Volcanic winter of 536
The volcanic winter of 536 was the most severe and protracted episode of climatic cooling in the Northern Hemisphere in the last 2,000 years. The volcanic winter was caused by an eruption, with several possible locations proposed in various continents. Most contemporary accounts of the volcanic winter are from authors in Constantinople, the capital of the Eastern Roman Empire, although the impact of the cooler temperatures extended beyond Europe. Modern scholarship has determined that in early 536 CE (or possibly late 535) an eruption ejected massive amounts of sulfate aerosols into the atmosphere, which reduced the solar radiation reaching the Earth's surface and cooled the atmosphere for several years. In March 536, Constantinople began experiencing darkened skies and cooler temperatures.
Summer temperatures in 536 fell by as much as 2.5 degrees Celsius (4.5 Fahrenheit degrees) below normal in Europe. The lingering impact of the volcanic winter of 536 was augmented in 539–540 when another volcanic eruption caused summer temperatures to decline as much as 2.7 degrees Celsius (4.9 Fahrenheit degrees) below normal in Europe. There is evidence of still another volcanic eruption in 547 which would have extended the cooler period. The volcanic eruptions, accompanied by the Plague of Justinian, which began in 541, caused crop failures, famine, and millions of deaths and initiated the Late Antique Little Ice Age, which lasted from 536 to 660.
Medieval scholar Michael McCormick has written that 536 was the worst year in history to be alive. "It was the beginning of one of the worst periods to be alive, if not the worst year." 
Byzantine historian Procopius recorded in 536 CE in his report on the wars with the Vandals, "during this year a most dread portent took place. For the sun gave forth its light without brightness … and it seemed exceedingly like the sun in eclipse, for the beams it shed were not clear".
- The sun's rays were weak, and it appeared a "bluish" colour.
- At noon, no shadows from people were visible on the ground.
- The heat from the sun was feeble.
- The moon, even when full, was "empty of splendour"
- "A winter without storms, a spring without mildness, and a summer without heat"
- Prolonged frost and unseasonable drought
- The seasons "seem to be all jumbled up together"
- The sky is described as "blended with alien elements" just like cloudy weather, except prolonged. It was "stretched like a hide across the sky" and prevented the "true colours" of the sun and moon from being seen, along with the sun's warmth.
- Frosts during harvest, which made apples harden and grapes sour.
- The need to use stored food to last through the situation.
- Subsequent letters (no. 26 and 27) discuss plans to relieve a widespread famine.
- "A failure of bread in the year 536 AD" – the Annals of Ulster
- "A failure of bread from the years 536–539 AD" – the Annals of Inisfallen
The mid-10th-century Annales Cambriae record for the year 537:
- "The Battle of Camlann, in which Arthur and Medraut fell, and there was great mortality in Britain and Ireland."[a]
Further phenomena were reported by a number of independent contemporary sources:
- Low temperatures, even snow during the summer (snow reportedly fell in August in China, which caused the harvest there to be delayed)
- Widespread crop failures
- "A dense, dry fog" in the Middle East, China and Europe
- Drought in Peru, which affected the Moche culture
Tree ring analysis by dendrochronologist Mike Baillie, of the Queen's University of Belfast, shows abnormally little growth in Irish oak in 536 and another sharp drop in 542, after a partial recovery. Ice cores from Greenland and Antarctica show evidence of substantial sulfate deposits in around 534 ± 2, which is evidence of an extensive acidic dust veil.
It was originally theorized that the climatic changes of 536 AD were caused by one of three possible sources, either:
In 2008, newly found evidence was obtained of sulfate deposits in ice cores. This evidence strongly supported the volcano hypothesis and ruled out the likelihood of an "extraterrestrial causal event" such as meteors or comets. The sulfate spike in the 6th century AD was even more intense than the spike which accompanied the lesser episode of climatic aberration in 1816, popularly known as the "Year Without a Summer", which has been connected to the explosion of the volcano Mount Tambora in Sumbawa.
In 1999, David Keys suggested that the volcano Krakatoa exploded at the time and caused the changes. It is suggested that an eruption of Krakatoa described as occurring in 416 by the Javanese Book of Kings actually took place in 535–536, there being no other evidence of such an eruption in 416.: 385
In 2010, Robert Dull, John Southon, and colleagues presented evidence suggesting a link between the Tierra Blanca Joven (TBJ) eruption of the Ilopango caldera in central El Salvador and the 536 event. Although earlier published radiocarbon evidence suggested a two-sigma age range of 408–536, which is consistent with the global climate downturn, the connection between 536 and Ilopango was not explicitly made until research on Central American Pacific margin marine sediment cores by Steffen Kutterolf and colleagues showed that the phreatoplinian TBJ eruption was much larger than previously thought. The radioactive carbon-14 in successive growth increments of a single tree that had been killed by a TBJ pyroclastic flow was measured in detail using accelerator mass spectrometry; the results supported the date of 535 as the year in which the tree died. A conservative bulk tephra volume for the TBJ event of ~84 km3 was calculated, indicating a large Volcanic Explosivity Index 6+ event and a magnitude of 6.9. The results suggested that the Ilopango TBJ eruption size, latitude, and age are consistent with the ice core sulphate records of Larsen et al. 2008. Later research suggested date 539/540 CE. However, a more recent study, examining other evidence, now dates the eruption to the year 431 CE.
A 2015 study further supported the theory of a major eruption in "535 or early 536", with North American volcanoes considered a likely candidate. It also identified signals of a second eruption in 539–540, likely to have been in the tropics, which would have sustained the cooling effects of the first eruption through to around 550.
In 2018, Harvard University researchers suggested the cause was a volcanic eruption in Iceland that erupted in early 536. However, the author of the previous study said to Science magazine that the evidence is insufficient to discard the North America hypothesis.
To date there is not yet a widely agreed upon single source volcano for the volcanic winter that began in 536 AD, and the possibility remains that the extreme cold of 536 AD - 540 AD may have been the result of multiple volcanic events during those years.
The 536 event and ensuing famine have been suggested as an explanation for the deposition of hoards of gold by Scandinavian elites at the end of the Migration Period. The gold was possibly a sacrifice to appease the gods and get the sunlight back. Mythological events such as the Fimbulwinter and Ragnarök are theorized to be based on the cultural memory of the event.
A book written by David Keys speculates that the climate changes contributed to various developments, such as the emergence of the Plague of Justinian (541–549), the decline of the Avars, the migration of Mongolian tribes towards the West, the end of the Sassanid Empire, the collapse of the Gupta Empire, the rise of Islam, the expansion of Turkic tribes, and the fall of Teotihuacán. In 2000, a 3BM Television production (for WNET and Channel Four) capitalized upon Keys' book. The documentary, under the name Catastrophe! How the World Changed, was broadcast in the US as part of PBS's Secrets of the Dead series. However, Keys and Wohletz's ideas lack mainstream acceptance. Reviewing Keys' book, British archaeologist Ken Dark commented that "much of the apparent evidence presented in the book is highly debatable, based on poor sources or simply incorrect. [...] Nonetheless, both the global scope and the emphasis on the 6th century AD as a time of wide-ranging change are notable, and the book contains some obscure information which will be new to many. However, it fails to demonstrate its central thesis and does not offer a convincing explanation for the many changes discussed".
The philologist Andrew Breeze in a recent book (2020) argues that some King Arthur events including the Battle of Camlann are historical, happening in 537 as consequence of the famine associated with the climate change of the previous year.
- Tierra Blanca Joven eruption
- 1257 Samalas eruption
- Great Famine of 1315–1317
- 1815 eruption of Mount Tambora, largest ever recorded
- Justinian I, Roman emperor at the time
- 1452/1453 mystery eruption
- Minoan eruption
- Year Without a Summer, 1816
- 946 eruption of Paektu Mountain
- Volcanism of Iceland
- The battle is dated 539 in some editions.
- Abbott, D. H.; Biscaye, P.; Cole-Dai, J.; Breger, D. (December 2008). "Magnetite and Silicate Spherules from the GISP2 Core at the 536 A.D. Horizon". AGU Fall Meeting Abstracts. American Geophysical Union, Fall Meeting 2008. Vol. 41. pp. 41B–1454. Bibcode:2008AGUFMPP41B1454A. Abstract #PP41B-1454.
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- Procopius; Dewing, Henry Bronson, trans. (1916). Procopius. Vol. 2: History of the [Vandalic] Wars, Books III and IV. London, England: William Heinemann. p. 329. ISBN 978-0-674-99054-8.
- Ochoa, George; Jennifer Hoffman; Tina Tin (2005). Climate: the force that shapes our world and the future of life on earth. Emmaus, PA: Rodale. ISBN 978-1-59486-288-5., gives this quote as "The Sun gave forth its light without brightness, like the moon during this whole year, and it seemed exceedingly like the Sun in eclipse".
- Cassiodorus; Hodgkin, Thomas, trans. (1886). The Letters of Cassiodorus. London, England: Henry Frowde. pp. 518–520. See: "25. Senator, Praetorian Praefect, to his deputy Ambrosius, an Illustris."
- Michel le Syrien; Chabot, J.-B., trans. (1901). Chronique de Michel le Syrien, Patriarche Jacobite d'Antoche [Chronicle of Michael the Syrian, Jacobite Patriarch of Syria] (in French). Vol. 2nd vol. Paris, France: Leroux. pp. 220–221. From pp. 220–221: "Or, un peu auparavant, en l'an 848, il y eut un signe dans le soleil. … , et le vin avait le goût de celui qui provient de raisins acides." (However, a little earlier, in the year 848 [according to the Greek calendar; 536/537 AD according to the Christian calendar], there was a sign in the sun. One had never seen it [before] and nowhere is it written that such [an event] had happened [previously] in the world. If it were not [true] that we found it recorded in most proven and credible writings, and confirmed by men worthy of belief, we would not have written it [here]; for it's difficult to conceive. So it is said that the sun was darkened, and that its eclipse lasted a year and a half, that is, eighteen months. Every day it shone for about four hours and yet this light was only a feeble shadow. Everyone declared that it would not return to the state of its original light. Fruits didn't ripen, and wine had the taste of what comes from sour grapes.)
- Gaelic Irish Annals translations
- Documents of Ireland
- The Annals of the Four Masters
- "Camlan | Robbins Library Digital Projects". Retrieved 2018-07-31.
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- Baillie, M.G.L. (1994). "Dendrochronology Raises Questions About the Nature of the AD 536 Dust-Veil Event." The Holocene fig. 3 p. 215.
- Larsen, L. B.; Vinther, B. M.; Briffa, K. R.; Melvin, T. M.; Clausen, H. B.; Jones, P. D.; Siggaard-Andersen, M.-L.; Hammer, C. U.; et al. (2008). "New ice core evidence for a volcanic cause of the A.D. 536 dust veil". Geophys. Res. Lett. 35 (4): L04708. Bibcode:2008GeoRL..3504708L. doi:10.1029/2007GL032450.
- Wohletz, Ken, Were the Dark Ages Triggered by Volcano-Related Climate Changes in the 6th Century? Archived 2003-06-18 at the Wayback Machine
- Baillie, M. G. L. (1999). Exodus to Arthur: Catastrophic Encounters with Comets. London: B.T. Batsford. ISBN 978-0-7134-8352-9.
- Rigby, Emma; Symonds, Melissa; Ward-Thompson, Derek (February 2004). "A comet impact in AD536?". Astronomy and Geophysics. 45 (1): 1.23–1.26. Bibcode:2004A&G....45a..23R. doi:10.1046/j.1468-4004.2003.45123.x.
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- "536 and all that", from Real Climate, March 2008.
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