A massive volcanic eruption brought gloom to the post-Roman world.
An enormous volcanic eruption in the sixth century seems to have triggered catastrophic global cooling, perhaps precipitating famine, cultural conflict and plague across the planet.
The theory offers an explanation for why historical records from the period make references to dimmer skies and a cooler climate. Documents from Ireland, for example, describe “a failure of bread” for a few years after 536 AD, and there are also accounts of summertime snow in China.
The Byzantine historian Procopius wrote that in 536 “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.”
Scientists have been studying evidence of this cold snap for more than 20 years. Tree rings from this time, for example, show that growth was slowed down by cold weather. The cause of these climate changes has been controversial. Now, however, it seems a volcano was to blame.
Climate cooling is consistent with the idea that the atmosphere became shrouded in a haze of dust, as is known to happen after large volcanic eruptions. A dust veil, causing cooling and spectacular sunsets, appeared after the eruptions of Tambora in Indonesia in 1815, and of Mount Pinatubo in the Philippines in 1991.
“Any normal interpretation of the data would point to a volcanic origin,” says Keith Briffa, a palaeoclimatologist at the University of East Anglia in the UK and an author of the new study. But no one has previously been able to find clear evidence for that, leaving room for other theories. In particular, palaeoecologist Mike Baillie at Queen’s University in Belfast has proposed that the dust veil was created by a meteorite impact.
Now Briffa and an international team of collaborators have found the characteristic fingerprint of a volcanic eruption in layers of ice in the Greenland and Antarctic ice sheets, narrowly dated to around 533-536. They think that the eruption happened in 535, and that its effects were felt in the Northern Hemisphere in the following year, they write in the journal Geophysical Research Letters1.
This fingerprint takes the form of sulphate ions, formed from the sulphur dioxide released by volcanoes. Meteorite impacts do not tend to release sulphur into the atmosphere unless they happen to hit sulphur-rich minerals.
The fact that a sulphate layer can be seen not only in the northern but also the southern polar region implies that the eruption probably happened close to the Equator, so that its dust was dispersed all over the Earth. But the north bore the brunt of the climate effects – there’s no sign of cooling in geological records from New Zealand, for example.
Briffa explains that, although it’s not hard to spot such sulphate contamination of ice in principle, the hard part is constraining the dates of the layers enough to link them to the tightly dated tree-ring evidence of cooling, and to distinguish them from sulphate that came from smaller but nearer eruptions. This careful detective work was carried out by Bo Vinther and his co-workers at the University of Copenhagen, Denmark.
'Most severe event'
The researchers say that the amount of sulphate they found implies that the eruption was even bigger than that of Tambora, perhaps releasing as much as 40% more dust. This, says Briffa, makes the event “the most severe volcanic cooling event in the Northern Hemisphere in the past 2,000 years”.
Some have suggested that its consequences were more dramatic than mere crop failure and overcast skies. Around 541, a plague pandemic rampaged from southern Asia to Denmark, wreaking havoc in the Byzantine Empire and possibly killing about 40% of the inhabitants of its capital Constantinople.
This so-called Justinian plague (after the Byzantine emperor Justinian I) might have been triggered by cold weather, during which it may be easier for plague-bearing fleas to reproduce. After hearing Baillie talk on the 536 AD event, British writer David Keys wrote a book in 1999 claiming that the cooling reshaped global history, playing a part in the rise of Islam and the migration of the Mongols. Keys proposed that the cooling was caused by an eruption of Krakatoa.
While historians think Keys’s case is tenuous, it now seems clear that a volcano did make itself felt around the world at that time. Now that the cause is clearer, says Briffa, “I think that historians may start to find more evidence of the event.”
Larsen, L. B. et al. Geophys. Res. Lett. doi:10.1029/2007GL032450 (2008).