They stretch out their "icy tongues" into lakes, sometimes below the water surface. A study of ice giants in the Himalayas shows that the extent to which this invisible portion of ice is disappearing from glaciers has apparently been underestimated up to now. In the case of glaciers that flow into lakes, the average mass loss is up to 6.5 percent greater than previously assumed. This could be similar in other regions of the world. The scientists say this result is important for predicting further glacier retreat, as well as for assessing water resources and the risk of dam failures.
The warm breath of climate change is gnawing alarmingly at the global ice masses. This becomes particularly clear when looking at the retreat of the glaciers in the mountains of the world. The ice giants not only indicate the warming trend, they are also important as water reservoirs in the mountain regions and their changes also increase the risk of debris floods and other natural disasters in the valleys. However, there is one aspect of some glaciers that makes it difficult to assess how they will change as a result of climate change: their ice masses flow into lakes.
Glaciers with lakes in sight
This is especially the case in the Himalayan region: there are more than 5000 glacial lakes. Until now, it has been difficult to estimate how much the ice on the glaciers that flow into the lakes is disappearing. Because the changes in the lakes have not been recorded in sufficient detail so far, and a significant part of the ice body may be below the water surface and therefore not recorded by satellite images. For this reason, there have only been imprecise estimates of the loss of ice mass from glaciers with lakes.
In order to provide more information, the international research team has now taken a closer look at the Himalayan region. His results are based on the evaluation of long-term satellite data in combination with survey results by research vessels in the case of some sample glacial lakes. These so-called bathymetric data provided information about the structures on site. "We used multitemporal satellite data and an empirical area-volume relationship to estimate glacial lake changes across the Himalayan range and to quantify subaqualic mass loss," the researchers write.
An average of 6.5 percent more loss
As the team reports, the data evaluations revealed that the number of glaciers with lakes in the Himalayan region increased by around 47 percent between 2000 and 2020. In terms of surface area, the water surfaces expanded by 33 percent and there are signs of an increase in volume of 42 percent. As for the loss of subaquatic ice, the researchers calculated about 2.7 gigatonnes of ice mass that turned to water between 2000 and 2020. That's roughly the weight of 570 million elephants. Apparently, this aspect has led to the previous underestimates of the total ice losses: They are actually on average about 6.5 percent larger than previously assumed. The researchers report that the loss in the central Himalayas was even ten percent greater than had emerged from previous analyses. At the top is Lake Galong, whose loss of glacial mass was underestimated by as much as 65 percent.
"These findings are of great importance for understanding the impact on regional water resources and tidal waves from erupting glacial lakes," says lead author Guoqing Zhang from the Chinese Academy of Sciences in Beijing. "Furthermore, by measuring the loss of mass from these glaciers, one can more accurately assess their annual mass balance compared to those that end on normal land surfaces. This then shows the accelerated mass loss of the glaciers throughout the Himalayas even more clearly," says the researcher.
The study suggests that the retreat of glaciers that end in glacial lakes may have been underestimated from a global perspective. "This underscores the importance of including subaqualic mass loss from glaciers ending in lakes in future mass change estimates and glacier evolution models, regardless of the study region," says co-author Tobias Bolch from Graz University of Technology.
For co-author Yao Tandong from the Chinese Academy of Sciences, the study results are of particular importance for the assessment of water availability: "By being able to calculate mass losses from glaciers more precisely, researchers are better able to estimate the future availability of water resources in the sensitive to predict mountainous regions,” says the scientist.
Source: Graz University of Technology, Institute of Tibetan Plateau Research of the Chinese Academy of Sciences. Specialist article: doi: https://doi.org/10.1038/s41561-023-01150-1