Water circulation and water resources of Asia's water tower: The past and future

The Asia's water tower (AWT), regarded as the third pole of the Earth, serves as the critical water supply unit for the Asian region, with numerous glaciers and lakes, giving birth to many Asian rivers, and providing water security about 800 million people. The sustainability of water resources in AWT is of great significance to the social and economic stability and sustainable development of the surrounding regions, which is being threatened by climate change. With the impact of global warming and anthropogenic activities, the hydrologic cycle has intensified, and the water vapor budget and water resources supply have undergone significant changes. This has led to an imbalance in regional water resource supply, severely threatening regional food security and socio-political stability. Therefore, the water resources stock and sustainability of AWT are facing severe challenges. It is of great importance to understand the hydrological cycle characteristics of the AWT and its modulating effect on the surrounding climate, comprehensively understand the water resources change and its impact, and cope with the imbalance and unequal distribution of water resources that the AWT will face. Introducing water vapor transport and water circulation of AWT based on the published work, we divided the water vapor circulation process of the AWT into two parts: External water vapor transport and internal water vapor circulation, and analyzed the mechanisms and water vapor source regions for external water vapor transport to AWT through the midlatitude westerlies, Indian monsoon, and Asian monsoon. Then, we introduced the main components and mechanisms of interaction in the internal circulation in AWT. We also evaluated the effects of water resources on AWT and surrounding areas, and deciphered how the AWT affected the surrounding areas by changing water supply, water vapor transport, etc., and how the changes may lead to extreme hydrometeorological events. Due to the impact of global warming, climate change has posed a serious threat to the security of water resources in AWT. We identified the changing rules of factors related to water resources in AWT. The main changes of these factors are as follows: Increased precipitation, accelerated glacier melting, expansion of lake areas, thawing and degradation of permafrost, increased runoff, and decreased terrestrial water storage. These changes have disrupted the balance of water resources in the AWT, significantly change the regional water cycle in the AWT, and it is expected that the imbalance of water resources in the AWT will accelerate in the future. In general, focusing on the main line of the water cycle process and its effects, water resources change and their future prediction, we analyzed the results on the changes and influencing factors of water resources components, such as climate status, glaciers, runoff, lakes and terrestrial water storage in AWT, and systematically summarized the progress of water resources in AWT under climate change. Finally, we discussed the challenges of water resource imbalance in AWT, and outlined future research directions. We proposed that we should focus on the effect on AWT and its surrounding areas under climate change, enrich observational data and improve the accuracy of model simulations, and cope with future risk and challenges through cross-domain collaborative system governance. We hope that this article can serve as reference for promoting the future management of water resources and sustainable socio-economic development of the countries around AWT, fully utilizing the human welfare services provided thereby.