Sikkim glacial lake outburst spotlights climate vulnerability of the Himalayas

Severe flooding of the Teesta river valley in the eastern Himalayan state of Sikkim has killed at least 14 people and left over a hundred missing since 4 October 2023. The floods have damaged buildings, highways and the Chungthang dam downstream, and impacted life in scores of small villages in the valley.

The first videos that emerged on social media suggested a cloudburst. Later, radar satellite images released by the Indian Space Research Organisation (ISRO), provided evidence that the size of the South Lhonak glacial lake had changed, confirming that the flash flood was triggered by a combination of glacial lake outburst flood (GLOF) and excess rainfall.

Melt water from the receding South Lhonak glacier at a height of about 7000 meters in Sikkim Himalayas formed a huge lake at its snout. The South Lhonak lake at an altitude of 5200 metres is one of the largest and fastest-growing proglacial lake in the region.

Though the exact cause of this outburst has not yet been ascertained, an avalanche or breaking of ice chunks (calving) at the glacier's snout may have possibly triggered the flood. ISRO satellite images reveal that more than half the lake’s expanse was reduced.

Glacial lakes are formed when glaciers retreat due to climate change, leaving behind meltwater in hollows. When ice melts, it creates lakes dammed by moraine ― material such as rocks, boulders and loose soil left behind by the retreating glacier. When the lake is past its capacity or impacted by external mass, it bursts causing a GLOF and wreaking havoc downstream.

Vulnerable mountains

Increased construction activity in the Teesta valley since 2006 has significantly heightened its vulnerability to flooding disasters. Two recent studies evaluated potential GLOF events that could originate from the South Lhonak lake. Mapping the downstream infrastructure at risk of flooding, the studies showed that dams, settlements and assets in the Teesta valley are exposed to the threat of GLOFs. The valley's largest town Chungthang is at high risk. We highlighted the importance of GLOF risk mitigation at South Lhonak due to the construction of the hydropower dam at Chungthang in 2015.

These studies should help develop GLOF risk management strategies and an early warning system in the valley.

South Lhonak lake was already recognised as potentially hazardous, with researchers warning of a GLOF. Scientists at the National Remote Sensing Centre warned in a 2013 paper that there was a 42% chance that Lhonak Lake could burst. The Himalayan region has been witness to many such flash floods in the past― the 1926 Jammu and Kashmir deluge, the 1981 Kinnaur valley floods in Himachal Pradesh and the 2013 Kedarnath outburst in Uttarakhand.

The South Lhonak GLOF has brought back to focus the region’s extreme vulnerability to climate change, which is altering glacier dynamics in the Himalayas. Glaciers are disappearing at an unprecedented rate, which means there will be devastating consequences for people and nature.

Kathmandu-based International Centre for Integrated Mountain Development said in a report that glaciers disappeared 65% faster in the 2010s than in the previous decade. If we don’t reduce greenhouse gas emissions, 80% of the current volume of Himalayas glaciers will be gone by 2100.