A combination of extreme rain in the northeast, retreating glaciers and poorly planned hydropower projects led to the current crisis in Sikkim, where a glacial lake outburst flood left at least four districts inundated and buried large swathes of land under silt and debris.

Read here: Sikkim floods: Weather hinders rescue operations as death toll rises to 27

Extreme rainfall in Sikkim, the north Sikkim region received around 39mm between Tuesday morning and Wednesday morning, triggered flash floods and is suspected to have triggered a glacial lake outburst flood (GLOF) on Lhonak Lake, that has so far claimed at least 27 lives.

The sudden surge of monsoon over eastern and northeastern India at a time when monsoon is in its withdrawal phase is a continuation of the erratic season observed in the country this year.

On September 30, HT reported that monsoon 2023 has been a story of swings, and as the months progressed, the variations only became more intense: 9% deficient in June, excess by 13% in July, deficient again by 36% in August and, finally in September, excess by 13%.

In its extended range forecast on Thursday, the India Meteorological Department said that the formation of consecutive low-pressure systems this month has intensified monsoon rainfall over east, northeast and parts of peninsular India. This, the forecast said, is expected to continue until October 18.

A low-pressure area is lying over central parts of Bangladesh, and a trough is running from Nagaland to southwest Bihar across the cyclonic circulation associated with the above low-pressure area, IMD said.

“Monsoon has not withdrawn from east India yet so when weather systems develop, we can see active monsoon conditions,” said M Mohapatra, director general, IMD.

Explaining the phenomena, Mahesh Palawat, vice president, climate and meteorology, Skymet Weather, said, “A low-pressure area formed over the eastern parts of the country but north-westerly winds did not allow it to travel across to northwest India. Instead, it is concentrated over east India. Moisture from the Bay of Bengal is feeding this system. The low-pressure system is stagnant over the eastern parts of the country causing heavy downpour.”

Due to this low-pressure area, the withdrawal of monsoon has been delayed, Palawat said.

But the erratic monsoon alone did not cause the Lhonak lake to burst on Wednesday morning.

South Lhonak glacier is among one of the fastest retreating glaciers, and the associated proglacial lake has become the largest and fastest-growing in Sikkim.

The glacier receded nearly 2 km in the 46-year period between 1962 and 2008. It further retreated by nearly 400 m from 2008 to 2019, a paper by the Department of Geology and Environmental Geosciences, University of Dayton, said in 2021. The paper warned of the increased susceptibility of the Lhonak lake to potential avalanche impacts as the lake grows in the future.

“Hazard intensity mapping based on the GLOF scenarios considered in the study shows that many settlements along the valley, including the largest town Chungthang, are at high risk… Due to the construction of the hydropower dam at Chungthang in 2015, GLOF risk mitigation in relation to South Lhonak and other critical lakes in the basin is of utmost importance,” the paper, published in Science Direct, said.

A GLOF is caused when the moraine (the debris accumulated over the years by a glacier) that usually functions as a dam, around the lake, is breached.

GLOFs can be triggered by extreme meteorological conditions, especially heavy rainfall events that cause degradation of the damming moraine and lead to overfilling of the lake.

And this is exactly what happened in Sikkim.

Around 1.30am on Wednesday, the overflowing Lhanak Lake burst, pushing the water downstream into the Chungthang dam, cleaving the structure, and flooding Mangan in North Sikkim, which bore the brunt of the disaster.

“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 disasters,” said Anil Kulkarni, glaciologist, a distinguished scientist at the Divecha Center for Climate Change and co-author of the Lhonak paper.

But the role of climate crisis was undeniable.

Read here: Residents pick up pieces after Sikkim floods leave behind trail of destruction

“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 said.

Experts warned that the risk of such incidents will only rise as the planet continues to warm, stressing the need for proper monitoring systems.

“We all know that in the warming world, the potential of cloudbursts, extreme rains, GLOF has gone up. Ocean warming has already increased moisture levels over the region. The low-pressure area near the region further increased the moisture, paving a conducive environment for torrential rains. Moreover, we do not have high-resolution monitoring systems that could tell us what happened, how it happened, and to what extent climate change aggravated the scenario. We know the Himalayan region is prone to cloudbursts, but we cannot identify the hotspot. Thus, a proper monitoring network is the need of the hour,” said Roxy Mathew Koll, Climate Scientist, Indian Institute of Tropical Meteorology in a statement on Friday.