The Neo-Assyrian empire was a mighty superpower that dominated the near east for 300 years before its dramatic collapse. Now researchers say they have a novel theory for what was behind its rise and fall: climate change.
The empire emerged in about 912BC and grew to stretch from the Mediterranean down to Egypt and out to the Persian Gulf.
But shortly after the death of the king Ashurbanipal around 630BC, the empire began to crumble, with the grand city of Nineveh sacked in 612BC. By the end of the seventh century BC, the empire’s fall was complete.
Now scientists say the reversal in the empire’s fortunes appears to coincide with a dramatic shift in its climate from wet to dry – a potentially crucial change in an empire reliant on crops.
“Nearly two centuries of high precipitation and high agrarian outputs encouraged high-density urbanisation and imperial expansion that was not sustainable when climate shifted to megadrought conditions during the seventh century BC,” the authors write.
In other words, while civil war, overexpansion and military defeat played a role in the empire’s collapse, the underlying driver could have been crop failures that led to economic collapse, exacerbating political unrest and conflict.
Prof Nicholas Postgate, an expert on Assyria from the University of Cambridge who was not involved in the study, said it was plausible that climate change helped finish the empire. “We don’t have any better explanation for what happened to the Assyrian empire during those times,” he said, adding that there was a dearth of written records from about 645BC to the sack of Nineveh.
To investigate the possible influence of climate, a team of scientists analysed two stalagmites taken from Kuna Ba cave in northern Iraq, looking at the ratio of two different types of oxygen atoms, known as isotopes, within the mineral deposits formed as water trickled into the cave. This ratio sheds light on levels of rainfall.
The team combined the results with thorium-230 dating to reveal that between 925BC and 550BC there were two distinct phases in the climate.
The first, lasting until about 725BC, was marked by wetter than average conditions – indeed, the team say that the period between 850BC and 740BC was one of the wettest of the 4,000-year span captured by the stalagmite, a window that fits with the expansion of the Neo-Assyrian empire.
The second phase was marked by increasingly dry conditions: between 675BC and 550BC, as the Neo-Assyrian empire was collapsing, the region was in the grip of a megadrought.
Prof Ashish Sinha of California State University, a palaeoclimatologist and the first author of the study, said that while the Neo-Assyrian empire was vast, computer models and modern rainfall data show much of it would have been affected by similar conditions as the Kuna Ba cave.
“If you do get these stark, severe droughts, they tend to affect a much broader region than just one location,” he said.
The climate trends were backed up by patterns in carbon isotope data, with a range of data from various caves and lakes across what was the empire offering broad support – albeit with some variation around dates. What is more, modern satellite data revealed crop productivity over northern Iraq is highly sensitive to small changes in rainfall when levels are low, and plummets across the region during droughts.
Writing in the journal Science Advances, the team conclude the shift to a megadrought could have had a devastating impact on society.
Modern crises, they say, back up their view, noting that severe droughts during 1999-2001 and 2007-2008 led to serious crop failures and the death of livestock and stoked social difficulties in northern Iraq.
“In the 20th century, the human [climate] forcing may be riding on top of the natural variability,” said Sinha. “That is why we think that the modern droughts are about as severe as, or maybe even getting more severe than, these droughts at 600BC.”
Prof James Baldini, an expert in analysing stalagmites who was not involved in the research, said the study left little doubt that the Neo-Assyrian empire enjoyed abundant rainfall followed by a megadrought.
“Although the causes of the collapse were undoubtedly multi-factorial– as the authors concede – I agree with the authors that drought was almost certainly a catalyst for the fall of the empire,” he added, noting similar climate patterns occurred during the growth and fall of the Classic Maya civilisation around the end of the first millennium AD.
“Populations grew in parallel with rainfall amount and food abundance, but once the rains abandoned the regions for prolonged periods of time, this triggered famine, political instability, civil wars, and foreign invasions,” he said.
Baldini added that the past can hold important lessons for the present – where fossil fuel use drives climate change.
“A recent study identified a severe drought as an underlying cause of the Syrian civil war, and it is increasingly clear that migration out of sub-Saharan Africa’s Sahel is driven by drought,” he said. “Hopefully we can learn from history, and rise to the challenges presented by climate change better than previous civilisations were able to.”