.D.? Slides from a presentation given by Dallas Abbott 2013
Changes in Cosmology
Tracing Old Norse Cosmology. The World tree, middle earth and the sun from archaeological perspective. By Anders Andrén. Series: Vägar till Midgård vol 16. Nordic Academic Press 2014 Twilight of the Gods? The ‘dust veil event’ of AD 536 in critical perspective. By Bo Gräslund and Neil Price In: Antiquity 2012 Vol 86: 428 -443 Fimbulvintern, Raganarök och klimatkrisen år 536-537 e. Kr. By Bo Gräslund In: Saga och sed : Kungl. Gustav Adolfs akademiens årsbok 2007, pp 93 – 123 Gildas and the Mystery Cloud of 536–7 By David Woods In: Journal of Theological Studies, 2010 Vol. 61, pp. 226 – 234 The Celtic Gods: Comets in Irish Mythology By Patrick McCafferty and Mike Baillie Tempus 2005 ISBN-10: 0752434446 ISBN-13: 978-0752434445
The Gold Hoards
The Year 536 and the Scandinavian Gold Hoards By Morten Axboe In: Medieval Archaeology 1999, Vol. XLIII, pp. 186 – 188 Amulet Pendants and a Darkened Sun By Morten Axboe In: KVHAA Konferenser 2001, Vol. 51 pp. 119 – 136
The changes in Landscape and Socio-Cultural Organisation
Iron Age Myth and Materiality. An archaeology of Scandinavia AD 400-1000. By Lotte Hedeager Routledge 2011 ISBN 978-0-415-60602-8 The Early Iron Age in South Scandinavia: Social Order in Settlement and Landscape By Frands Herschend Series: OPIA 46 Department of Archaeology and Ancient History, Uppsala University 2009