The 2023 Sikkim disaster exposes growing risk of GLOFs in a warming Himalaya

On the night of October 4, 2023, a glacial lake in North Sikkim burst without warning, unleashing a deadly torrent down the Teesta River valley. The disaster, triggered by climate-driven glacial melt, left a trail of devastation and renewed alarm over the growing threat of Glacial Lake Outburst Floods (GLOF) in the eastern Himalayas.

Global climatic change has impacted the overall conditions of the northern mountain state in India. Research indicates that rapid increases in land surface warming (+0.03 degrees Celsius per year) have been observed in recent decades. Between 1951 and 2012, these rates surpassed the global mean surface temperature rise (0.011°C/year), according to a 2014 report by the Intergovernmental Panel on Climate Change (IPCC). 

GLOFs are one of the reasons Indian Himalayan states are facing devastating flash floods. Rising temperatures have significantly contributed to glacier thinning and retreat in the Indian Himalayan region, which eventually causes an increase in glacial lakes and rising Himalayan river water levels. The El Niño climate phenomenon, along with global warming, has caused record temperature rises in the ongoing and previous year. 

Glaciers and glacial lakes serve as one of the most important water resources for downstream communities, but over the years, the increase in the number and area of glacial lakes has also become a major threat to lives and infrastructure in downstream areas in the form of GLOF events.

The North Sikkim GLOF struck the downhill areas due to an abrupt rupture of South Lhonak glacial lake. The outburst spilled a massive amount of water down the Teesta River valley, devastating homes, infrastructure and lives in its path. It was fuelled by heavy rains and accelerated glacial melting linked to climate change. Bridges, hydropower plants and entire communities were washed away by the river, causing extensive destruction across North Sikkim and parts of the lower valleys. Hundreds were displaced, resulting in significant losses of life, livelihoods and ecological damage.

What caused the disaster

In recent decades, glacier retreat and ice loss have led to an increasing number and areal extent of glacial lakes in most high mountain regions around the world. A similar pattern has also been observed in Sikkim. According to our study on the growth of glacial lakes using satellite images and geographic information system tool applications, many lakes came into existence at different times after 1965 and have been expanding at varying rates over the last 30 years, particularly between 1978 and 2005.

One of the fastest-growing glacial lakes in the Eastern Himalayas is the South Lhonak lake. The failure of the side wall on the north-western side of the lake caused massive calving of ice blocks from the snout of the South Lhonak Glacier. Both processes generated extensive waves in the lake that eventually resulted in massive floods downstream on October 4, 2023. This wave, carrying a huge volume of water, eroded the outlet, making it much wider and deeper.

Our research shows area expansion of five potential glacial lakes from 1977 to 2022: Lake South Lhonak, Lake Gurudongmar (A), Lake Gurudongmar (B), Lake Tso Lhamo, Lake Khangchung Tso. Among these, the area extension of Lake South Lhonak lake has shown exponential growth in terms of area with approximately 1.67 square kilometres during the outbursting year.

The other lakes are in relatively good condition due to mitigation measures being cautiously implemented by the Sikkim government, in collaboration with Centre’s Department of Science and Technology and Indian Space Research Organisation.

However, South Lhonak’s proximity to downstream settlements (about 50 km away) made it particularly dangerous. An early warning system (EWS) installed near the lake was temporarily inactive due to extreme weather. Despite this, Prabhakar Rai, director of Sikkim State Disaster Management Authority (SSDMA), reported that many residents were evacuated promptly after local authorities issued warnings.

Aftermath of the disaster

The 2023 Sikkim GLOF caused widespread devastation, particularly in the Teesta River basin. We from Indian Institute of Technology Bombay conducted a post-disaster field study and identified Lachen, Chungthang and Toong Naga as the worst-affected areas following discussions with SSDMA, Rural Development Department, Directorate of Economics, Statistics, Monitoring and Evaluation, and Sikkim government representatives.

As a result of the flash flood, well-constructed reinforced cement concrete buildings accounted for 79 per cent of the destroyed housing stock. The primary cause of the damage was the unusual volume of water and the proximity of these buildings to the river. 

During the field visits, it was observed that the majority of buildings were located right beside the Teesta River; due to such violation of norms, many houses were completely damaged. There were 128 pucca houses totally damaged, 303 semi-pucca houses severely damaged and 858 partially damaged. In addition, 138 kutcha houses were completely damaged, followed by 35 severely damaged and 23 partially damaged, as per the post-disaster needs assessment report of SSDMA.

Out of the state's total 178 health facilities, 17 are located close to the Teesta basin and were affected by the flash floods. According to the Health Department, the tragedy affected seven medical facilities located in Namchi (Melli), Gangtok (Makha) and Mangan (Naga, Dikchu, Lachen, Lachung and Chungthang). The flash mud flood also caused significant damage to numerous highways, schools and other sources of income.

Timely response and impact assessment

Due to the prompt action of the SSDMA, rural management & development department (RDD) and other emergency departments, thousands of people were relocated at the earliest during the disaster; however, several went missing, including a few army personnel, according to RDD director Tenzing D Denzongpa. 

The Teesta riverbed rose by 8-12 metres due to sediment deposition. The lake outburst released approximately 65 million cubic metres of water, widening the Teesta’s floodplain and depositing sediment downstream. The path of the flash flood triggered several landslides along the Teesta riverbanks, causing damage to riverbanks and nearby houses, said the director of SSDMA. The flash flood moved at a rapid pace in a linear manner, destroying all standing structures including annual and perennial crops in its way.

According to the SSDMA, 618 farmers were affected, with 497 hectares of agricultural land fully damaged and 9.76 hectares partially damaged. Along both banks of the Teesta River, the districts of Mangan, Gangtok, Pakyong and Namchi were impacted by the flash floods. However, there were no reports of land or crop damage in the Pakyong area. 

The main cause of land destruction in the other districts was erosion and the accumulation of silt and large rocks, which severely damaged agricultural land. Roads such as National Highway 10, which connects Sikkim with West Bengal, were severely damaged due to bank erosion and multiple landslides, resulting in closures for several days.

The route leading to North Sikkim remained completely closed for several months. Army personnel were able to supply necessities to locals via the Indo-China route during this time, according to Denzongpa.

Awareness programmes

During the field visit, it was observed and documented that the Sikkim government has consistently taken the initiative to conduct awareness campaigns across the state, including in remote regions. Some of these initiatives are:

• Capacity-building workshops like Building Resilient Communities: Women and Sustainable Ecosystems in NER, organised by the Sikkim State Commission for Women in collaboration with the National Commission for Women.

• Participatory Rural Appraisal exercises (for example, evacuation mapping for vulnerable villages like Lachung Dzumsa).

• State-level mock drills to test emergency response protocols.

• Community awareness campaigns on GLOF risks, involving students and local residents.

These are just a few of the programmes run by the SSDMA; several other capacity-building and resilience exercises, as well as mock drills, have been held periodically.

The cost of a warming Himalaya

The 2023 Sikkim GLOF underscores the escalating threat of high-altitude hazards in the Eastern Himalayas due to climate change. Rising temperatures (+0.03°C/year) have accelerated glacier retreat, expanding glacial lakes like South Lhonak, which breached after decades of growth. 

The event underscored the growing vulnerability of mountain communities to high-altitude hazards amid a warming climate, where glacial lakes serve as vital water resources and are simultaneously a ticking time bomb too. 

Despite proactive efforts by the Sikkim government, including awareness programmes, drills and EWS, challenges such as the temporary inactivity of the EWS at South Lhonak lake reveal gaps in preparedness. The disaster’s aftermath, marked by unprecedented destruction and ecological harm, highlighted the urgent need for enhanced climate adaptation strategies, robust early warning infrastructure and stricter enforcement of safety norms to protect downstream settlements. 

Along with this, community-led preparedness needs to be more focused on the remote and high altitude areas in the states of Himalayas. 

As glacial lakes continue to expand across the Himalayas, the 2023 Sikkim GLOF serves as a stark warning of the cascading consequences of climate change, demanding a unified response to build greater resilience in these fragile ecosystems.

Debasmita Majumder is PhD senior scholar, Centre for Climate Studies, IIT Bombay; D Parthasarathy is professor, Centre for Climate Studies, IIT Bombay; and Trupti Mishra professor, Centre for Climate Studies, IIT Bombay

Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth