PART TWO ...continued from Part One

Avalanches and Floods

For several years we have been integrating findings from various catchments into a model that describes all the processes influencing Himalayan water flow; other scientists are doing the same in other mountain regions worldwide. Climate change presents some conspiring factors. One is elevation-dependent warming, in which mountains warm faster than lower-lying plains because of atmospheric feedbacks such as cloud formation, increased humidity and higher albedo as snowpacks retreat. Global warming of 1.5 degrees C means 2.1 degrees C of warming in the Himalayas.

Another factor is seasonality. A warmer atmosphere holds more moisture, which leads to more mountain precipitation. And more of it falls as rain than as snow, landing on rocky surfaces that were previously covered by ice, quickly running into rivers. According to research published July 2020, most models predict a wetter climate, although conditions can vary widely in the region. Compared with glaciers in a steady state, melting glaciers will provide more river water in the short term but less in the long term as glaciers retreat uphill and ice thins. We estimate that Langtang will hit peak water supply around 2060; after that, the supply will drop steadily.

We will achieve even greater understanding as we erect more sensors, making the observational network denser, particularly at high altitude, and as we integrate the data with extremely detailed models. Satellite remote sensing can also help us better estimate precipitation patterns between sensors across the basin, allowing us to fine-tune our models. Other research teams are making similar progress in mountainous terrains. Abundant data sets are available for the Alps, for example, and for the Andes. Coverage is getting better in the Himalayas as researchers at Kathmandu University and Tribhuvan University shift their attention to higher ground, taking measurements across the Annapurna and Everest mountain ranges.

My team has analyzed supply worldwide by using hydrological simulations, too. Our Nature study ranks mountain water towers worldwide. We consider a water tower “important” if it is rich in glaciers, snow or lakes and if water demand is great from people downstream. Significant water towers include the Colorado, the Fraser in western Canada and the Negro in Argentina, as well as the European Alps feeding the Rhine and Po Rivers.

Our modeling shows that Asia's ranges, which feed major rivers such as the Amu Darya and Indus, are the most important in the world. They are also among the most vulnerable: the models project strong rates of warming there, as well as rapidly growing populations and economies that will increase water demand tremendously. Average water availability is not likely to decrease until midcentury, in part because of greater monsoon rainfall, but the longer-term forecast is grim. We predict that 50 to 60 percent of the ice volume will be lost by the end of the century unless the world radically reduces it greenhouse gas emissions.

The big challenge for high-mountain Asia in the short term will be coping with changes in the timing of river flow and with natural hazards. In some basins, snow melt may start several weeks earlier than before, requiring farmers to change crops or sowing schedules. With snowpack providing less of a buffer, swollen rivers will increase across a region that is already facing heavy flooding every year.

Extreme rainfall in the mountains is also causing more landslides, particularly during monsoons. Greater melting is filling glacial lakes to the brim, causing disastrous floods when rocky ridges that dam the lakes burst because of the immense water pressure behind them. In the past two decades natural disasters such as avalanches, landslides and sudden floods have caused thousands of casualties and billions of dollars' worth of economic damage. Future increases in extreme rainfall and warming will exacerbate these hazards. Damages will rise, too, as growing populations build towns and hydropower dams at increasingly high altitudes.

Although these overall trends are clear, each region must be studied in detail to provide people there with useful information. One anomaly is a Central Asian region connecting the eastern Pamir and Karakoram ranges with the western Kunlun Shan. Glaciers there are stable or even gaining mass, which we see almost nowhere else on the planet. Data collected within the past year or so reveal that greater farming and irrigation in the nearby Tarim Basin may play a role. Irrigation water withdrawn from groundwater and surface sources evaporates into the atmosphere, and transpiration by crops adds even more moisture. This vapor condenses over the mountains and falls as snow—a critical reminder that human actions can alter natural systems.

Mountain Policies

Research on the high-altitude water cycle is already creating awareness of the importance of mountain water to billions of people worldwide. Officials should start acting now to safeguard it.

The first step is to include mountains in broader discussions about preserving Earth's natural resources. Locally, leaders can create national parks to protect peaks from development. They can set policies to reduce emissions of pollution and black carbon to lessen airborne debris. And they can build reservoirs to store rainwater and snowpack that melts rapidly in the spring—as long as they analyze how a structure's size and effects on water flow could interfere with ecosystems. An excellent example comes from Langtang Valley. The upper village had no electricity until 2019, when a nongovernmental organization and the community built a hydropower plant that now provides electricity, and therefore Internet, to villagers.

Neighboring countries can work together to reduce water demand; treaties can cover competing withdrawals from rivers flowing down from high peaks, which frequently cross national boundaries. Ministers from eight countries that touch the Hindu Kush Himalayan region set a strong example in October 2020 when they met for a mountain summit and signed a declaration pledging to use science to improve mountain policies, listen to advice from the region's highly diverse population and speak as a unified voice in global negotiations. Millions of people in Afghanistan, Bangladesh, Bhutan, China, India, Myanmar, Nepal and Pakistan depend on the Hindu Kush Himalayas for water, and rain patterns and crop yields there are already changing.

The world's high peaks are transforming rapidly. Within the next few decades many people living downstream will have to adapt to more weather extremes, greater natural risks and shifts in water supply. Scientists, engineers and policymakers should join forces and act now to ensure that sustainable mountain water resources will be available for future generations.

Walter Immerzeel is a physical geography professor at Utrecht University in the Netherlands, where he leads the mountain hydrology group. He has lived in Nepal and has been measuring snow and ice high in the Himalayas since 2002.

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