Anatomy of a disaster: What the 2023 Sikkim glacial flood tells us about climate risk in the Himalayas

Vaishnavi Chandrashekhar

Plans to rebuild the Teesta III dam destroyed by a glacial lake flood are being opposed by environmentalists and local communities.

Icebergs, growlers and bergy bits breaking off glaciers at Cape York, Greenland.

Pic credit: Wikimedia Commons

On the night of October 3, 2023, a landslide on the flanks of South Lhonak Lake, located high in the Himalayan state of Sikkim, triggered a 20-metre-high tsunami in the lake that helped breach its earthen boundary, releasing millions of cubic metres of water downstream. Water raged down the hill—eroding riverbank, setting off mini-landslides, and picking up the force and debris needed to destroy the Teesta-III dam, the state’s biggest hydropower project, located at Chungthang 67.5km downstream. 

Now, India plans to rebuild the dam. Should it? 

That question—and answer—is important not only for Teesta III but for the future of dozens of hydropower projects and all kinds of other development in the Himalayas that are at potential risk from melting glaciers due to global warming. The UN rang the alarm on this issue on the first ever World Glaciers Day on March 21, introduced as part of 2025’s International Year of Glaciers’ Preservation. 

The past three years has seen the largest loss of glacier ice on record, the UN noted, with potential consequences for natural disasters in the high mountains such as avalanches, landslides, and glacial lake bursts as well as for water supply to two billion people globally. The Hindu Kush Himalayas, which stretch from Afghanistan to Nepal, is home to thousands of glaciers, and whatever happens to them has an outsize impact since they are the source of major rivers, including the Indus, the Ganges, and the Brahmaputra, in the world’s most populous region.

Disasters like the one in Sikkim highlight the potential devastation caused by landslides and glacial lake outbursts floods or GLOFs. As a glacier melts, the water pools into a glacial lake. The build-up of water from heavy rainfall or snowmelt or a landslide or quake can lead the lake to suddenly breach its boundaries—a GLOF. One study projects almost a three-fold rise in GLOF risk and hazard in high mountain Asia, especially in the eastern Himalayas. Another study found that half of the 15 million people globally exposed to exposed to the impact from potential GLOFs, live in just four countries: China, Peru, Pakistan, and India. 

Two decades ago, such events felt almost hypothetical. No longer. In August 2024, a glacial lake burst caused flooding of Thame village in the Everest region in Nepal, destroying homes, a school and a clinic, and displacing some 135 people. In May 2022, a GLOF in Gilgit-Baltistan in Pakistan displaced 1,000 families, many of whom have migrated semi-permanently for work in towns or villages downhill. And in 2021, a rock-and-ice avalanche in Chamoli district in Uttarakhand killed some 200 people and destroyed two under-construction hydropower facilities. 

The good news is the region is waking up to these risks. In September, India’s National Disaster Management Authority announced that it was installing high-tech warning systems for 190 glacial lakes at risk of bursting, including six in Sikkim. The National Glacial Lake Outburst Floods Risk Mitigation Programme will include multiple expeditions to the lakes, many of which have only been studied by satellite so far. Bhutan, which has over 500 glacial lakes, is also working on an early warning system for GLOFs in high-risk areas. 

And in December, the Intergovernmental Centre for Integrated Mountain Development (ICIMOD), a multi-country organisation in Kathmandu, launched the Hindu Kush Himalayan Disaster Risk Reduction (DRR) Hub, aimed at improving understanding, information sharing, and action across the region. “Hindu Kush Himalayas, also known as the Third Pole, provide ecosystem services to a quarter of the global population. But it is a global hotspot for disasters,” said ICIMOD head Pema Gyamtsho at the launch. The organisation noted last week that only 38 of the thousands of glaciers are monitored in situ, and called for expansions in glacier monitoring and data sharing across the region. 

But while India and other countries are starting to install monitoring and warning systems to mitigate the impact of any future disaster, it’s not clear whether they are willing to scale back development in the Himalayas, especially of hydroelectric projects. In greenhouse gas emission terms, hydropower is clean energy---and is thus an important piece of the Indian government’s plan to install 500 gigawatts of renewable energy capacity by 2030. 

Many of these projects are on rivers that flow from the Himalayas. A 2022 study warned of risk to more than 650 hydro projects being planned or constructed close to glaciers or in the path of lake bursts. With 47 hydro facilities, the Teesta basin has a high density of such projects.

Yet the remoteness of these areas means risks are easy to ignore. And, as a recent important study of the Sikkim event concluded, current models may be inadequate to assess some of the threats. The study, carried out by 34 researchers from around the world, found that long-term climate warming leading to loss of ice as well as local topographical factors contributed to the GLOF at Lhonak. “This multihazard cascade exhibits the complex interactions between climate change, glacier mass loss, and human infrastructure in mountainous regions,” said researchers led by Ashim Sattar. They call for a “paradigm shift” in modelling techniques for GLOFs, as well as improvements in risk management and infrastructure development in the high mountains. 

Sattar and his colleagues’ detailed analysis of the South Lhonak GLOF is instructive about the way in which we tend to underestimate risk as well as the potential impact of these events.  

The Lhonak glacial lake was known to be a threat—a 2021 study found it had tripled in size between 1990-2019, and warned of the risk of the lake bursting and flooding areas downstream. Official agencies had also submitted reports on the potential risk. Yet, as Sattar’s recent study showed, almost 60% of the buildings damaged by the disaster was built in the past decade---which means that risk was likely not communicated to the people who would be most affected. (Another study found that the people who had migrated here for work from neighbouring states are the most vulnerable to such disasters.) 

The disaster also highlights the far-reaching impact of “multi-hazard cascading events”. The glacial lake flood led to erosion of the banks of the Teesta, leading to 45 secondary landslides, most of them beyond 100 km downstream of the lake, the study found. The most affected buildings were located below Chungthang, beyond 200km from the lake. The most heavily inundated zone was even farther downstream, between 290-385km away in Bangladesh. And the impact was compounded by other events: heavy rain the next day in Bangladesh also contributed to flooding. 

The toll of the disaster: at least 55 deaths, 74 missing, and over 7000 displaced; hundreds of cattle, sheep, and goats lost; 25,900 buildings damaged; 276 km2 of agricultural land flooded; 31 major bridges affected along with 20 pedestrian ones.  

Moreover, the risk is not over. The erosion of the riverbanks has weakened them, making them “susceptible to future collapse, particularly near roads and settlements”, Sattar’s study notes. Future landslides and GLOFs from upstream of South Lhonak pose a risk to this weakened terrain. Better early warning systems along with “enhanced infrastructure resilience and rigorous land-use management practices are essential to mitigate GLOF risks,” the study concludes, and not just in one country but from a transboundary perspective. 

No wonder then that Sikkim’s local communities and opposition parties as well as   environmentalists have opposed what they call the ‘hasty clearance’ of the new Teesta III dam. Even the state BJP unit has expressed its concerns to the prime minister. “This proposal should be treated as a fresh proposal in the context of changing rainfall patterns, the GLOF vulnerabilities,” said Himanshu Thakkar, of the South Asia Network on Dams, Rivers and People. Given the scale and history of this project, he argued, the new dam needs “a fresh clearance, with fresh impact assessments and fresh public hearing.” News reports say the last public consultations for the dam were held when the project was originally proposed---almost twenty years ago. The climate has changed since then.

THE IMPORTANCE OF GLACIERS

✧ There are more than 275 000 glaciers in the world, covering an area of around 700 000 km².

✧ Glaciers are important 'water towers',  storing about 170 000 km3 of ice, which amounts to approximately 70% of the global freshwater.

✧ More than 2 billion people, including many Indigenous Peoples, rely on melt from glaciers and snow for their freshwater, including for their food security, livelihood, cultural and domestic needs.

✧ Glaciers are sensitive indicators of climate change and are retreating globally due to rising temperatures.

✧ Glaciers are projected to vanish in one-third of the current sites by 2050.

✧ The ongoing decline in glaciers contributes to global sea-level rise, with today’s sea level about 20 cm higher than in 1900.

✧ As glaciers shrink and snow cover changes, less water availability is expected to contribute to greater competition for water resources, especially in seasonally dry regions.

✧ Continuous glacier retreat also leads to extreme events and new and evolving disaster risks for downstream populations and vulnerable transport and energy infrastructure, such as glacier lake outburst floods, landslides or enhanced erosion and sediment.

✧ The disappearance of glaciers would also result in a substantial loss of cultural heritage and spiritual connection to the landscape and nature.

✧ In their ice, glaciers contain an important record of the past climate and environment. The disappearance of glaciers results in the irreversible loss of unique archives of human, environmental, and climate history.

Source: United Nations

Earth Shifts, a monthly column on The Migration Story, will analyse the impact of global green goals amid mounting climate uncertainties on lives and livelihoods.

Vaishnavi Chandrashekhar is an environment and science journalist based in Mumbai.