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Irreversible declines in freshwater storage predicted in parts of Asia by 2060


The Tibetan Plateau, known as the “water tower” of Asia, provides fresh water to nearly two billion people who live downstream. New research by scientists from Penn State, Tsinghua University and the University of Texas at Austin predicts that climate change, under a weak climate policy scenario, will cause an irreversible decline in freshwater storage in region, constituting a total water supply collapse for Central Asia and Afghanistan and a near total collapse for northern India, Kashmir and Pakistan by mid-century.

“The prognosis is not good,” said Michael Mann, Distinguished Professor of Atmospheric Sciences, Penn State. “In a ‘business as usual’ scenario, where we fail to significantly reduce the burning of fossil fuels in the coming decades, we can expect a near collapse – that is, a nearly 100% loss – of water availability in the downstream regions of the Tibet Plateau. I was surprised at the magnitude of the predicted decline, even under a modest climate policy scenario.”

According to the researchers, despite its importance, the impacts of climate change on past and future terrestrial water storage (TWS) – which includes all groundwater and groundwater – in the Tibetan Plateau have been largely under-explored.

“The Tibetan Plateau provides a substantial share of the water demand of nearly two billion people,” said Di Long, an associate professor of hydrological engineering at Tsinghua University. “Terrestrial water storage in this region is crucial in determining water availability, and it is very sensitive to climate change.”

Mann added that it lacked a solid reference for the TWS changes that have already occurred in the Tibetan Plateau. Furthermore, he said, the absence of reliable future projections from TWS limits any guidance on policy-making, despite the fact that the Tibetan Plateau has long been considered a climate change hotspot.

To fill these knowledge gaps, the team used “top-down” – or satellite-based – and “bottom-up” – or ground-based – measurements of water mass in glaciers, lakes and underground sources, combined with machine learning techniques to provide a benchmark of observed changes in the TWS over the past two decades (2002-2020) and projections over the next four decades (2021-2060).

Mann explained that advances in the Gravity Recovery and Climate Experiment (GRACE) satellite missions have provided unprecedented opportunities to quantify large-scale TWS changes. Yet, previous studies have not explored the sensitivity of GRACE solutions using ground-independent data sources, leading to a lack of consensus regarding changes in TWS in the region.

“Compared to previous studies, establishing consistency between the top-down and bottom-up approaches is what gives us confidence in this study that we can accurately measure the declines in TWS that have already occurred in this critical region,” he said. he declares.

Next, the researchers used a new neural network-based machine learning technique to relate these observed changes in total water storage to key climate variables, including air temperature, precipitation, humidity, cloud cover and incoming sunlight. Once they “trained” this artificial neural network model, they were able to study the likely impact of predicted future climate changes on water storage in this region.

Among their findings, published today (August 15e) in the journal Natural climate changethe team found that climate change over the past decades has led to severe depletion of TWS (-15.8 gigatonnes/year) in some areas of the Tibetan Plateau and substantial increases in TWS (5.6 gigatonnes/ yr) in others, probably due to the competing effects of retreating glaciers, degradation of seasonally frozen soils, and expanding lakes.

The team’s projections for the future TWS under a moderate carbon emissions scenario – specifically, the mid-range SSP2-4.5 emissions scenario – suggest that the entire Tibetan Plateau could experience a net loss of carbon. ‘about 230 gigatonnes by the middle of the 21st century (2031-2060) compared to the beginning of the 21st century (2002-2030).

Specifically, projections of excessive water loss for the Amu Darya basin — which supplies water to Central Asia and Afghanistan — and the Indus basin — which supplies water to the north India, Kashmir and Pakistan — show a 119% and 79% decline in water supply capacity, respectively.

“Our study provides insight into the hydrological processes affecting high mountain freshwater supplies that serve large downstream Asian populations,” Long said. “By examining the interactions between climate change and TWS in the historical and future time frame to 2060, this study serves as a foundation to guide future research and the management by governments and institutions of improved adaptation strategies.”

Indeed, Mann added, “Substantial reductions in carbon emissions over the next decade, as the United States is now on the verge of achieving through the recent Cut Inflation Act, can limit additional warming and associated climate change behind the projected collapse of Tibet Plateau water towers But even in the best-case scenario, further losses are likely unavoidable, requiring substantial adaptation to dwindling resources water supply in this vulnerable and densely populated region of the world.

Mann noted that more alternative sources of water supply, including intensified groundwater extraction and transfer projects, may be needed to address amplified water scarcity in the future.

Other authors of the Tsinghua University paper include Xueying Li, Xingdong Li, Fuqiang Tian, ​​Zhangli Sun, and Guangqian Wang. Bridget Scanlon, senior researcher at the University of Texas at Austin, is also an author.

The National Natural Science Foundation of China and the Second Tibetan Plateau Scientific Expedition and Research Program supported this study.