The impact of Asian water towers on the evolution of Southeast Asian civilization over the past millennium

Chen Feng, a researcher at the Institute of International Rivers and Ecological Security at Yunnan University, and together with relevant scholars from the Institute of Atmospheric Physics of the Chinese Academy of Sciences, the University of Cambridge in the United Kingdom, the University of Mainz in Germany, and the University of Arizona in the United States, have made progress in the field of runoff changes in many transboundary rivers in the southern part of the Tibetan Plateau.

The research results were published online in Nature Geoscience on November 14, 2023, Beijing time, titled “Southeast Asian ecological dependency on Tibetan Plateau streamflow over the last millennia”.

Researcher Chen Feng of Yunnan University and Associate Researcher Man Wenmin of the Institute of Atmospheric Physics of the Chinese Academy of Sciences are the co-first authors, and Academician Chen Fahu, Chief Scientist of the project, Professor He Daming and Researcher Chen Feng of Yunnan University are the co-corresponding authors.

As the “third pole of the earth”, the Qinghai-Tibet Plateau is known as the “water tower of Asia”, the birthplace of many cross-border rivers in Asia, affecting more than half of the world’s population and giving birth to a rich and colorful Asian civilization. In the past 40 years, under the background of global warming, the runoff output of the Tibetan Plateau has shown an increasing trend, which has led to an increasing concern about the change of water resources in high Asia. Accurately quantifying the changes in water resources of the Qinghai-Tibet Plateau under the background of climate change and scientifically judging the trend of river runoff in the future are the basis and premise for the effective utilization of water resources. However, the climatic and topographic conditions of the Tibetan Plateau are extremely complex, and the uncertainties of various observations and model simulations in the region have long constrained the research on this issue. In addition, most of the studies on water resources in high Asia focus on the changes of water resources themselves, and lack the depiction of the influence of hydrological processes in the high mountains of Asia on the evolution of river basin civilization. Therefore, there is an urgent need to use hydroclimatic proxy data to reveal the changes of water resources in transboundary basins from a long-term perspective, and to enhance people’s understanding of the role of water resources in Asia’s socio-economic development over the past millennium.

Due to the influence of the longitudinal river valley topography in the eastern part of the Tibetan Plateau, many large cross-border river headwaters such as the Brahmaputra River, the Mekong River and the Salween River are relatively concentrated, and have similar hydroclimatic conditions. This similarity of hydroclimatic conditions provides the possibility to reconstruct past runoff changes using a large-scale treering data network. Based on the tree-ring width data sensitive to dryness and wetting changes in the southern Tibetan Plateau, this study reconstructed the changes in the total runoff of the Brahmaputra River, the Mekong River, and the Salween River from September to July of the previous year since 1000 AD, identified the mechanism affecting the flow changes of these rivers, and predicted the trend of runoff in the next 100 years using the relevant model data.

The results show that: (1) Runoff changes over the Tibetan Plateau affect the socio-economic development of Southeast Asia. The comparison between modern observational records and runoff reconstruction shows that the impact of runoff on vegetation productivity (NDVI) in the lower reaches of the basin is mainly concentrated in the dry season (Fig. 1). From the rise of the Bagan Dynasty and the construction of Angkor Wat during the Middle Ages (high runoff stage) to the weakening of Myanmar and the abandonment of Angkor Wat during the Xiaoice Age (low runoff stage), it is suggested that the runoff from the Tibetan Plateau driven by climate change may play an important role in the evolution of civilization in the basin (Fig. 2). In the evolutionary history of agricultural civilization in Southeast Asia, in addition to abundant precipitation in the local monsoon season, runoff from the distant Qinghai-Tibet Plateau can supplement the regional water shortage in the dry season and form a high regional agricultural productivity.

Figure 1(a) spatially correlates the reconstructed runoff with the average NDVI data from 1982 to March 2019. (b) Correlation between the runoff reconstruction series and the NDVI data of the dry season from April to May 1982 to 2019. (c) A conceptual model of the response of various systems in the transboundary watershed of high Asia to runoff change

Fig. 2 Comparison of regional paleohydroclimatic records of reconstructed runoff from 1050 to 1500

(2) The change of runoff on the Tibetan Plateau is affected by regional climate and large-scale sea-land-atmosphere interaction. Among them, the North Atlantic Multidecadal Variability (AMV) and the Pacific Decadal Oscillation (PDO) are important internal variables of the climate system that have caused changes in runoff over the past millennium. Therefore, improving the predictive power of internal variability, including AMV and PDO, is expected to improve the prediction of runoff on the decadal scale.

(3) Under the background of modern climate warming, the runoff of the Tibetan Plateau will show a significant upward trend in the future, and it is possible to reach and exceed the high runoff level of the medieval warm period. However, this growth may not be able to compensate for the rapid socioeconomic and demographic growth in Southeast Asia, which means that the water scarcity crisis is likely to continue to intensify. Therefore, how the countries in the basin can strengthen cooperation to overcome the water scarcity crisis is a question worth further pondering. By breaking down disciplinary boundaries, this study advances the understanding of the scientific question of “the chain response of the earth system on the Tibetan Plateau driven by global change and regional human activities”.

This study was co-funded by the National Natural Science Foundation of China (NSFC) Center for Basic Earth System Science (41988101) and the Second Tibetan Plateau Scientific Expedition (2019QZKK010206). (Source: Web of Science)

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