Climate Change, Not Mongol Invasion, Destroyed Central Asia’s River Civilizations

The Aral Sea basin in Central Asia and its major rivers, the Amu Darya and Syr Darya, were the center of advanced river civilizations, and a principal hub of the Silk Roads over a period of more than 2,000 years. The region’s decline has been traditionally attributed to the Mongol invasion of the early-13th century CE. But a new study, published in the Proceedings of the National Academy of Science, challenges this long-held view.

Toonen et al. challenge the long-held view that the fall of Central Asia’s river civilizations was determined by warfare and the destruction of irrigation infrastructure during the Mongol invasion. Image credit: National Library, Berlin.

“While the great river civilizations of the Old World have been the subject of archaeological and scientific study for more than a century, the ancient irrigation-based urban cultures that developed along the great rivers of Central Asia are virtually unknown,” said co-lead authors Dr. Willem Toonen of the Vrije Universiteit Amsterdam, University of Lincoln’s Dr. Mark Macklin and their colleagues.

“In the 1950-60s, archaeologists demonstrated that the Amu Darya and Syr Darya rivers, that flow northwest from the Pamir and Tien Shan Mountains and drain to the Aral Sea, were the centers of flourishing urban societies from prehistory to the late Middle Ages.”

“The 50,000-km² area of floodwater irrigated land was estimated to have been twice that of Mesopotamia.”

“The region’s stagnation at the end of the Medieval period is generally attributed to a combination of the destructive early-13th century CE Mongol invasion and the progressive decline of the Silk Roads trade network.”

“However, the hydroclimatic and hydromorphic contexts of these changes are largely unknown with only a handful of sites having been radiometrically dated.”

Geomorphology and archaeology of the Arys river catchment and Otrar oasis in Kazakhstan: (A) the Arys river catchment and research areas; (B) Otrar oasis with main irrigation canals and archaeological sites; (C) the Arys River at low flow; (D) the Badam River during peak discharge; (E) aerial view of irrigation canal bifurcations southeast of Altyn; (F) aerial view of Otrar; (G) irrigation canal bifurcation with a fortified settlement; (H) ancient irrigation canal near Arys town; (I) abandoned canals (white arrows) and abandoned agricultural field plot (black arrows) east of Kuik Mardan. Image credit: Toonen et al., doi: 10.1073/pnas.2009553117.

The researchers focused on the archaeological sites and irrigation canals of the Otrar oasis, a UNESCO World Heritage site that was once a Silk Road trade hub located at the meeting point of the Syr Darya and Arys rivers in southern Kazakhstan.

They investigated the region to determine when the irrigation canals were abandoned and studied the past dynamics of the Arys river, whose waters fed the canals.

They found that despite the documented destruction of settlements, many sites in the Otrar oasis persisted until the drought-related contraction in the 9th century CE.

The Mongol invasion and destruction of the oasis in 1219 CE, however, came after more than 200 years of reducing rainfall, with evidence of large-scale canal abandonment.

“Our research shows that it was climate change, not Genghis Khan, that was the ultimate cause for the demise of Central Asia’s forgotten river civilizations,” Dr. Macklin said.

“We found that Central Asia recovered quickly following Arab invasions in the 7th and 8th centuries CE because of favourable wet conditions.”

“But prolonged drought during and following the later Mongol destruction reduced the resilience of local population and prevented the re-establishment of large-scale irrigation-based agriculture.”

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Willem H.J. Toonen et al. A hydromorphic reevaluation of the forgotten river civilizations of Central Asia. PNAS, published online December 14, 2020; doi: 10.1073/pnas.2009553117