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Speaking to News18, the study’s first author, Ashim Sattar, said South Lhonak glacier experienced rapid mass loss in the past four years, and the moraine’s instability was evident for years before the 2023 glacial lake outburst flood (GLOF)
Flood-affected locality at Singtam in Gangtok district of Sikkim in 2023. File image/PTI
South Lhonak Lake, which experienced Sikkim’s worst disaster in 2023, remains at a high risk of future glacial lake outburst floods (GLOFs), as the northern lateral moraine continues to deform. An international team of scientists has shown that satellite images from October 2023 to June 2024 revealed movement of up to 15 metres per year in the collapsed areas.
“The changed slope may cause more failures, as the moraine is now steeper than before the 2023 collapse,” warn scientists who just published their investigation into the deadly glacial lake outburst on October 3, 2023. The team also concluded that the disaster was exacerbated by years of warming, which made the moraine dam highly unstable.
“The instability of the moraines had been evident years before the event, with shifts of up to 15 metres per year,” the study’s first author, Ashim Sattar, assistant professor at the Indian Institute of Technology (IIT), Bhubaneswar, told News18. The final collapse of this northern moraine was linked to glacier retreat, lake growth, and water from a stream draining nearby glaciers.
Rapid retreat in last four years
The South Lhonak glacier has undergone rapid mass loss in recent decades, like many other Himalayan glaciers, due to long-term rising temperatures and local changes. However, over the past four years, which also experienced the three warmest summers on record (2020, 2022, and 2023), the mass loss increased to -0.58 ± 0.33 metres per year, causing the lake to expand by up to 100 metres annually. As the glacier retreated, the lake also grew bigger.
Additionally, the researchers estimate that the adjoining permafrost warming reached about 100 metres below the surface, close to the slide detachment depth of 85 m. “Climate change is one of the long-term drivers of this disaster. The glaciers are sensitive to rising temperatures, and they are responding to it by retreating faster than usual. Or else, the South Lhonak glacier would have supported the lateral moraine and prevented its fast displacement,” elaborated Sattar.
The team of scientists from as many as nine countries spent months reconstructing the exact time of the onset of the disaster using high-resolution satellite images, digital elevation models, and numerical simulations. They used seismic data to determine the exact time of the moraine collapse and published their findings in the peer-reviewed journal Science.
Wake-up call for climate change
Christian Huggel from the University of Zurich, one of the co-authors, highlighted that the event is a stark reminder of the vulnerability of high mountain regions to the effects of climate change.
“The thawing of permafrost and the instability of rock, ice, and moraine structures pose major risks. The case of South Lhonak Lake is a reminder to take climate risks in mountain regions worldwide more seriously,” he said, adding that collaboration among researchers from different disciplines helped in understanding the full extent of this event.
With more than 2,400 lakes larger than 0.1 km² in the Himalayas and many growing rapidly, the scientists warned of similar disasters in the future as rising temperatures increase the risk of glacial lake outbursts. They also called for stronger regulation of hydropower development in high-risk areas, better monitoring of glacial lakes, early warning systems, and robust adaptation strategies.
What exactly happened on Oct 3, 2023?
Over 14.7 million cubic metres of frozen moraine material, which was left behind by a moving glacier, had collapsed into South Lhonak Lake on October 3, 2023, which triggered a tsunami-like impact wave up to 20 metres high. The ensuing glacial lake flood (GLOF) drained nearly 50 million cubic metres of water—enough to fill 21,000 Olympic-sized swimming pools—and 270 million cubic metres of sediment, leaving a trail of destruction along the 385-kilometre-long valley.
It inundated massive infrastructure, roads, buildings, and villages, including the 1,200-megawatt (MW) Teesta-III hydropower dam on the Teesta River at Chungthang, killing 55 people, while 70 others were reported missing. The massive erosion and sedimentation also left serious consequences for farmers and local businesses. The South Lhonak Lake itself is located at 5200 m above sea level and is one of the largest, fastest-growing, and most hazardous lakes in Sikkim, with the potential to cause significant downstream damage in the event of a GLOF.
