The extreme weather events of 535–536 were the most severe and protracted short-term episodes of cooling in the Northern Hemisphere in the last 2,000 years. The event is thought to have been caused by an extensive atmospheric dust veil, possibly resulting from a large volcanic eruption conjectured to be either in Asia, the Americas, Europe, or other locations. Its effects were widespread, causing unseasonable weather, crop failures, and famines worldwide.
Byzantine historian Procopius recorded in 536 AD 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".
In 538, Roman statesman Cassiodorus described the following to one of his subordinates in letter 25:
Michael the Syrian (1126–1199), a patriarch of the Syriac Orthodox Church, recorded that during 536–537 the sun shone feebly for a year and a half.
The mid 10th-century Annales Cambriae record for the year 537:
Further phenomena were reported by a number of independent contemporary sources:
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 has been conjectured that the changes were caused by ashes or dust thrown into the air after the eruption of a volcano (a phenomenon known as "volcanic winter"), or after the impact of a comet or meteorite. The evidence of sulfate deposits in ice cores strongly supports the volcano hypothesis; the sulfate spike is even more intense than that 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 1984, R. B. Stothers postulated that the event might have been caused by the volcano Rabaul in what is now New Britain, in Papua New Guinea.
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 2009, Dallas Abbott of Columbia University's Lamont–Doherty Earth Observatory in New York published evidence from Greenland ice cores that multiple comet impacts may have caused the haze. The spherules found in the ice might originate from terrestrial debris ejected into the atmosphere by an impact event.
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 C.E. However, a more recent study, examining other evidence, now dates the eruption to the year 431 C.E.
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.
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.
The decline of Teotihuacán, a large city in Mesoamerica, is also associated with the droughts related to the climate changes, with signs of civil unrest and famines.
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 AD), 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 historian 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.