'invisible' glacier loss in greater Himalaya

A new study involving scientists from the University of St Andrews reveals that the mass loss of lake-terminating glaciers in the greater Himalaya has been significantly underestimated due to the inability of satellites to see glacier changes occurring underwater.

The research, which utilizes multi-temporal satellite data and bathymetric measurements, offers valuable insights into the growing issue of subaqueous mass loss in the Himalaya and provides critical implications for the region's future projection of glacier disappearance and water resources.

Published in Nature Geoscience, the study, conducted by an international team including researchers from the Chinese Academy of Sciences, Graz University of Technology in Austria, the University of St Andrews, and Carnegie Mellon University in the USA, demonstrates that a previous assessment underestimated the total mass loss of lake-terminating glaciers in the greater Himalaya by 6.5%. The most significant underestimation of ten per cent occurred in the central Himalaya, where glacial lake growth has been the most rapid. A particularly interesting case is Galong Co in this region, with a high underestimation of 65%.

This oversight was largely due to the limitations of satellite imaging in detecting underwater changes, which has led to a knowledge gap in our understanding of the full extent of glacier loss. From 2000 to 2020, proglacial lakes in the region increased by 47% in number, 33% in area, and 42% in volume. This expansion resulted in an estimated glacier mass loss of around 2.7 Gt, equivalent to 570 million elephants, or more than 1000 times the total number of elephants living in the world. This loss was not considered by previous studies as the utilized satellite data can only measure the lake water surface but not the ice underwater replaced by water.

"These findings have important implications for understanding the impact of regional water resources and glacial lake outburst floods," says lead author Guoqing Zhang from the Institute of Tibetan Plateau Research at the Chinese Academy of Sciences. "By accounting for the mass loss from lake-terminating glaciers, we can more accurately assess the specific annual mass balance of these glaciers compared to land-terminating ones, which further highlights the accelerated glacier mass loss across the greater Himalaya."

The study also highlights the need to understand the mechanisms driving glacier mass loss and the underestimated mass loss of lake-terminating glaciers globally, which is estimated to be around 211.5 Gt, or roughly 12%, between 2000 and 2020.

"This emphasizes the importance of incorporating subaqueous mass loss from lake-terminating glaciers in future mass-change estimates and glacier evolution models, regardless of the study region," explains co-corresponding author Tobias Bolch from Graz University of Technology, Austria, who worked at St Andrews until the end of 2022.

"In the long run, we expect mass loss from lake-terminating glaciers to continue being a major contributor to total mass loss throughout the twenty-first century as glaciers with significant mass loss may disappear more rapidly compared to existing projections," adds David Rounce, another co-author from Carnegie Mellon University.

"By more accurately accounting for glacier mass loss, researchers can better predict future water resource availability in the sensitive mountain region," said co-author Tandong Yao, who also co-chairs Third Pole Environment (TPE), an international science programme for interdisciplinary study of the relationships among water, ice, climate, and humankind in the region and beyond.