Climate change and migration are often seen as modern concerns, but new research suggests similar dynamics were already shaping societies more than a thousand years ago. An interdisciplinary study examining the final century of China’s Tang dynasty argues that repeated droughts and floods may have played a key role in weakening the empire and contributing to its eventual collapse in 907 CE.
The research, published in Nature Communications Earth and Environment, was carried out by an international team of scholars, including researchers from the University of Basel. By combining climate data, historical sources and modelling of supply networks, the team examined how environmental pressures between 800 and 907 affected society and politics in northern China.
Reading the climate in tree rings
Map of the Tang Dynasty of China, c. 669 CE, by Simeon Netchev / World History Encyclopedia
The study focuses on the region around the Yellow River, one of the most important agricultural and political centres of the Tang Empire. Founded in 618 CE, the Tang dynasty presided over one of the most prosperous and culturally influential periods in Chinese history. By the ninth century, however, the empire was facing growing instability.
To understand the environmental conditions during this period, the researchers turned to climate proxy data, especially long-term records derived from tree rings. These natural archives allow scientists to reconstruct past weather patterns because the width of tree rings reflects the conditions in which the tree grew.
In wet years, trees grow more quickly and produce wider rings, while dry years leave narrower ones. Because some trees can live for centuries, they preserve a detailed record of changing climate conditions over long periods.
The researchers used existing tree-ring datasets from the Yellow River basin to reconstruct river runoff and hydrological patterns during the ninth century. These reconstructions helped them estimate how droughts and floods affected water availability in the region.
“The runoff eventually reaches further downstream and influences the amount of water available, for example for irrigating the fields,” explained the study’s lead author, Michael Kempf, who conducted the research at the University of Basel and has since moved to the University of Cambridge.
Agriculture under pressure
According to the study, the ninth century saw an increase in hydroclimatic extremes, including both droughts and flooding events. These shifts had serious consequences for agriculture.
“Hydroclimatic extremes have a very direct influence on crop failure and grain storage conditions,” Kempf said. Poor harvests could quickly push supply systems to their limits, particularly when shortages of seed grain made it difficult for farmers to recover from a bad year.
The researchers argue that agricultural changes may have made the situation worse. Over time, farmers increasingly cultivated wheat and rice instead of millet. While wheat and rice were widely valued foods, they required more water and were less resilient during droughts than millet.
Kempf suggests that cultural factors may have played a role in this shift. Millet may have been regarded as a less prestigious crop compared with wheat or rice. However, the transition also meant that agricultural systems became more vulnerable to prolonged dry periods.
“As long as there is enough water, this is not a problem,” Kempf said. “But during prolonged dry periods, shortages occur.”
A fragile supply system
Environmental pressures did not affect only farmers. They also threatened the logistical networks that supported the Tang military.
Frontier garrisons along the empire’s northern borders depended heavily on transported grain. Historical evidence suggests that military farms produced only a fraction of the food required by these forces, forcing the government to rely on long-distance supply systems to provision troops and their families.
When droughts or floods disrupted river transport routes or damaged infrastructure, these supply networks became difficult to maintain. In some cases, supply corridors collapsed altogether, leaving frontier troops vulnerable.
The study also highlights the importance of key supply hubs, such as Shengzhou, where grain from northern regions like Taiyuan was stored and redistributed to military garrisons. Flood events in the early ninth century damaged fortifications and infrastructure in several frontier areas, further weakening the system.
Migration and political instability
Food shortages and deteriorating living conditions had broader consequences for society. According to the researchers, environmental pressures may have contributed to population movements within the empire.
“Of course, people were weakened and therefore more vulnerable,” Kempf said. “Due to the military pressure on the outer border regions, they migrated south, where they believed they would find better conditions.”
Such movements could destabilize already fragile political structures. The late Tang period was marked by growing autonomy among regional military governors, known as fanzhen, who increasingly acted independently of the imperial court. The combination of environmental stress, logistical failures and political fragmentation created conditions that made the empire harder to govern.
The ninth century also witnessed major uprisings, including the Huang Chao Rebellion in the 870s, which devastated large areas of China shortly before the dynasty collapsed.
Climate as one factor among many
Despite the striking correlations between environmental changes and political instability, the researchers caution against attributing the fall of the Tang dynasty to climate alone.
“Our results are approximations,” Kempf noted. “The actual conditions at that time cannot be reconstructed with certainty. It’s a complex interplay of many different factors.”
Instead, the study suggests that climatic stress acted as an additional pressure on a political system already facing military and administrative challenges. Environmental shocks could push vulnerable systems past a tipping point when other stresses were already present.
The researchers conclude that understanding how past societies responded to climatic pressures may offer valuable insights for the present. In both historical and modern contexts, environmental changes can interact with social and political systems in ways that produce unexpected and far-reaching consequences.
The article, “Hydroclimatic instability accelerated the socio-political decline of the Tang Dynasty in northern China,” by M. Kempf, M. L. C. Depaermentier, R. N. Spengler III, M. D. Frachetti, F. Chen, J. Luterbacher, E. Xoplaki and U. Büntgen, is published in Nature Communications Earth & Environment. Click here to read it.
Climate change and migration are often seen as modern concerns, but new research suggests similar dynamics were already shaping societies more than a thousand years ago. An interdisciplinary study examining the final century of China’s Tang dynasty argues that repeated droughts and floods may have played a key role in weakening the empire and contributing to its eventual collapse in 907 CE.
The research, published in Nature Communications Earth and Environment, was carried out by an international team of scholars, including researchers from the University of Basel. By combining climate data, historical sources and modelling of supply networks, the team examined how environmental pressures between 800 and 907 affected society and politics in northern China.
Reading the climate in tree rings
The study focuses on the region around the Yellow River, one of the most important agricultural and political centres of the Tang Empire. Founded in 618 CE, the Tang dynasty presided over one of the most prosperous and culturally influential periods in Chinese history. By the ninth century, however, the empire was facing growing instability.
To understand the environmental conditions during this period, the researchers turned to climate proxy data, especially long-term records derived from tree rings. These natural archives allow scientists to reconstruct past weather patterns because the width of tree rings reflects the conditions in which the tree grew.
In wet years, trees grow more quickly and produce wider rings, while dry years leave narrower ones. Because some trees can live for centuries, they preserve a detailed record of changing climate conditions over long periods.
The researchers used existing tree-ring datasets from the Yellow River basin to reconstruct river runoff and hydrological patterns during the ninth century. These reconstructions helped them estimate how droughts and floods affected water availability in the region.
“The runoff eventually reaches further downstream and influences the amount of water available, for example for irrigating the fields,” explained the study’s lead author, Michael Kempf, who conducted the research at the University of Basel and has since moved to the University of Cambridge.
Agriculture under pressure
According to the study, the ninth century saw an increase in hydroclimatic extremes, including both droughts and flooding events. These shifts had serious consequences for agriculture.
“Hydroclimatic extremes have a very direct influence on crop failure and grain storage conditions,” Kempf said. Poor harvests could quickly push supply systems to their limits, particularly when shortages of seed grain made it difficult for farmers to recover from a bad year.
The researchers argue that agricultural changes may have made the situation worse. Over time, farmers increasingly cultivated wheat and rice instead of millet. While wheat and rice were widely valued foods, they required more water and were less resilient during droughts than millet.
Kempf suggests that cultural factors may have played a role in this shift. Millet may have been regarded as a less prestigious crop compared with wheat or rice. However, the transition also meant that agricultural systems became more vulnerable to prolonged dry periods.
“As long as there is enough water, this is not a problem,” Kempf said. “But during prolonged dry periods, shortages occur.”
A fragile supply system
Environmental pressures did not affect only farmers. They also threatened the logistical networks that supported the Tang military.
Frontier garrisons along the empire’s northern borders depended heavily on transported grain. Historical evidence suggests that military farms produced only a fraction of the food required by these forces, forcing the government to rely on long-distance supply systems to provision troops and their families.
When droughts or floods disrupted river transport routes or damaged infrastructure, these supply networks became difficult to maintain. In some cases, supply corridors collapsed altogether, leaving frontier troops vulnerable.
The study also highlights the importance of key supply hubs, such as Shengzhou, where grain from northern regions like Taiyuan was stored and redistributed to military garrisons. Flood events in the early ninth century damaged fortifications and infrastructure in several frontier areas, further weakening the system.
Migration and political instability
Food shortages and deteriorating living conditions had broader consequences for society. According to the researchers, environmental pressures may have contributed to population movements within the empire.
“Of course, people were weakened and therefore more vulnerable,” Kempf said. “Due to the military pressure on the outer border regions, they migrated south, where they believed they would find better conditions.”
Such movements could destabilize already fragile political structures. The late Tang period was marked by growing autonomy among regional military governors, known as fanzhen, who increasingly acted independently of the imperial court. The combination of environmental stress, logistical failures and political fragmentation created conditions that made the empire harder to govern.
The ninth century also witnessed major uprisings, including the Huang Chao Rebellion in the 870s, which devastated large areas of China shortly before the dynasty collapsed.
Climate as one factor among many
Despite the striking correlations between environmental changes and political instability, the researchers caution against attributing the fall of the Tang dynasty to climate alone.
“Our results are approximations,” Kempf noted. “The actual conditions at that time cannot be reconstructed with certainty. It’s a complex interplay of many different factors.”
Instead, the study suggests that climatic stress acted as an additional pressure on a political system already facing military and administrative challenges. Environmental shocks could push vulnerable systems past a tipping point when other stresses were already present.
The researchers conclude that understanding how past societies responded to climatic pressures may offer valuable insights for the present. In both historical and modern contexts, environmental changes can interact with social and political systems in ways that produce unexpected and far-reaching consequences.
The article, “Hydroclimatic instability accelerated the socio-political decline of the Tang Dynasty in northern China,” by M. Kempf, M. L. C. Depaermentier, R. N. Spengler III, M. D. Frachetti, F. Chen, J. Luterbacher, E. Xoplaki and U. Büntgen, is published in Nature Communications Earth & Environment. Click here to read it.
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