The ices of Greenland and Antarctica bear the fingerprints of a monster: a gigantic volcanic eruption in 539 or 540 A.D. that killed tens of thousands and helped trigger one of the worst periods of global cooling in the last 2,000 years. Now, after years of searching, a team of scientists has finally tracked down the source of the eruption.We actually have quite good sources from the Roman Empire about this event. Procopius wrote:
The team’s work, published in Quaternary Science Reviews, lays out new evidence that ties the natural disaster to Ilopango, a now-dormant volcano in El Salvador. Researchers estimate that in its sixth-century eruption, Ilopango expelled the equivalent of 10.5 cubic miles of dense rock, making it one of the biggest volcanic events on Earth in the last 7,000 years.
And it came about during this year that a most dread portent took place. For the sun gave forth its light without brightness, like the moon, during this whole year, and it seemed exceedingly like the sun in 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 free neither from war nor pestilence nor any other thing leading to death. And it was the time when Justinian was in the tenth year of his reign [536/37]. (Procopius, Wars, IV.xiv.5-6, Tr. Dewing).This was 536 or 537 AD. The temperature dropped, harvests failed, people starved. But something much more sinister happened:
In terms of the historical consequences of all this, some have theorized that the climatic shift caused a migration of a certain breed of rats into the ports of Upper Egypt, which subsequently led to the Justinianic plague.The rats were adapted to a cooler climate in the upper Nile river valley, but the cooler weather allowed them to migrate all the way to the Nile delta. They brought with them fleas which brought Yersinia Pestis - the bacterium that causes bubonic plague. William Rosen has a very interesting book titled Justinian's Flea: The first great plague and the end of the Roman Empire. It describes the plague and how it put an end to Justinian's attempt to reconquer the western provinces that Rome had lost to the barbarians. 5,000 people a day perished in Constantinope, hollowing out both the tax system and army recruitment. After the plague finally burned out, the Empire simply didn't have the strength to complete the reintegration of the west.
The excellent History of Byzantium podcast has a (typically excellent) episode on Justinian's plague.
The moral of this story (and of others, like how the eruption of Mt. Tamboura gave rise to the "Year without a summer") is that the danger from climate change is not warming - and especially not gradual warming. Rather, it is sudden cooling that can (and has, repeatedly) killed millions.