Could climate change thousands of years ago have changed the direction of the ancient Silk Road?

Professor Chen Jianhui's team at Lanzhou University recently published a research paper in Science Bulletin, proposing that climate change directly and indirectly drove the migration of the main transportation routes in the middle section of the Silk Road from the Tarim Basin to the northern foothills of the Tianshan Mountains from the Southern and Northern Dynasties to the Sui and Tang Dynasties (420-907).

Previous studies have shown that climate change can trigger social crises, such as plague, famine, and war. Most of these studies focus on the causal relationship between climate change and the rise and fall of civilizations in the temporal dimension, but pay less attention to the impact of climate change on the spatial evolution of human civilization. The historical event of the migration of the main transportation route of the ancient Silk Road from the Tarim Basin to the northern foothills of the Tianshan Mountains provides a rare opportunity for conducting such research.

The north and south sides of the Tianshan Mountains were important passages on the ancient Silk Road: During the Han Dynasty (202 BC-220 AD), the Silk Road mainly extended westward along the ancient oasis cities on the north and south edges of the Tarim Basin, forming the "North-South Road" of the ancient Silk Road (Figure 1, yellow line). From the Southern and Northern Dynasties to the Sui and Tang Dynasties (420-907 AD), a trade route along the northern foothills of the Tianshan Mountains quietly emerged and gradually replaced the "North-South Road" to become the main transportation route in the middle section of the Silk Road, known as the "New North Road" of the Silk Road (Figure 1, red line).

Figure 1 The main traffic routes in the eastern and central sections of the ancient Silk Road. The yellow line is the "North-South Road" and the red line is the "New North Road"

The rise of the "New North Road" is of great significance: it not only promoted the formation of the "Turk-Sogdian Community", facilitated the connection between the Central Plains Dynasty and the nomadic regimes in Central Asia, but also promoted the extension of the Silk Road trade from the Pacific coast to the Atlantic coast. However, the reason for the northward migration of the Silk Road route is still unclear, and whether climate change played a key role in the occurrence of this event is a scientific question worth exploring. The answer to this question is also expected to provide new insights into the impact of climate change on the spatial changes of human civilization.

Water resources are the most important environmental factor restricting human activities in the study area, and are strongly affected by changes in temperature and precipitation: temperature can affect runoff by regulating snowmelt, while precipitation is the direct provider of water resources. Existing paleoclimate studies have basically reached a consensus on the precipitation changes in the region at that time, but there is still much controversy about its temperature change history.

The research team selected Shuanghu, an alpine lake with little interference from human activities (Figure 1, red five-pointed star), as the research object, and used midges that are sensitive to temperature as a proxy indicator to quantitatively reconstruct the temperature change sequence in Xinjiang over the past 2000 years with high resolution (Figure 2c & d); integrating the hydrological and climate data of existing research points (Figure 1, blue dots), the ages of the newly measured site points after field investigations (Figure 1, white five-pointed stars), and the frequency series of wars newly obtained through literature sorting, the team explored the impact of climate change from the Southern and Northern Dynasties to the Sui and Tang Dynasties (420-907) on the changes in the Silk Road transportation routes.

Fig. 2 Reconstruction of chironomid-temperature changes in the Shuanghu area over the past 2000 years and comparison with sediment TOC and C/N

The comprehensive analysis results show that during the period of about 420-600 AD, the low temperature in Xinjiang (Figure 3a) reduced the amount of meltwater from ice and snow, while the low precipitation (Figure 3b) further reduced the water supply, which together led to water shortage (Figure 3c & d), triggering the Silk Road route to migrate from the Tarim Basin (North-South Road) to the northern foothills of the Tianshan Mountains (New North Road) where water resources are more abundant and stable, reflecting the direct impact of climate change on the changes of the Silk Road. Interestingly, during the subsequent period of about 600-850 AD, the warming and wetter climate (Figure 3a-d) did not slow down the northward migration of the route. The possible reason was that the conflict between Tubo and the Tang Dynasty in the Tarim Basin intensified and wars broke out frequently (Figure 3e & f), which led to the obstruction of the "North-South Road" Silk Road trade. The rise and expansion of Tubo (Figure 3g) was closely related to the suitable climate of the Qinghai-Tibet Plateau during the same period, reflecting the indirect impact of climate change on the changes of the Silk Road.

Figure 3 Climate, environment and human activity background of the Silk Road route changes

The research results reveal that climate change can drive the spatial changes of human civilization in two different ways: direct impact (deterioration of the living environment) and indirect impact (geopolitical conflict). This not only has reference value for understanding the migration of ancient peoples between Eurasia, but also provides a scientific basis for addressing the challenges facing geopolitics in arid areas under the background of global warming.

Haipeng Wang, Jianhui Chen, Menghan Qiu, Zhilin Shi, Shengda Zhang, Guanghui Dong, Shuai Ma, Tao Ai, Guan Ren, Fahu Chen. Climate change drove the route shift of the ancient Silk Road in two distinct ways. Science Bulletin, 2024, 69(8): 1153-1160. DOI: 10.1016/j.scib.2024.02.025