Climate change played a key role in the “catastrophic” 2023 floods in the Himalayan state of Sikkim in India, a new study says.
The breach of one of the “largest, fastest-growing and most hazardous” glacial lakes in Sikkim, the South Lhonak lake, led to cascading floods that killed 55 people and washed away a 1,200 megawatt (MW) hydropower dam.
The event was identified as a glacial lake outburst flood (GLOF), which is a sudden release of water from a lake fed by glacial melt.
The research, published in Science, explores the many drivers of the GLOF, its extensive impacts and policy implications going forward.
“There are many, many factors that came together here,” the study’s lead author tells Carbon Brief, but the “main driver” was the destabilising effect caused by thawing permafrost.
The research also finds that the South Lhonak lake has been expanding for decades, due to meltwater from the glacier above, with its area growing 12-fold between 1975 and 2023.
The paper concludes that the GLOF highlights the “complex interactions” between climate change, glacier mass loss and human infrastructure in mountainous regions.
It also demonstrates the importance of “robust monitoring systems and proactive measures to minimise devastating consequences and enhance resilience”, the authors add.
Flood cascade
Sikkim is a small Himalayan state in north-east India, bordering China in the north, Bhutan in the east, Nepal in the west and the state of West Bengal in the south.
Part of the eastern Himalaya, Sikkim is host to more than 90 glaciers and Kanchenjunga, the world’s third-highest peak. Sikkim serves as the origin and upper river basin for the Teesta river, one of the largest tributaries of the Brahmaputra river system.
On the night of 3 October 2023, a ridge of frozen rock and other debris on the side of the South Lhonak glacier – called a “lateral moraine” – collapsed into the glacial lake. This set off a tsunami-like wave nearly 20 metres high that breached the front of the lake, sending 50m cubic metres of water – almost half the lake’s volume – downstream.
According to the study, the GLOF’s peak discharge “vastly exceeds” the magnitude of any meteorological flood in the region’s history, equivalent to a “rare” one-in-200-year event.
Dr Ashim Sattar, a glaciologist at the Indian Institute of Technology, Bhubaneswar and the lead author of the study, tells Carbon Brief the sheer scale of impact is not always evident in satellite images. He explains:
“Here, 270m cubic metres of sediment was eroded, enough to fill 108,000 Olympic swimming pools. The South Lhonak Lake itself is 2.2km long. Just walking around it will make you sweat.”
Two hours later, the GLOF and huge volumes of eroded sediment reached the village of Chungthang 35km away, destroying the 1,200MW Teesta-III hydropower project on impact and damaging four other dams downstream.
Exclusive visuals from completely damaged Chungthang Dam in Sikkim
Received from Pokhraj Rai Ji pic.twitter.com/x9gFxs1PC6
— Weatherman Shubham (@shubhamtorres09) October 12, 2023
As the GLOF travelled, it set off 45 secondary landslides, many of them deep-seated and up to 150 metres in depth, with impacts not just in Sikkim, but also in neighbouring West Bengal and Bangladesh.
In all, the flood cascade damaged 25,900 buildings, 31 major bridges and flooded 276km2 of agricultural land. The most heavily inundated zone was in Bangladesh 300km away, where intense cyclonic rainfall – initially attributed as a main GLOF driver – exacerbated flooding.
The figure below, taken from the study, shows before-and-after images and illustrations of the moraine collapse and the flood’s path from Sikkim to Bangladesh, where floodwaters finally discharged into the Brahmaputra river.
Dr Jakob Steiner, a geoscientist at the University of Graz and a member of the Himalayan University Consortium, who was not involved in the study, says the assessment captures the “cascading” impacts of GLOFs and their interaction with other complex, climatic factors in great detail. He tells Carbon Brief:
“Even if the glacial lake is relatively small, it can trigger other movements downstream and that can have far-reaching consequences, even for hydropower plants miles away from any lakes. So the message [of the study] is that you’re not safe anywhere and, hopefully, that’s a message that policymakers will get. Institutionally, however, we are not yet prepared to receive that kind of message.”
What caused the flood?
To study such a complex and multifaceted event, researchers combined satellite imagery, meteorological data, field observations and numerical modeling.
Study lead Sattar tells Carbon Brief that “capturing this entire process into one model is very tricky and complex”.
Throughout the paper, the authors emphasise the “multi-hazard” nature of the disaster, explaining that multiple short- and long-term changes in the climate and terrain converged to create the conditions needed for the event.
However, Sattar tells Carbon Brief that the “main driver” of the GLOF was the long-term impact of rising temperatures on permafrost – the perennially frozen ground that makes up much of the mountain’s slope.
According to the authors, decades of rising temperatures have led to permafrost thaw, which caused “extensive, rapid deformation” of the slope for years preceding the collapse. The paper estimates that permafrost warming has reached a depth of 100 metres below the surface of the soil.
The study also identifies the expansion of the South Lhonak lake as an important driver. The authors find that the South Lhonak glacier, which sits above the lake, has been melting for decades. Meltwater from the glacier flows directly into the lake, which has been gradually filling up.
The charts below show the annual mass balance of the glacier (left) – where a negative number indicates a shrinking glacier – and the increasing area of the lake (right) between 1951 and 2023.
The research finds that the lake has been expanding by 0.32km2 per year over 1975-2023. It notes there has been a “doubling” in the rate of expansion over the past two decades.
The authors suggest that rising temperatures are responsible for the glacier losing mass, as the annual average temperature in the region has been increasing by 0.08C per decade since the 1950s.
The long-term permafrost thaw and growth of the lake means that, by October 2023, the region was in a state of “increased sensitivity” to a multi-hazard cascade, the paper says.
The authors say the final “trigger” was the intense rainfall that hit Sikkim on 3-4 October. Though the rainfall was “typical” for the region and season, the authors say that it “saturated the soil and increased the vulnerability of slopes to failure”.
Dr Stephan Harrison – a researcher from the University of Exeter – tells Carbon Brief that the study is “very significant” and is “written by some of the leading scientists in the field”.
Dr Miriam Jackson is the programme coordinator for the cryosphere initiative at the International Centre for Integrated Mountain Development, and was not involved in the study. She echoes Harrison’s praise, but warns about the “lack of good data” in the region for these sorts of studies. She says:
“We desperately need more data on the status of glaciers and glacial lakes, more meteorology measurements at high elevation and more data on the status of frozen ground in the Hindu Kush Himalaya.”
Harrison and Jackson gave conflicting answers about whether GLOFs are increasing or decreasing globally. However, both pointed to the lack of data on GLOFs, noting that datasets are incomplete or unavailable in many regions and emphasised the need to get better records before definitive answers can be drawn.
Hydropower rush
The Sikkim GLOF event joins a chain of recent disasters in high-mountain Asia that have destroyed hydropower plants. Given the sheer “physical magnitude” of these events and their impacts, the study highlights “potential limits to adaptation” in the Himalaya, warning that “even the most diligent and comprehensive suite of disaster risk reduction strategies [is] unlikely to entirely prevent” loss and damage.
The study draws attention to a “surge” of hydropower development in the Himalayan region near glacial lakes, which it attributes to a rising demand for “stable and renewable energy”.
With more than 650 projects planned or under construction in high-mountain Asia, it warns that many dams are “moving closer to these hazard-prone areas” and this could “exacerbate” GLOF impacts. The Teesta basin, for instance, hosts the highest density of hydropower projects in the Himalayan region, with 47 dams planned, including the reconstruction of the Teesta-III project.
While dams themselves are “susceptible” to a wide array of high-mountain hazards, they also increase the exposure of communities, workers and infrastructure investments to a “greater likelihood” of GLOFs in the future, according to the paper.
Comprehensive risk assessments, stringent building standards, regulating land use and regional cooperation among river-sharing countries are among the measures suggested by the study to reduce GLOF risks.
Sattar says governments “can make a start” by developing “basin-scale” early-warning systems. However, he cautions that structural measures such as draining glacial lakes “are easy to say, but difficult to do” in harsh terrain.
Meanwhile, geoscientist Steiner says it is critical that the key role played by infrastructure development in damage caused by GLOFs is not downplayed – noting that a failure to do so risks “absolv[ing] local institutions of their responsibility”. He concludes:
“As scientists, we find it important to show that climate change is involved, but we have to be aware that the science we create is very, very political… [A] big part of the disaster is not climate change; it’s institutional failures, it’s infrastructural failures.
“If nobody takes the responsibility and everyone just says: ‘it’s my neighbour and not me’, then we are truly in deep shit. Maybe we already are.”
The post ‘Catastrophic’ 2023 lake outburst in India driven by glacial melt and permafrost thaw appeared first on Carbon Brief.
‘Catastrophic’ 2023 lake outburst in India driven by glacial melt and permafrost thaw
Dr. Stacy Jupiter is the Executive Director of the Wildlife Conservation Society’s Global Marine Program. Melissa Wright is Bloomberg Ocean Initiative Lead at Bloomberg Philanthropies.
For years, the dominant story on coral reefs has been one of inevitable loss, with news headlines focusing on mass bleaching, ecosystem collapse, and catastrophic tipping points. As ocean temperatures continue to rise, many people have come to see the decline of the world’s reefs as unavoidable.
The threats are real and urgent, but new evidence points to a more complicated and useful conclusion: some reefs still have a meaningful chance to survive and recover, provided they are protected.
A major new analysis, published today with the support of Bloomberg Philanthropies, identifies more than 165,000 square kilometers of coral reefs, across 71 countries and 100 territories and jurisdictions, with the strongest potential to withstand and recover from climate impacts.
Drawing on more than 45,000 coral surveys, along with decades of climate and ocean data, the research finds that three times more reefs may be capable of surviving the climate crisis than previously understood. That has major implications for reef-dependent communities, food security, coastal protection, fisheries, tourism, and national economies.
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Essential natural infrastructure for communities
The findings make clear that reefs will not all respond to climate impacts in the same way. Some are located in rare underwater cool spots that can help shield them from extreme heat. Some show greater resistance to bleaching and other climate-related stress. Others recover more quickly after severe disturbances. These differences matter because they show where protection can have the greatest long-term impact.
More than 500 million people depend on reefs for food, livelihoods, and coastal protection. For those communities, climate-resilient reefs are not an abstract conservation priority. They are essential natural infrastructure. They help protect coastlines, sustain fisheries, support local economies, and reduce climate risk. Because ocean currents move coral larvae and marine life between reef systems, some of these reefs may also help regenerate wider reef ecosystems after climate shocks.
This should change how governments, funders, and conservation partners prioritize action.
Climate change remains the greatest long-term threat to coral reefs. At the same time, many of the pressures pushing reefs closer to collapse are immediate and local. Sewage pollution, deforestation, agricultural runoff, destructive fishing practices, and poorly managed coastal development continue to damage reefs that are already under stress. Recent research shows that water pollution and fishing pressure are now among the leading local threats affecting nearly two-thirds of the world’s coral reefs.
These pressures can be reduced. Governments and local partners are already working to improve reef management, cut pollution, strengthen enforcement, and protect critical ecosystems. Those efforts need to move faster, alongside much stronger action to reduce greenhouse gas emissions.
Prioritising climate-resilient reefs
The new maps of climate-resilient reefs give governments, communities, and reef managers a clearer basis for action. They show where reefs have the strongest potential to persist over time, and where protection can deliver the greatest benefits for people, coastlines, and economies.
Right now, only around 28 percent of the identified climate-resilient reefs fall within protected or conserved areas. If these reefs are among the most capable of surviving climate impacts and helping regenerate broader reef systems, they should be prioritized for protection, management, and investment.
The case for action is practical as well as ecological. Healthy reefs can reduce wave energy by up to 97 percent, helping protect coastlines from storms, flooding, and erosion. They support fisheries that feed millions of people, sustain tourism jobs and local economies, and help reduce climate risk for vulnerable coastal communities.
For many families, a healthy reef means food, income, and protection when storms hit. For Indigenous Peoples and coastal communities, reefs are also tied to culture, heritage, identity, and traditional knowledge systems.
Ocean conservation must catch up
Governments are beginning to recognize the urgency of protecting climate-resilient reefs. At last year’s UN Ocean Conference in Nice, 11 countries signed a declaration committing to stronger protection of these reefs, including action to address destructive fishing, pollution, and unsustainable coastal development.
As leaders meet in Kenya this week to discuss the challenges facing the world’s ocean, more governments should join the declaration and help build a broader coalition committed to safeguarding these critical ecosystems.
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Some countries are already showing what this leadership can look like. Brazil has included corals in its national climate plans. The Bahamas is embedding reef protection into national policy and local stewardship systems. The declaration offers a way to build on these efforts and scale them globally.
But commitments will not be enough. Success will depend on implementation. That means stronger protection and management, reduced local pressures, increased investment, and meaningful support for the Indigenous Peoples and local communities stewarding these ecosystems.
The science is clear. Many reefs still have the capacity to persist and recover. The question is whether policy and investment will move quickly enough to protect them, so they can continue sustaining communities, economies, and coastlines for generations to come.
The post Coral reefs are not doomed – but policy must catch up with the science appeared first on Climate Home News.
Coral reefs are not doomed – but policy must catch up with the science
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