The breach of one of many “largest, fastest-growing and most hazardous” glacial lakes in Sikkim, the South Lhonak lake, led to cascading floods that killed 55 folks and washed away a 1,200 megawatt (MW) hydropower dam.
The occasion was recognized as a glacial lake outburst flood (GLOF), which is a sudden launch of water from a lake fed by glacial soften.
The analysis, printed in Science, explores the numerous drivers of the GLOF, its intensive impacts and coverage implications going ahead.
“There are lots of, many components that got here collectively right here,” the examine’s lead creator tells Carbon Temporary, however the “most important driver” was the destabilising impact brought on by thawing permafrost.
The analysis additionally finds that the South Lhonak lake has been increasing for many years, as a result of meltwater from the glacier above, with its space rising 12-fold between 1975 and 2023.
The paper concludes that the GLOF highlights the “advanced interactions” between local weather change, glacier mass loss and human infrastructure in mountainous areas.
It additionally demonstrates the significance of “sturdy monitoring methods and proactive measures to minimise devastating penalties and improve resilience”, the authors add.
Flood cascade
Sikkim is a small Himalayan state in north-east India, bordering China within the north, Bhutan within the east, Nepal within the west and the state of West Bengal within the south.
A part of the japanese Himalaya, Sikkim is host to greater than 90 glaciers and Kanchenjunga, the world’s third-highest peak. Sikkim serves because the origin and higher river basin for the Teesta river, one of many largest tributaries of the Brahmaputra river system.
On the evening of three October 2023, a ridge of frozen rock and different particles on the facet of the South Lhonak glacier – referred to as a “lateral moraine” – collapsed into the glacial lake. This set off a tsunami-like wave almost 20 metres excessive that breached the entrance of the lake, sending 50m cubic metres of water – virtually half the lake’s quantity – downstream.
In accordance with the examine, the GLOF’s peak discharge “vastly exceeds” the magnitude of any meteorological flood within the area’s historical past, equal to a “uncommon” one-in-200-year occasion.
Dr Ashim Sattar, a glaciologist on the Indian Institute of Know-how, Bhubaneswar and the lead creator of the examine, tells Carbon Temporary the sheer scale of affect will not be at all times evident in satellite tv for pc photos. He explains:
“Right here, 270m cubic metres of sediment was eroded, sufficient to fill 108,000 Olympic swimming swimming pools. The South Lhonak Lake itself is 2.8km lengthy. Simply strolling round it’ll make you sweat.”
Two hours later, the GLOF and big volumes of eroded sediment reached the village of Chungthang 68km away, destroying the 1,200MW Teesta-III hydropower undertaking on affect and damaging 4 different dams downstream.
Because the GLOF travelled, it set off 45 secondary landslides, a lot of them deep-seated and as much as 150 metres in depth, with impacts not simply in Sikkim, but additionally in neighbouring West Bengal and Bangladesh.
In all, the flood cascade broken 25,900 buildings, 31 main bridges and flooded 276km2 of agricultural land. Probably the most closely inundated zone was in Bangladesh 300km away, the place intense cyclonic rainfall – initially attributed as a most important GLOF driver – exacerbated flooding.
The determine beneath, taken from the examine, reveals before-and-after photos and illustrations of the moraine collapse and the flood’s path from Sikkim to Bangladesh, the place floodwaters lastly discharged into the Brahmaputra river.
An outline of the glacial lake flood cascade over 3-4 October, with (A) depicting the flood-impacted stretch of the Teesta river, landslides, hydropower crops and main settlements, (B) illustrating the moraine collapse and breach and (C) evaluating pre- and post-GLOF satellite tv for pc imagery of the lake. Supply: Sattar et al. (2025)
Dr Jakob Steiner, a geoscientist on the College of Graz and a member of the Himalayan College Consortium, who was not concerned within the examine, says the evaluation captures the “cascading” impacts of GLOFs and their interplay with different advanced, climatic components in nice element. He tells Carbon Temporary:
“Even when the glacial lake releases comparatively much less water, it could possibly set off different actions downstream and that may have far-reaching penalties, even for hydropower crops miles away from any lakes. So the message [of the study] is that you simply’re not secure anyplace and, hopefully, that’s a message that policymakers will get. Institutionally, nonetheless, we aren’t but ready to obtain that form of message.”
What prompted the flood?
To review such a fancy and multifaceted occasion, researchers mixed satellite tv for pc imagery, meteorological information, area observations and numerical modelling.
Examine lead Sattar tells Carbon Temporary that “capturing this whole course of into one mannequin may be very tough and complicated”.
All through the paper, the authors emphasise the “multi-hazard” nature of the catastrophe, explaining that a number of short- and long-term adjustments within the local weather and terrain converged to create the circumstances wanted for the occasion.
Nevertheless, Sattar tells Carbon Temporary that the “most important driver” of the GLOF was the long-term affect of rising temperatures on permafrost – the perennially frozen floor that makes up a lot of the mountain’s slope.
In accordance with the authors, many years of rising temperatures have led to permafrost thaw, which prompted “intensive, speedy deformation” of the slope for years previous the collapse. The paper estimates that permafrost warming has reached a depth of 100 metres beneath the floor of the soil.
The examine additionally identifies the growth of the South Lhonak lake as an vital driver. The authors discover that the South Lhonak glacier, which sits above the lake, has been melting for many years. Meltwater from the glacier flows instantly into the lake, which has been step by step filling up.
The charts beneath present the annual mass stability of the glacier (left) – the place a unfavorable quantity signifies a shrinking glacier – and the growing space of the lake (proper) between 1951 and 2023.
Annual mass stability of South Lhonak glacier (left) and growing space of South Lhonak lake (proper) between 1951 and 2023. Information supply: Sattar et al. (2025)
The analysis finds that the lake has been increasing by 0.32km2 per 12 months over 1975-2023. It notes there was a “doubling” within the price of growth over the previous 20 years.
The authors recommend that rising temperatures are accountable for the glacier dropping mass, because the annual common temperature within the area has been growing by 0.08C per decade because the Nineteen Fifties.
The long-term permafrost thaw and development of the lake implies that, by October 2023, the area was in a state of “elevated sensitivity” to a multi-hazard cascade, the paper says.
The authors say the ultimate “set off” was the extreme rainfall that hit Sikkim on 3-4 October. Although the rainfall was “typical” for the area and season, the authors say that it “saturated the soil and elevated the vulnerability of slopes to failure”.
Dr Stephan Harrison – a researcher from the College of Exeter – tells Carbon Temporary that the examine is “very vital” and is “written by a number of the main scientists within the area”.
Dr Miriam Jackson is the programme coordinator for the cryosphere initiative on the Worldwide Centre for Built-in Mountain Improvement, and was not concerned within the examine. She echoes Harrison’s reward, however warns in regards to the “lack of excellent information” within the area for these types of research. She says:
“We desperately want extra information on the standing of glaciers and glacial lakes, extra meteorology measurements at excessive elevation and extra information on the standing of frozen floor within the Hindu Kush Himalaya.”
Harrison and Jackson gave conflicting solutions about whether or not GLOFs are growing or lowering globally. Nevertheless, each pointed to the shortage of information on GLOFs, noting that datasets are incomplete or unavailable in lots of areas and emphasised the necessity to get higher data earlier than definitive solutions could be drawn.
Hydropower rush
The Sikkim GLOF occasion joins a sequence of latest disasters in high-mountain Asia which have destroyed hydropower crops. Given the sheer “bodily magnitude” of those occasions and their impacts, the examine highlights “potential limits to adaptation” within the Himalaya, warning that “even probably the most diligent and complete suite of catastrophe danger discount methods [is] unlikely to thoroughly forestall” loss and injury.
The examine attracts consideration to a “surge” of hydropower improvement within the Himalayan area close to glacial lakes, which it attributes to a rising demand for “steady and renewable power”.
With greater than 650 initiatives deliberate or underneath development in high-mountain Asia, it warns that many dams are “transferring nearer to those hazard-prone areas” and this might “exacerbate” GLOF impacts. The Teesta basin, as an illustration, hosts the best density of hydropower initiatives within the Himalayan area, with 47 dams deliberate, together with the reconstruction of the Teesta-III undertaking.
Whereas dams themselves are “prone” to a big selection of high-mountain hazards, in addition they improve the publicity of communities, employees and infrastructure investments to a “better probability” of GLOFs sooner or later, in line with the paper.
Complete danger assessments, stringent constructing requirements, regulating land use and regional cooperation amongst river-sharing nations are among the many measures prompt by the examine to cut back GLOF dangers.
Sattar says governments “could make a begin” by growing “basin-scale” early-warning methods. Nevertheless, he cautions that structural measures corresponding to draining glacial lakes “are simple to say, however troublesome to do” in harsh terrain.
In the meantime, geoscientist Steiner says it’s crucial that the important thing function performed by infrastructure improvement in injury brought on by GLOFs will not be downplayed – noting {that a} failure to take action dangers “absolv[ing] native establishments of their duty”. He concludes:
“As scientists, we discover it vital to indicate that local weather change is concerned, however we now have to bear in mind that the science we create may be very, very political… [A] massive a part of the catastrophe will not be local weather change; it’s institutional failures, it’s infrastructural failures.
“If no one takes the duty and everybody simply says: ‘it’s my neighbour and never me’, then we’re really in deep shit. Perhaps we already are.”
This story was printed with permission from Carbon Temporary.
