Water supply of 2,000 million people has become unbalanced

The Third Pole, which is centred on the Tibetan Plateau, stores most of the world's frozen water after Antarctica and the Arctic. As a reliable water supply for nearly 2 billion people, it is known as the "Asian Water Tower." However, according to a new study published in the popular science journal Nature Reviews Earth & Environment, the situation is changing.

Water supply

Even as demand for its water continues to grow, the frozen assets of the Asian Water Tower are melting, with more liquid water in the northern endorheic basins and less in the southern exorheic basins. " Such an imbalance is expected to pose a major challenge to the supply and demand of water resources in downstream regions," explained Professor Yao Tandong, lead author of the study and co-chair of Third Pole Environment.

The study, titled The Asian Water Tower Imbalance, describes how it has become unbalanced. Rapid warming has shifted the balance of the Asian Water Tower's "stock mix" between solid water from glaciers and liquid water from lakes and river runoff. Changes in the atmospheric circulations that shape the region's climate have also altered the way its "inventory" is distributed. Almost a year ago a study warned that the so-called "Third Pole" could lose up to 40% of water in the thaw if global warming is not limited.

According to Dr. Tobias Bolch, one of the study's co-authors and a researcher at the University of St. Andrews, the North Asian Water Tower loses less solid water but gains more liquid water. "We think that the changing mass of westerly winds and the Indian monsoons have contributed to more rainfall in the north and less in the south," said Professor Gao Jing of the Tibetan Plateau Research Institute (ITP), Academy China of Sciences (CAS).

The Asian Water Tower imbalance is likely to manifest as an imbalance in water supply and demand in downstream communities, where the north has higher supply and the south has higher demand.

warming of Asian Water Tower

According to Professor Yoshihide Wada, one of the study's co-authors and a researcher at the International Institute for Applied Systems Analysis, Austria, the total water supply for the Asian Water Tower is projected to increase, but "a particularly strong increase" is expected for the north part.

In contrast, the greatest demand for water is estimated to be in the southern Indus basin. Study co-author and researcher from Utrecht University Prof. Walter Emergill linked this demand to irrigation, which accounts for more than 90% of the water use in the entire region. He said the densely populated Indus and Ganges Brahmaputra river basins "boast the world's largest irrigated agricultural area."

As a result of these trends, seasonal water availability in the Indus and Amu Darya river basins will change and the Yellow and Yangtze river basins will increase. This north (endorheic)-south (exorheic) disparity is also expected to increase with the warming of the climate in the future. “There is a great need for actionable policies for sustainable water resource management in this region,” said study co-author and researcher at Peking University and CAS, Prof. Piao Shillong said.

While these general trends are clear, scientists still need more information to help guide the public's response to changes in the Asian Water Tower. Ohio State University co-author Prof. "We need more accurate predictions of future water supplies to assess mitigation and adaptation strategies for the region," said Lonnie Thompson and co-chair of Third Pole Environment.

• During 1980–2018, the warming of the Asian Water Tower (AWT) was 0.42 °C per decade, twice the global average rate.

• Annual precipitation in the AWT increased by 11 mm per decade in endorheic basins and 12 mm per decade in exorheic basins, despite decreased precipitation in some large river basins.

• From 2000 to 2018, total glacier mass in the AWT decreased by about 340 Gt whereas total water mass in lakes increased by 166 Gt.

• Changes in the westerlies and the Indian monsoon led the AWT to develop an imbalance characterized by water gains in endorheic basins and water losses in exorheic basins.

• Ubiquitous increases in precipitation and river run-off are projected in the future of the AWT; however, these changes cannot meet the accelerating water demands of downstream regions and countries.

• Comprehensive monitoring systems, advanced modelling capacity and sustainable water management are needed to develop adaptation policies for the AWT through collaboration between upstream and downstream regions and countries.

The Hindu Kush-Karakoram-Himalayan system, called the Third Pole because it is the world's largest reservoir of frozen water after the polar regions, provides a reliable water supply to nearly 2 billion people. Marked atmospheric warming has changed the balance of this so-called Asian water tower and altered water resources in downstream countries. In

According to the study, they synthesize observational evidence and model projections that describe an imbalance in the Asian water tower caused by the accelerated transformation of ice and snow into liquid water. This phase change is associated with a south-north disparity due to the Spatio-temporal interaction between westerly winds and the Indian monsoon.

Alterations in freshwater resources in endorheic or exorheic basins exhibit a corresponding spatial imbalance. Global warming is expected to amplify this imbalance, alleviating water scarcity in the Yellow and Yangtze river basins and increasing scarcity in the Indus and Amu Darya river basins.

However, the future of the Asian water tower remains highly uncertain. Accurate predictions of future water supply require the establishment of comprehensive monitoring stations in data-sparse regions and the development of advanced coupled atmosphere-cryosphere-hydrology models. Such models are needed to inform the development of viable policies for the sustainable management of water resources.

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