The glaciers of High Mountain Asia feed, among other things, the major Asian river systems such as the Indus, Ganges, Yangtze and Mekong - an enormous water reservoir for 250 million people. But because these high-altitude regions are remote and difficult to access, there are few direct observations of the changes in their ice masses. Models developed for glaciers in other regions, for example in the Alps, often do not reflect the distinct characteristics and processes of the Asian glaciers very well.
In the last few years, thanks to Earth-observation satellites, large datasets have become available at regional scales. Research led by Evan Miles from WSL has now used these data to disentangle the the yearly mass changes across more than 5000 of the largest glaciers in High Mountain Asia. The team, led by WSL glaciologist Francesca Pellicciotti, spends several months each year on HMA glaciers.
They used these results to assess the portion of yearly ice loss which is compensated by snowfall and avalanching for each glacier. The results represent average conditions for the period from 2000 to 2016; they were published in the scientific journal Nature Communications.
Most glaciers lose much more ice than they form
The results allow us to answer a number of important questions about the future of these glaciers. The picture is not exactly bright: 70 per cent of the glaciers were losing ice on an annual basis, and the majority of glaciers had only small areas accumulating ice. Only the glaciers around the Karakoram and Kunlun Mountains, located around the borders and contested areas of India, Pakistan, and China, were gaining mass due to recent increases in snowfall.
The loss of ice mass is of great importance for the water supply of the lower-lying land areas, where millions of people live and need the glaciers’ meltwater for agriculture. Calculations show that accumulation makes up for less than half of annual melt for the majority of glaciers. "For many glaciers, the ice is just melting away – accumulation can’t keep up," says Evan Miles, lead author of the study. “As a result, the majority of glaciers are simply not sustainable in their current geometry.”
By 2100, about one-fifth of the region’s ice will melt, even if the climate does not warm further, Miles said, and this will lead to long-term changes in meltwater supplied to mountain rivers. Impending climate change is not included in the figures from 2000 to 2016. It will further increase glacier melt and broadly alter water supply throughout the region.
“The silver lining for now” says Evan Miles, “is that key mountain river systems most important for downstream populations and sensitive to future changes are at the foot of glaciers that were growing during our study period.” Due to their high population density and location in arid regions, rivers such as Amu-Darya, Indus, Syr-Darya and Tarim Interior are particularly vulnerable to the loss of glacial meltwater. “But these glaciers are also sensitive to continued climate warming, and their melt is now already overcoming accumulation.”
The next steps for the research team will be to try to understand how debris-covered glaciers in particular behave across this vast, diverse region. This will further improve the prediction of expected meltwater volumes.