HOW CHANGING CLIMATES SHAPE THE FATE OF EMPIRES...

An early morning mist veils the view from the Bavarian Forest onto the Alps near the town of Lalling, Bavaria, Germany, on Dec. 12, 2015. (Armin Weigel/EPA)

Time and again, scholars have pointed out the frightening potential impacts of climate change on human civilizations, linking global warming to everything from forced migration to outbreaks of war. And while such effects are a growing concern in today’s warming world, research now increasingly suggests that previous civilizations were likely molded by a shifting climate as well...

A new paper, just published Monday in the journal Nature Geoscience, proposes a link between a marked cooling event in the fifth and sixth centuries AD and a period of dramatic social change across Europe and Asia, including a pandemic plague outbreak, food shortages, political turmoil in China, migrations and even the rise of the Islamic empire. It’s impossible to say for sure whether the climatic shift was actually responsible for all the upheaval that went on during that time, the researchers say — but since the two periods coincided, the scientists are proposing that a connection is likely.

“There are a lot of things that occurred at the same time, and now it’s certainly very difficult to disentangle to what degree was it caused by climatic fluctuations,” said the study’s lead author, Ulf Büntgen, who heads a research group specializing in tree-ring science at the Swiss Federal Institute for Forest, Snow and Landscape Research. “But we just cannot exclude it anymore.”

The study hinges on a research project that examined tree ring data in the European alps and the Altai-Sayan mountains in Asia to see what kinds of climate shifts had occurred across the two continents over the past 2,000 years. By looking at the rings of preserved ancient trees, scientists can tell a lot about what kinds of temperature changes happened during the tree’s life, Büntgen said.

“Trees growing at the upper tree line are very sensitive to small changes in summer temperature,” he said. “If it’s a cooler summer, the rings are more narrow.”

Büntgen and his colleagues’ most recent work focused on larch tree specimens collected from the Altai-Sayan mountains. But the work built on a previous paper, published in 2011, which conducted similar studies on trees from the European alps. Significantly, the temperature reconstructions they created from both regions matched closely.

“This together, because the agreement is so high, allowed us to get an idea of what happened in Eurasia over the last two millennia,” Büntgen said. “The most striking period was what we now, for the first time, call the ‘Late Antique Little Ice Age.’ ”

This Late Antique Little Ice Age was a period of pronounced cooling, which started in the year 536 AD and lasted until about 660 AD. And the researchers believe they’ve also pinpointed what caused it.

A recent study of ice core data from both Greenland and Antarctica, which Büntgen was also involved in, indicated that a series of volcanic eruptions right at the beginning of the cooling event. These eruptions would have poured ash and other particulate matter into the atmosphere, deflecting sunlight away from the Earth and so helping to induce a temporary cooling event.The researchers hypothesize that the cooling event was sustained for such a long time by a combination of other factors, including certain oceanic processes that helped trap heat and what is believed to have been a solar minimum around the same time period.

“This together — volcanoes, ocean-atmosphere coupling, plus solar minimum — these are the three natural forcings that caused this century-long depression, which is rather large-scale,” Büntgen said. “We have evidence that this was affecting most of if not all of the northern hemisphere.”

Having made these observations, Büntgen and his team collaborated with an interdisciplinary group of historians and anthropologists to look back at what events were happening in human history during the cool period. They realized that a number of remarkable changes were happening in civilizations across Europe and Asia during this time.

In Europe, the colder climate is believed to have contributed to a decrease in agricultural output, which resulted in widespread food shortages. Around the same time, there was an outbreak of plague in the Byzantine Empire, which started around 541 AD and eventually achieved pandemic status as it spread throughout Europe, killing millions and contributing to the weakening of the Empire. 

The researchers have hypothesized that the agricultural declines may have helped the plague bacteria spread from Asia into Europe via wildlife moving into the increasingly abandoned agricultural fields, although they acknowledge in the paper that much remains unknown about the plague’s origin and its link to climate during this time.

Additionally, historians believe there was a great deal of political turmoil in central Asia during this time period, with particular conflict within the regimes governing northern China. Meanwhile, it appears that Slavic populations were spreading across Europe. These events may have also been triggered by social instability caused by famine, crop shortages and disease outbreaks.

The authors have even suggested that there may be a link between climate and the eventual rise of the Islamic Empire. Changes in precipitation patterns, caused by the cooling, may have helped enable the growth of scrub vegetation on the Arabian peninsula, they note in the paper, adding that“larger camel herds may have facilitated transportation of the Arab armies and their supplies during the substantial conquests in the seventh century, during which the reconstructed fraction of human land use seems relatively high in the Arabian Peninsula.”

Altogether, the historians have painted a picture of a volatile era during which many changes were rapidly occurring at the same time across a variety of civilizations. The fact that these changes were preceded by a major climatic shift may be less than coincidental — but the researchers are careful to point out that it’s impossible to establish a definite causal link between the Late Antique Little Ice Age and all of the subsequent social chaos.

“We have to be very very careful that we are not deterministic — 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,” Büntgen said.

However, the researchers have support from other experts as well. John Haldon, a historian at Princeton University, published an accompanying perspective piece in Nature Geoscience on Monday explaining his take on the paper. In it, he acknowledges that that the complexities of human societies can rarely be explained by any one causal factor. But, he adds, there’s a good case for the important influence of the Late Antique Little Ice Age on human social change, even though it was likely not the only factor at play.

He writes, “In the case of the Late Antique Little Ice Age, we seem to have a pretty clear a priori case for assuming that a dramatic period of cooling, that can be dated reasonably precisely and that coincided with substantial societal change across a large part of the Earth’s surface, had a causal impact.”

He also suggests that such studies are useful because they can help give scientists a better idea of how different types of societies respond differently to stress — as he puts it, “how and why some societal systems are more resilient or flexible than others.”

This kind of knowledge could be useful in the future as scientists attempt to gain a better understanding of how human societies might react to future changes in the climate. And the key to such research is continued collaboration among interdisciplinary groups of researchers, Büntgen said.

“Exactly at this human-climate interface, I would say what we need are case studies,” he added. “We select certain particular periods and regions — so case studies — for which we have both a very good understanding about historical fluctuations in climate and documentary evidence of what really happened. These are perfect examples for cross-disciplinary work.”

February 8, 2016

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