3.1 External forcing on the regional climate
Our results suggest that these long-term humidity trends are driven by changes in solar insolation (Fig. 3a). On the one hand, reduced summer insolation leads to a weakening of the monsoon system and thus to less precipitation in the areas affected by the monsoonal system and higher precipitation in the adjacent northern regions (sometimes referred to “Arid Central Asia”) including Mongolia47 (Supplementary Fig. 1). This effect can be explained with weak subsidence and more prominent convection. On the other hand, a long-term reduced summer insolation (~6 Wm−2 from the Mid- to the Late Holocene, Fig. 3a) will inevitably lead to lower temperatures, reduced evaporation and more effective ecosystem moisture (Supplementary section S7).
However, superimposed on this long-term trend we find a strong resemblance of our EI record with short-term fluctuations of the high-resolution TSI record (Fig. 3b, e). The solar minima at ~800 BCE, 400 BCE, as well as ~700 CE and after 1300 CE all coincide with prominent minima displayed by our EI record. We interpret this as strong empirical evidence for solar forcing being the most important factor influencing the local hydrological conditions at our site.
TSI varies by about 1 Wm−2, which modifies the radiative forcing at the Earth’s surface and the mean global temperature by ~0.17 Wm−2 and ~0.07°C, respectively48. Lean49 suggested a higher sensitivity for the mid-latitudes and regional temperature modifications of up to 2°C. A temperature decrease at this magnitude should entail a significant decrease of evaporation, which directly affects the regional moisture balance (see section “Hydrological modelling” below). In addition, reduced TSI during solar minima favored northern hemispheric cooling, which strengthened latitudinal temperature gradients and the Westerly jet streams48,50,51. Then, the North Atlantic Oscillation (NAO) tends to be in a distinct negative mode (Fig. 3c), which forces the Westerly jets to the South, advecting more moisture to “Arid Central Asia”, and thus, our research site21,51 (Supplementary Fig. 1). In contrast, higher TSI leads to reduced latitudinal temperature gradients, which coincides with a distinct positive NAO mode (Fig. 3b, c) as well as a northward shift of the Westerly jets21,51, which favors dry conditions at Lake Telmen.
3.3 Climate impact on human history in Mongolia
Sustained humid conditions likely enabled the expansion of fertile grasslands and thus, increased ecosystem carrying capacity14,17,54 – allowing to raise larger numbers of livestock and horses for both meat and dairy production9,11. Particularly in the dry and seasonal steppe environment, domestic livestock herds experience “economies of scale” – wherein smaller herds are more vulnerable to loss from disease, predation, or weather, and larger herds are more resilient55. With productive areas distributed unequally across the landscape, and some herders inevitably subject to disaster and loss, periods of environmental productivity appear to encourage the formation of larger steppe social networks17. As the key engine of preindustrial 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 livestock56. Together, these and other factors likely helped create an uncommonly close causal link between environmental dynamics and sociopolitical developments in the Mongolian grasslands.
The onset of humid conditions at 1200 BCE (Figs. 2, 3) coincides with drastic social changes across Central Mongolia, including the first emergence of horse culture and evidence for widespread social integration across the eastern steppe. In contrast to earlier pastoralists, who were apparently constrained largely to mountain margins, DSK and other late Bronze Age herders made use of open grassland and desert regions8. At some sites, hundreds or even thousands of horse burials testify to the expanded ecological and social significance of horses57. The epicenter of this dramatic emergence of horse culture appears to have been Central Mongolia, with large funerary and monument complexes emerging in the Central Mongolian Khangai Mountain Range (including Zavkhan province)58–60. 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 NAO weakening during the grand solar minimum is associated with a general climate and environmental crisis51, triggering human migrations and the collapse of cultures in large parts of northern Europe61,62, we find the opposite causal link in Mongolia – between increased effective ecosystem moisture and positive socio-environmental impacts due to enhanced biomass production and an expansion of fertile grasslands15,16,63. During the grand solar minimum from 800 to 600 BCE (Fig. 3d, I.)64 in key social changes, and the emergence of the first integrated pastoral empires took place during a prolonged period of humid conditions, as indicated by our EI. As the DSK culture waned, Mongolia witnessed an expansion of the Slab Burial culture, whose sites also yield the first direct evidence of riding tack65, royal equestrian burials and the earliest evidence for horsemanship appear in the archaeological record at Arzhan, in Tuva, and early mounted Scythian groups spread westward out of interior Asia66.
From this first expansion of horse culture, a prolonged period of humid conditions in central Mongolia supported the convergence of Mongolia’s first united pastoral polities. The Xiongnu Empire thrived particularly between 200 BCE and 100 CE (Fig. 3f)10,67,68, when climate conditions were also predominantly humid. 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 Mediterranean7,10,12,68. 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 empire6,8,12,31,67.
This prolonged period of favorable climate-human interaction seems to have persisted across the early first millennium CE. This record shows that humid conditions were no guarantee of persistent political stability, as some important polities rose and fell in the Mongolian steppe. However, after the Xiongnu state failed ca. 100 CE, both the Rouran Khaganate (ca. 400 CE) and the first Turkic Khaganate (ca. 550 CE) formed during periods of favorable grassland conditions in Central Mongolia10. This run ended with the onset of the LALIA around 600 CE (Fig. 3b-f).
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. Very harsh and long winters seem to have caused high livestock mortality, an increase in warfare activity, famines, and cultural re-organization during the LALIA6,10,27,28,32. Based on our record, dry climate conditions prevailed during the MCA in Central Mongolia, and conditions remained unfavorable until the end of the MCA 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 in ca. 603 CE69. During subsequent centuries, Mongolia cycled through a comparatively tumultuous period of political instability, with brief periods of steppe integration like the Second Turkic (ca. 680-740 CE) and Uyghur (ca. 750-850 CE) Khanates, interspersed with periods of domination by external powers like the Tang and Khitan states10,70.
Finally, 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. Our EI shows a shift to humid conditions since 1100 CE and a positive effective moisture balance at the MCA-LIA transition around 1300 CE (Fig. 3e). 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 (Fig. 3f)13,14,54.
We conclude that solar forcing played an important role in controlling regional climate at Lake Telmen over the past 4000 years. 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 disease1–4.
While these moisture fluctuations seem to have exerted an important impact on the rise and fall of Mongolian steppe cultures over the past 4000 years, in light of the paleoclimate record we expect 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 is already experiencing a 2°C temperature increase since 196371, and will likely exceed TSI-induced temperature fluctuations in the near-future. Previous studies have shown a rapid loss of lakes52, melting mountain ice72, persistent soil moisture deficits73,74, and an increased frequency of droughts73,75,76 and heavy rainstorms15,77,78. 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 floods77,78. Although, modeling results show a low probability that future drought intensities will exceed those of the last two millennia76, present-day climate changes already cause enhanced socio-environmental consequences15,75,77, and it is uncertain whether and how modern pastoralists will to adapt to the future climate.