Irreversible declines in freshwater storage projected in Asia by mid-century

A total collapse of water supply for central Asia and Afghanistan and a near-total collapse for northern India, Jammu & Kashmir and Pakistan by the middle of the century is predicted as a worst-case scenario in a comprehensive study on water storage in the Tibetan Plateau released recently.

The Tibetan Plateau, known as the ‘water tower’ of Asia, supplies freshwater for nearly two billion people who live downstream.

New research led by scientists at Penn State University, Tsinghua University and the University of Texas at Austin projects that climate change, under a scenario of weak climate policy, will cause irreversible declines in freshwater storage in the region.

“The prognosis is not good,” said Michael Mann, distinguished professor of atmospheric science at Penn State.

“In a ‘business as usual’ scenario, where we fail to meaningfully curtail fossil fuel burning in the decades ahead, we can expect a near collapse — that is, nearly 100 per cent loss — of water availability to downstream regions of the Tibetan Plateau. I was surprised at just how large the predicted decrease is even under a scenario of modest climate policy.”

According to the researchers, despite its importance, the impacts of climate change on past and future terrestrial water storage (TWS) — which includes all the above- and below-ground water — in the Tibetan Plateau have largely been underexplored.

“The Tibetan Plateau supplies a substantial portion of the water demand for almost two billion people,” said Di Long, associate professor of hydrologic engineering, Tsinghua University. “Terrestrial water storage across this region is crucial in determining water availability, and it is highly sensitive to climate change.”

Mann added that a solid benchmark for the TWS changes that have already occurred in the Tibetan Plateau has been lacking. In addition, he said, the absence of reliable future projections of TWS limits any guidance on policymaking, 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 below-ground sources, combined with machine learning techniques to provide a benchmark of observed TWS changes over the past two decades (2002-2020) and projections over the next four decades (2021-2060).

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

“Compared to previous studies, establishing consistency between 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.

Next, the researchers used a novel neural net-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 net model, they were able to look at how projected future changes in climate are likely to impact water storage in this region.

Among their results, published in the journal Nature Climate Change recently, the team found that climate change in recent decades has led to severe depletion in TWS (-15.8 gigatons/year) in certain areas of the Tibetan Plateau and substantial increases in TWS (5.6 gigatons/year) in others, likely due to the competing effects of glacier retreat, degradation of seasonally frozen ground, and lake expansion.

The team’s projections for 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 about 230 gigatons by the mid-21st century (2031-2060) relative to an early 21st century (2002-2030) baseline.

More specifically, excess water loss projections for the Amu Darya basin — which supplies water to central Asia and Afghanistan — and the Indus basin — which supplies water to Northern India, Kashmir and Pakistan — indicate a decline of 119 per cent and 79 per cent in water-supply capacity, respectively.

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