Scientists have long known that Europe and Asia went through a cold snap some 1,500 years ago, but a new study has found evidence that the chill might have had a major impact on history. By measuring tree ring data in Russia, an international team of researchers has created a chronology of the temperatures from the cooling period, which they dub the “Late Antique Little Ice Age.” Along with highlighting its potential causes, they argue that the temperature drop might have played a role in a string of cataclysmic events including pandemics, food shortages, mass migrations and the rise and fall of empires.

Ancient historical records show that the year 536 A.D. was marked by the sudden appearance of a “dust veil” or “dry fog” that dimmed the sun and thrust the Northern Hemisphere into a period of bitter cold. “And it came about during this year that a most dread portent took place,” wrote the Byzantine chronicler Procopius. “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 that it shed were not clear nor such as it is accustomed to shed.” Other sources mention crop failures, summertime snowfalls, and fruits withering “at an unseasonable time.”

The mysterious dust cloud is now believed to have been the beginning of a cold spell triggered by a trio of catastrophic volcanic eruptions that took place in 536, 540 and 547 A.D. The eruptions would have spewed a massive amount of ash and aerosols into the atmosphere, blocking the sun’s rays and causing a substantial drop in temperatures. Scientists previously thought that the chill lasted only a few years, but a new study in the journal Nature Geoscience has suggested that the volcanic activity ushered in an unprecedented “Late Antique Little Ice Age” (LALIA) that extended for over a century and contributed to widespread famines, plagues, political upheaval and even the fall of empires.

New tree-ring width measurements from the Russian Altai mountains indicate a drastic cold period 1,500 years ago. (Credit: Vladimir S. Myglan)
New tree-ring width measurements from the Russian Altai mountains indicate a drastic cold period 1,500 years ago. (Credit: Vladimir S. Myglan)

The study’s authors reached their conclusions after using tree ring data to reconstruct summer temperatures in central Asia for the last two millennia. Working with a team of more than a dozen climatologists, naturalists, historians and linguists, the Swiss Federal Research Institute’s Ulf Büntgen measured the width of the rings of more than 650 ancient trees from the Altai-Sayan Mountains in Russia. Trees grow new rings each summer, and their variations in size can provide vital information about how climate has changed over time. “Trees growing at the upper tree line are very sensitive to small changes in summer temperature,” Büntgen explained to the Washington Post. “If it’s a cooler summer, the rings are more narrow.”

After assembling a chronology from the Altai-Sayan samples, Büntgen and his colleagues found that central Asia went through a period of extended cooling from around 536 to 660 A.D. This closely matched the data from a similar research project that Büntgen and others conducted in the Austrian Alps a few years ago. Taken together, the two studies suggest that the newly christened “Late Antique Little Ice Age” was even more extreme than the more famous “Little Ice Age” that unfolded between 14th and 19th centuries. The trio of volcanic eruptions would have been the main factor, but the authors say the drop in temperatures was probably sustained by increases in sea ice and the effects of a solar minimum—a long interval of diminished sunspot activity. “This was the most dramatic cooling in the Northern Hemisphere in the past 2,000 years,” Büntgen said in a press release.

The mini ice age decreased average summer temperatures in central Asia by as much as four degrees Celsius and resulted in 13 of the 20 coldest decades of the past two millennia, and it may have contributed to a period of widespread chaos in the Northern Hemisphere. The researchers note that crop failures and famine were reported across Europe and Asia during the 6th and 7th centuries. Meanwhile, just five years after the 536 A.D. volcanic eruption, the Byzantine kingdom of Justinian was rocked by a virulent plague that claimed some 50 million lives and weakened the last vestiges of the Roman Empire. The origins of the pandemic are still unclear, but the authors suggest that it may have entered Europe after bacteria-carrying wildlife wandered there in search of grazing lands.

Cross section of tree trunk. (Credit: Dustin Diaz/EyeEm/Getty Images)
Cross section of tree trunk. (Credit: Dustin Diaz/EyeEm/Getty Images)

The plunging temperatures also coincided with a period of mass migration across Eurasia. The researchers argue that the cold may have led the proto-Slavic peoples to first move from the greater Carpathian region to more hospitable climes in Eastern Europe. Farther east, ice age-induced food shortages may have caused nomadic steppe peoples to migrate toward China, sparking infighting and political strife. Some of these steppe groups would later ally themselves with the Eastern Romans and help topple the mighty Sasanian Empire in Persia.

Interestingly, the Late Antique Little Ice Age might have been a boon to a few cultures. An increase in precipitation could have watered the arid landscape of the Arabian Peninsula, bolstering the growth of scrub vegetation and contributing to the rise of the early Arab-Islamic empire. “Larger camel herds may have facilitated transportation of the Arab armies and their supplies during the substantial conquests in the seventh century,” the authors write.

Despite their assertions that the Late Antique Little Ice Age contributed to an extended period of historical tumult, the researchers note that the extent of its impact is difficult to measure. “We have to be very, very careful that we are not deterministic,” Büntgen told the Washington Post. “We are never saying that the cooling was the main driver, we’re just say the cooling was one additional environmental factor out of many others.” While he and his colleagues argue that their research “fits in well with the main transformative events that occurred in Eurasia” during the 6th and 7th centuries, they write that more case-by-case assessments are required to increase scientists’ understanding of the potential links between climate change and historical events.