Climate change ultimate trigger for South Lhonak glacial lake outburst flood
Photo: EastMojo The Glacial Lake Outburst Flood (GLOF) that took place at South Lhonak lake in Sikkim earlier this month was a result of cascading events triggered by climate change, a researcher has said.
Remya SN, assistant professor at the School for Sustainable Futures, Amrita Vishwa Vidyapeetham, Kerala, has said that a detailed analysis showed that the incident reported from North of Sikkim in the wee hours of October 4, 2023 unfolded as a result of “complex interplay of natural forces” accompanied by cascading hazards.
Remya told Down To Earth that satellite images indicated that an ice avalanche from the South Lhonak glacier, the source of the lake, triggered initial disturbance. The event then led to impacting the glacial lake by sudden increase in its water levels and subsequently breaching the moraine.
This coincided with a sudden cloudburst, further increasing the water levels in the lake and making the situation more critical, she stated in her observations.
The researcher conducted a multi-sensor analysis of the lake before and after the event using Planet Labs imagery (optical) and Sentinel-1A imagery (radar).
Images from Planet Labs were sourced from September 27 and October 6 and 7 to observe the changes before and after the event. The Sentinel-1 imagery processed from the European Space Agency (ESA) were also procured for September 25 and October 7.
The volume of the lake was noticed to be around 63 million cubic metres (pre-event).
Post the event on October 4, the lake displayed immediate changes in its characteristics with notable alterations. “Images demonstrated that a huge chunk of glacier ice from the South Lhonak glacier had fallen into the lake and the lake's area had receded from its terminus, and the water level had lowered,” the researcher said.
She added that the huge ice mass that fell into the lake might have generated catastrophic waves which resulted in the damage and breach of the terminal moraine.
“This phenomenon is attributed to the failure of the terminal moraine, which was damming the lake, resulting in the formation of a wider channel through which the lake water drained,” she noted.
She said that the chunks of ice can still be seen on the lake which is evidence of glacier ice avalanche.
Remya said that the images from October 7 (Sentinel-1) revealed a different look of the lake, giving a false impression that the lake's surface area had drastically reduced.
“A detailed analysis is needed to assess the water level, but the total surface area of the lake had not changed significantly. To be precise, from the Planet Imagery, South Lhonak Lake covered an area of 170 hectares on September 27, and this reduced slightly to 150 hectares on October 7,” she said.
It is observed that the geomorphological changes that occurred as a result of breach of the terminal moraine drained water downstream.
Remya stated the incident is a striking reminder of cascading hazard events triggered by climate change. “Climate change is causing the average global temperature to rise, which is leading to glacial retreat. As glaciers retreat, they leave behind moraine dammed / proglacial lakes. These lakes can be dangerous because they are often dammed by unstable moraines,” she added.
Max Van Wyk de Vries, United Kingdom-based senior research associate in Multihazard Risk Modelling, concurred.
On micro-blogging platform X, formerly Twitter, the expert wrote, “It is clear from the timelapse that major, metre scale movement occurred on this moraine prior to collapse. I used optical feature tracking to measure this: A large area of the lateral moraine was moving several metres per year, with the fastest areas moving at greater than 10 metres / year.”
He stated the destabilisation of these lateral moraines and opening of the lake basin is closely related to glacier frontal retreat. “So its link to climate warming is clearer here than in most cases,” Vries tweeted.
He further pressed the need for defining natural and unnatural disasters. Using the example of the South Lhonak lake incident, he said the hazard was a naturally occurring process known to scientists for years and hence the lake opening and landslide cannot be separated from human activity.
We need to think carefully about our definition of “natural hazards”, he said.
Source: Remya SN, Vishnu Nandan (University of Calgary)
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