One emerging repeated pattern in history is that climate destroys a civilization and then, after a civilization is already in its death throes, "barbarians" rush in to fill the vacuum, while famine begets plagues as well. A few of many examples make the point.
It happened in the Indus River Valley Civilization, Mesopotamia, Egypt, the Levant around 2000 BCE that also was pivotal in further Indo-European expansion into Europe.
There were parallel events of this type in East Asia and Southeast Asia (both island and mainland).
It happened with Bronze Age collapse in the Mediterranean basin and Europe around 1200 BCE.
The fall of the Roman Empire was, in part, to due climate issues.
This happened to the Ancient Puebloans around 1000 CE and related event also pushed Na-Dene peoples out of Canada and into what became the American Southwest severely impacting tribal civilizations with Meso-American roots like the Utes.
Something along these lines facilitated the dramatic Mongol Empire expansion by destroying Central Asian river civilizations (which you'd probably never even heard of) of a region also known as the Turan, clearing the way for barbarian invasions in the late Middle Ages, and that is the subject of a new paper on the subject in PNAS.
While Genghis Khan and Mongol invasion is often blamed for the fall of Central Asia's medieval river civilizations, new research shows it may have been down to climate change. Researchers conducted analysis on the region and found that falling water levels may have led to the fall of civilizations around the Aral Sea Basin, as they depended on the water for irrigation-based farming. . . ."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."The research 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 present southern Kazakhstan.The researchers 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. The abandonment of irrigation systems matches a phase of riverbed erosion between the 10th and 14th century CE, that coincided with a dry period with low river flows, rather than corresponding with the Mongol invasion.
Via Science Daily.
The paper and its abstract are as follows:
Our paper challenges 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. An integration of radiometric dating of long-term river dynamics in the region with irrigation canal abandonment shows that periods of cultural decline correlate with drier conditions during multicentennial length periods when the North Atlantic Oscillation had mostly positive index values. There is no evidence that large-scale destruction of irrigation systems occurred during the Arab or Mongol invasion specifically. A more nuanced interpretation identifies chronic environmental challenges to floodwater farming over the last two millennia, punctuated by multicentennial-length periods with favorable hydromorphic and hydroclimatological conditions that enabled irrigation agriculturists to flourish.AbstractThe 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 y. The region’s decline has been traditionally attributed to the devastating Mongol invasion of the early-13th century CE.
However, the role of changing hydroclimatic conditions on the development of these culturally influential potamic societies has not been the subject of modern geoarchaeological investigations. In this paper we report the findings of an interdisciplinary investigation of archaeological sites and associated irrigation canals of the Otrār oasis, a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage site located at the confluence of the Syr Darya and Arys rivers in southern Kazakhstan. This includes radiometric dating of irrigation canal abandonment and an investigation of Arys river channel dynamics.
Major phases of fluvial aggradation, between the seventh and early ninth century CE and between 1350 and 1550 CE coincide with economic flourishing of the oasis, facilitated by wet climatic conditions and higher river flows that favored floodwater farming. Periods of abandonment of the irrigation network and cultural decline primarily correlate with fluvial entrenchment during periods of drought, instead of being related to destructive invasions. Therefore, it seems the great rivers of Central Asia were not just static “stage sets” for some of the turning points of world history, but in many instances, inadvertently or directly shaped the final outcomes and legacies of imperial ambitions in the region.
Toonen, W.J.H., et al., "A hydromorphic reevaluation of the forgotten river civilizations of Central Asia." Proceedings of the National Academy of Sciences, (2020) DOI: 10.1073/pnas.2009553117