Himalayan glacial lake outbursts are a growing threat to South Asia

A satellite image shows the Himalayan Glacial Lake after it burst its banks and caused flash floods, in South Lhonak Lake, Sikkim, India, October 6, 2023.

India has already experienced the scale of damage that glacial lake outburst flood (GLOF) or glacier-breaking events can cause. The February 2021 glacier breach induced flash floods in Uttarakhand’s Rishi Ganga Valley, killing over 200 people, nearly washing away two hydropower plants, while also damaging Raini village, a historical border village where the Chipko movement was once active.

Two years later, a significant GLOF originated in South Lhonak in Sikkim on October 4, 2023. It led to the catastrophic collapse of the Teesta III hydroelectric dam at Chungthang in North Sikkim, causing widespread devastation downstream.

According to World Meteorological Organizations’ The State of the Climate in Asia 2023 report released on Tuesday, which sourced data from the National Emergency Response Centre of India (NDMI), the GLOF led to over 100 deaths and more than 70 missing individuals. Around 4,500 persons were evacuated, and nearly 90,000 people were affected. Additionally, about 2,000 houses were damaged.

The risk of dangerous GLOFs set to increase

Climate scientists have been warning about the risk of melting glaciers in the Himalayas that can catch disaster agencies, state and district authorities off guard in sudden and massive disasters. Anil Kulkarni, glaciologist and distinguished scientist at the Divecha Center for Climate Change had flagged this link last October. “We knew this was coming. We had warned the government that the breach can happen anytime for Lhonak. In this particular case it will be premature to say rainfall triggered the GLOF because we really need data of how much it rained in the higher altitudes around 5000 m above sea level where Lhonak is. Extreme rains may have triggered flash floods in Teesta for sure causing multiple,” he had said.

“Climate crisis is impacting glaciers and we need to monitor such lakes in Himachal Pradesh, Uttarakhand and Sikkim which are all vulnerable. It’s important to consider if hydropower projects should be located in these zones or you will end up losing thousands of crores in a go as we have seen for the Teesta hydropower project. I have been recommending climate assessments in addition to environment impact assessments for such projects,” Kulkarni added.

The Indian Space Research Organisation (Isro) on Monday announced that a little over one in every four glacial lakes larger than 10 hectares in area (100,000 sq metres) in the Himalayas have increased in size since 1984, heightening the risk of a GLOF. Long term satellite imagery covering the Himalayas has revealed that of the 2,431 glacial lakes larger than 10 hectares in area identified in 2016-17 and 676 glacial lakes have expanded notably since 1984. And 130 of the 676 are within India , with 65, seven, and 58 lakes located in the Indus, Ganga, and Brahmaputra River basins respectively, Isro said.

This is cause for major worry for downstream communities. Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD) released a 610-page report by 210 authors who studied the ecology of the Hindu Kush and Himalayan region. The report predicts that a third of glaciers in the Hindukush Himalaya (HKH) region will thaw if average global temperatures rise by 1.5 degrees Celsius. Glaciers have thinned, retreated, and lost mass across the HKH since the 1970s, except for parts of the Karakoram, eastern Pamir, and western Kunlun. These trends are projected to continue, with possibly large consequences for the timing and magnitude of glacier melt runoff and glacier lake expansion.

“Since the 1990s, glacial lakes show a clear increase both in number and in area. Several glacial lakes in the extended HKH are potentially hazardous — and as glaciers continue to retreat, the risk of dangerous GLOFs may increase further,” ICIMOD’s Summary of the Hindu Kush Himalaya Assessment Report said.

Another concerning factor is how rapid glacial melt and rise in GLOFs will impact the hydropower infrastructure in the Himalayan region. Hydropower infrastructure can change the timing and location of river flow, thereby disrupting natural flow regimes, in turn harming local irrigation, fisheries, and ecosystems.

“Such conflicts arise especially in the mid-hills and the mountains – the location of most current and foreseeable hydropower sites,” states the ICIMOD report.

The HKH has a total of 500 gigawatts (GW) of hydropower potential, of which only a small fraction is actually developed. The hydropower sector in HKH faces major challenges due to glacial melt induced by climate change. Glaciers across the region are retreating, leading to changes in future hydrological regimes. At the same time, risks of glacial lake outburst floods and landslides are increasing, putting both existing and planned hydropower plants at risk.

Its important to add that most deaths in the Uttarakhand February 2021 glacier disaster were of workers who were trapped in the Rishiganga (35 MW) and Tapovan Vishnugad (520 MW), which were nearly completely destroyed.

Melting of glaciers poses transboundary threats

Glaciers in high-mountain Asia have lost significant mass over the past 40 years, at an accelerating rate. In 2023, record-breaking high temperatures and drier conditions in

the eastern Himalayas and the Tien Shan exacerbated mass loss. The ICIMOD reports a rapid disappearance of glaciers, which is at a 65% faster rate in the 2010s compared to the previous decade.

For the glaciological year 2022/2023, 20 out of 22 glaciers observed in the HMA region show continued negative mass changes. Record-breaking high temperature and dry conditions in the East Himalaya and most of the Tien Shan exacerbated mass loss for most glaciers. During the period 2022–2023, Urumqi Glacier No. 1, in eastern Tien Shan, recorded its second most negative mass balance since measurements began in 1959.

Glacier ice mass is sensitive to changes in regional temperature, precipitation, and surface radiation. The melting of glaciers affects sea level, regional water cycles and the occurrences of local hazards such as GLOF. The HMA region is the high-elevation area centred on the Tibetan Plateau; it contains the largest volume of ice outside of the polar regions, with glaciers covering an area of approximately 100,000 km2.

Over the last several decades, most of these glaciers have been retreating, with the altitudes of the equilibrium lines (the lower topographic limit of the glaciers) gradually rising. In the past 40 years, four glaciers in the HMA region with more than 30 years of ongoing mass-balance measurements have recorded significant mass losses, with an increase in the rate of mass loss since the mid-1990s, according to the WMO.

This type of disaster is increasingly observed because of climate change-induced glacier retreat and highlights the compounding and cascading risks faced by vulnerable mountain communities. Glacial lakes formed by retreating glaciers, exemplified by the reduced expanse of South Lhonak Lake, pose threats that are transboundary, spanning across regions in Bhutan, India, Nepal and Pakistan. These events underscore the urgent need for global climate action to mitigate the increasing risks faced by mountainous regions.

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