The Third Pole (TP) is the high-mountain region of Asia that contains the largest glacial area outside of the Arctic and Antarctic. This region is known as the ‘Asian Water Tower’ because it provides invaluable freshwater resources for large human populations in Asia. The TP is the source of many large Asian rivers, including inland rivers (e.g., Amu Darya, Syr Darya, and Tarim) and exorheic rivers that flow into the Pacific Ocean (e.g., Yangtze, Yellow, and Mekong) and Indian Ocean (e.g., Indus, Brahmaputra, Ganges, and Salween). Some of the water towers, such as the Indus, are highly vulnerable to environmental and socio-economic changes. The basins of these TP rivers are inhabited by 1.9 billion people who depend on mountain water for their livelihoods, energy, agriculture, domestic use, and industry, representing a vitally important source of freshwater for China, southern Asia, southeast Asia, and central Asia.
In the past half-century, the warming rate of the TP (0.3‒0.4 ℃/10a) has been approximately twice that of the global average, described as the elevation-dependent warming phenomenon. Possibly in response to a warming world, the westerlies have strengthened and shifted poleward over the past 50 years. Furthermore, the large-scale circulations dominate the precipitation changes around the TP and in turn influences the changes of glacier. Previous studies find that the patterns of precipitation across the TP have changed considerably, with increasing convective precipitation over the westerlies-dominated central TP, and decreasing precipitation over the monsoon-dominated southern and eastern TP. Rapid warming across the TP has intensified glacial retreat, with an accelerating rate of mass loss since 2000 and contrasting patterns observed between Himalayan and Karakoram glaciers. The warming across the TP has also caused permafrost degradation and affected the hydrological cycle of this region. Combined with these climatic changes, the rapid growth of the populations and economies of Asian countries in recent decades has caused enhanced anthropogenic impacts on the TP river systems, such as water withdrawal from the Amu Darya and Syr Darya for irrigation of cotton in central Asia. These changes to the multi-sphere system (atmosphere‒cryosphere‒biosphere‒hydrosphere‒anthroposphere) of the TP have affected intra-annual and interannual variability in river runoff, impacting the sustainability of water resources for downstream ecosystems and society.
Routine ground-based hydro-meteorological observations are very scarce in the high-mountain river basins of the TP owing to the complex topography and harsh environment, which limits the study of long-term runoff changes. In addition, it is challenging to compile existing runoff observations for transboundary rivers, as many countries have strict data sharing policies. Consequently, it is currently difficult to quantify the overall changes in TP runoff over the past half-century from a coherent, regional perspective.
To solve this difficult question, we combined both in-situ observations (10 rivers) and cryosphere‒hydrology model simulations (2 rivers with large anthropogenic impacts) to study the long-term (1960‒2016) changes in mountain-outlet runoff for 12 river basins. We find that the mountain-outlet runoff generally experienced significant increases for the westerlies-dominated rivers (Indus, Amu Darya, Syr Darya, Tarim, Heihe, and, Shule) and insignificant declines for the monsoon-dominated rivers (Ganges, Brahmaputra, Mekong, and Salween) in the past half-century, largely driven by the enhanced westerlies and weakened Indian summer monsoon. Although the changing rates of runoff can be mostly explained by the varying precipitation minus evapotranspiration, the total water storage changes (e.g., regional glacier melting, groundwater depletion) cannot be neglected. After the year 1997, the contrasting changes in the westerlies- and monsoon-dominated regions have been remarkably accelerated, necessitating proactive adaptations to sustain regional water, ecology, and food security.
The full paper titled"Acceleration of diverging runoff trends on the Third Pole" can be found at https://doi.org/10.1038/s43247-025-02854-5.