The onset of humid conditions around 1200 BCE in the area of modern-day Mongolia appears to have been a key driver in the expansion of the great equestrian empires of the Eastern Steppes, researchers report.
The repeated expansion of East Asian steppe cultures has been a key driver of Eurasian history.
The rise of transcontinental, pastoral empires linking eastern and western Eurasia across the steppes had a tremendous transformative effect on human societies, facilitating the spread of people, goods, and ideas — as well as domestic animals, plants, and catastrophic disease.
The Mongolian steppe was first occupied by pastoral people around 3000 BCE, when early herders appear to have migrated to the region from western Asia.
Around 1200 BCE, domestic horses were used first for transport by mobile herders and other Bronze Age culture groups.
The emergence of a horse culture changed mobility for the steppe cultures, leading to the rise of important nomadic societies such as the Xiongnu, from around 200 BCE to 100 CE, and the Great Mongol Empire, which rose to global dominance under Genghis Khan in the early 13th century CE.
For these pastoral empires, extensive and productive grasslands provided the engine for both economic and political power.
However, in the dry and harsh steppes of eastern Eurasia, minor climate variations can have large impacts on the water balance, biomass production, and ecosystem carrying capacity.
The close coupling between precipitation, temperature and domestic animal productivity gave rise to the hypothesis that climate changes may have played an important role in the way human history unfolded in Central Asia.
Researchers, writing in the journal Scientific Reports, noted that while climate is proposed as an important driver of these poorly understood cultural expansions, paleoclimate records from the Mongolian Plateau can be difficult to interpret.
The study team used a combination of geochemical analyses and comprehensive radiocarbon dating to establish what they describe as the first robust and detailed record of paleohydrological conditions for the Lake Telmen region of Mongolia, covering the past 4000 or so years.
Analysis by Julian Struck and his fellow researchers showed that humid conditions coincided with solar minima – the part of the 11-year solar cycle when sunspot and solar flare activity is at a minimum.
Their hydrological modeling confirmed the high sensitivity of the lake to paleoclimate changes.
Careful comparisons with archaeological and historical records suggest that solar minima led to reduced temperatures, less evaporation, and higher biomass production across the vast semi-arid grasslands of eastern Eurasia, expanding the power base for pastoral economies and horse cavalry.
The sustained humid conditions that arose likely enabled the expansion of fertile grasslands, allowing people to raise larger numbers of livestock and horses for both meat and dairy production.
While smaller herds are more vulnerable to loss from disease, predation, or weather, larger herds often prove more resilient.
The researchers said periods of environmental productivity appear to have encouraged the formation of larger steppe social networks.
As the key engine of pre-industrial transport and warfare in Eurasia, horses directly impacted the military and transport capacity of steppe societies, while long military campaigns also often required grazing areas for other livestock.
“Together, these and other factors likely helped create an uncommonly close causal link between environmental dynamics and sociopolitical developments in the Mongolian grasslands,” they said.
The onset of humid conditions around 1200 BCE coincided with drastic social changes across central Mongolia, including the first emergence of a horse culture and evidence of widespread social integration across the eastern steppe.
In contrast to earlier pastoralists, who were apparently constrained largely to mountain margins, the late Bronze Age herders in the area made use of the open grasslands and desert regions.
At some sites, hundreds or even thousands of horse burials testify to the expanded ecological and social significance of horses.
“The epicenter of this dramatic emergence of horse culture appears to have been central Mongolia,” they said, “with large funerary and monument complexes emerging in the Khangai Mountain Range.
“Our results suggest that the expansion of the region’s first culture, which spread as far as Trans Baikal, Tuva, Kazakhstan, Xinjiang, and China, was supported by wet conditions driven by a solar minimum.”
While the grand solar minimum from 800 to 600 BCE, coupled with a known weakening of the North Atlantic Oscillation – a weather phenomenon over the North Atlantic – was associated with a general climate and environmental crisis triggering human migrations and the collapse of cultures in parts of northern Europe, the study found the opposite in Mongolia.
Instead, there was an increase in effective ecosystem moisture and positive socio-environmental impacts because of better growing conditions, and an expansion of fertile grasslands.
It was during the grand solar minimum that key social changes and the emergence of the first integrated pastoral empires took place during a prolonged period of humid conditions.
Mongolia witnessed an expansion of the Slab Burial culture that yielded the first direct evidence of riding tack, and royal equestrian burials. The earliest evidence for horsemanship appears in the archaeological record at Arzhan, in Tuva, and early mounted Scythian groups spread westward out of interior Asia.
From this first expansion of horse culture, the prolonged humid conditions in central Mongolia supported the convergence of Mongolia’s first united pastoral polities (organized societies).
“The Xiongnu Empire thrived particularly between 200 BCE and 100 CE, when climate conditions were also predominantly humid at Lake Telmen and Lake Khar Nuur.
“Extensive fertile grasslands favored pastoralism, while this period also saw the adoption of agriculture, the establishment of village-like settlements, increased gene flow with East and Central Asia, and extensive trade relations were established as far as the Mediterranean.”
Complemented by new military and organizational techniques, climatic and environmental conditions favorable for animal pastoralism enabled the Xiongnu to form a large and powerful politically structured empire.
“This prolonged period of favorable climate-human interaction seems to have persisted across the early first millennium CE.”
The authors noted that humid conditions were no guarantee of persistent political stability, as some important groups rose and fell in the Mongolian steppe.
However, after the Xiongnu state failed around 100 CE, both the Rouran Khaganate (around 400 CE) and the first Turkic Khaganate (around 550 CE) formed during periods of favorable grassland conditions in central Mongolia.
This run ended with the onset of the Late Antique Little Ice Age – a long-lasting northern hemisphere cooling period around 600 CE.
The record shows a distinct decline in the evaporation index during this period, likely indicating cooler conditions at Lake Telmen, indicating dryer conditions and the development of a cold steppe during this period.
“Just as solar minima appear to have been crucial to the first formation of pastoral empires, solar maxima may have had a disruptive effect on social integration in ancient Mongolia,” they said.
“Very harsh and long winters seem to have caused high livestock mortality, an increase in warfare activity, famines, and cultural re-organization during the Late Antique Little Ice Age.”
Based on the records, dry climate conditions prevailed in central Mongolia during the warmer Medieval Climate Anomaly. Conditions remained unfavorable until the end of this period around 1300 CE.
Under these conditions, failing grassland biomass may have undercut the economic and social power base of the first Turkic Khaganate, and contributed to its disintegration around 603 CE, they said.
During subsequent centuries, Mongolia cycled through a comparatively tumultuous period of political instability, they said, interspersed with periods of domination by external powers like the Tang and Khitan states.
“Our record supports previous arguments that moisture balance also played an important role in the emergence and success of the largest pastoral empire — the Great Mongol Empire of Genghis and Khubilai Khan,” they wrote.
Their research showed a shift to humid conditions since 1100 CE and a positive effective moisture balance around 1300 CE.
“This likely favored the union of nomadic tribes under Genghis Khan and the formation of the Mongol Empire, which began during the early 13th century and reached its greatest spatial extent during the late 13th through the mid-14th century.”
Changes in solar radiation ultimately played an important role in controlling the regional climate around Lake Telmen over the past 4000 years, they concluded.
“We have shown that even small changes in temperature and precipitation have a huge impact on the effective ecosystem moisture balance and thus, biomass production and the expansion of fertile grasslands.
“This apparent causal link between favorable climate conditions and positive socio-environmental impacts for herding cultures in the Mongolian steppe likely had tremendous impact on the broader trajectory of human history in Eurasia, as the cyclical emergence of pastoral cultural networks and empires helped to forge some of the first pan Eurasian trade networks, spreading goods, plants, and animals, people, ideas, and even catastrophic pandemic disease.”
Given the findings, the researchers voiced the view that the near-future consequences of global warming will put the ecosystems and livelihood of the pastoral population in Central Asia at great risk.
Mongolia has already experienced a 2°C temperature increase since 1963. Previous studies have shown a rapid loss of lakes, melting mountain ice, persistent soil moisture deficits, more droughts and heavy rainstorms.
“Increased rainfall may not counteract the impact of rising temperatures. Instead, rainfall may exacerbate ongoing land degradation as these short-term heavy rainstorms exceed the soil’s infiltration capacity and cause surface runoff, soil erosion, and even floods.”
It is uncertain whether and how modern pastoralists will adapt to the future climate, they said.
The study team comprised Struck, Marcel Bliedtner, Paul Strobel, William Taylor, Sophie Biskop, Birgit Plessen, Björn Klaes, Lucas Bittner, Bayarsaikhan Jamsranjav, Gary Salazar, Sönke Szidat, Alexander Brenning, Enkhtuya Bazarradnaa, Bruno Glaser, Michael Zech and Roland Zech, variously affiliated with the Friedrich Schiller University Jena in Germany; the German Research Centre for Geosciences; the University of Trier in Germany; the Dresden University of Technology and Max Planck Institute for the Science of Human History, both in Germany; the University of Bern in Switzerland; the Mongolian University of Life Sciences; the Martin Luther University Halle-Wittenberg in Germany; and the University of Colorado-Boulder Museum of Natural History.
Struck, J., Bliedtner, M., Strobel, P. et al. Central Mongolian lake sediments reveal new insights on climate change and equestrian empires in the Eastern Steppes. Sci Rep 12, 2829 (2022). https://doi.org/10.1038/s41598-022-06659-w