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Climbing the Asian Water Tower

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Characterising the water tower of Asia

The high mountain Asia water cycle contains clues about how climate change may impact water supply and natural hazards for the millions below. Blending research approaches, including some technological firsts, CAT has produced the data necessary for accurate modelling.

Climate Change and Environment

High mountain Asia (HMA), including the Himalayas, is Asia’s water tower. Water is stored in snow and glaciers, and 10 major rivers, including the Ganges, originate here. As around 20 % of the world’s population live in river basins below the HMA, understanding high altitude water cycles is critically important. Changes determine water supply but can also cause damage, such as the recent Uttarakhand flood. Yet, the region’s remoteness and inaccessibility make measurements of glacier, snow, mountain weather and hydrology difficult. The CAT project, supported by the European Research Council combined high-altitude field observations with remote satellite sensing of glacier mass and snow. “We were also the first to use drones to monitor glacial dynamics, now widely used in the region,” explains Walter Immerzeel, professor of Geosciences at the University of Utrecht and coordinator of the EU-funded project. This data informed detailed hydrometeorological models able to make projections of the likely hydrological impact of climate change-induced changing mountain weather patterns. Early project results fed into a key study, published in ‘Nature’, which found that even if the 1.5 °C threshold is met, 36 % of the ice volume will disappear by the century’s end. If not met, CAT’s projections ranged between 49 % and 64 %. Additionally, as HMA warms more rapidly than the global average, a 1.5 °C increase globally would result in a 2.1 °C regional increase.

Glacier areas, volumes and changes

The CAT team focused particularly on rock and sediment debris covered glaciers, prevalent in the Himalayas. The debris impacts melt rates, as thick layers insulate ice, reducing melt; while thin layers darken the surface, reducing surface reflection and so accelerating melt. Drones were flown every 6 months to survey the glaciers. The data was used for models that reconstructed the surface elevation of the glaciers and tracked changes and variability in melt. A network of precipitation gauges took rain and snow measurements. Measuring snow properties such as sublimation rates, temperatures and depths helped establish the interaction with atmospheric variables and speed of melting, crucial for validating atmospheric simulations. “By understanding how large-scale atmospheric weather patterns, alongside debris-covered glaciers and snow sublimation, influence the high-altitude water cycle, we can learn how glaciers and snow packs in the region will likely respond to climate change and impact water availability,” adds Immerzeel. Contrary to expectations, a key finding was that climate change will result in more water in the coming decades as glacier melt and precipitation increase. Another finding relates to an HMA region where the Karakoram, Pamir and Kunlun Shan mountain ranges meet and where the glaciers are growing and gaining mass. “This was caused by increased irrigation using groundwater reserves. We showed how in the Tarim Basin in China, irrigation directly impacted glacial growth as more water evaporates and atmospheric moisture increases, causing more clouds and snowfall in the mountains,” says Immerzeel.

Preparing for extremes

The team is now working with the International Centre for Integrated Mountain Development in Kathmandu, a knowledge institute of key decision makers. A notable outcome is ‘The Hindu Kush Himalaya Assessment: Mountains, Climate Change, Sustainability and People’, a flagship report featuring CAT’s findings. “The key regional challenge is dealing with mountain weather extremes, potentially bringing more floods, landslides and glacier collapses. I am now focused on the role of climate change and the increased vulnerability of people and infrastructure,” concludes Immerzeel.

Keywords

CAT, Asia, water, climate change, snow, ice, glacier, drone, debris, melt, sublimation

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