What is a GLOF?

As a glacier retreats, a phenomenon more common now as temperatures continue to soar, it leaves behind a glacial lake. As the ice melts, the water collects in the depression that once formed the glacier. This water is dammed by moraine (loose regolith and rock), or unstable ice. Over time, as the glacial lake swells, the pressure on the moraine dam containing it increases.

Often these dams break, due to reasons as varied as landslide debris falling on it, heavy rainfall increasing the lake volume, or fragmentation at source. What results is a sudden tsunami-like gush of water, which rushes down mountain slopes.

“There has been a substantial increase in the number of glacial lakes as the glaciers are melting due to global warming. When the glaciers advance, they erode the river bed, leading to the deepening of the river. Also, climate change has resulted in erratic precipitation, including events like cloudbursts, as we saw in Sikkim, leading to the partial breach of the moraine dam,” said Farooq Azam, glaciologist, Indian Institute of Technology - Indore, in a statement.

Last documented event

While there was speculation that the 2021 flooding in Chamoli district of Uttarakhand was due to a similar glacial event, the last such disaster – where there was consensus on the reason being a GLOF – was in 2013 in Kedarnath.

In June that year, the state received an unusual amount of rainfall, and the Chorabari glacier melted into a lake that burst into the Mandakini River. In the following days, the official death toll was placed close to 6,000.

How common is it?

GLOF as a climate risk is so ill-perceived that Google searches for any western media outlet pages on it throws up articles on Golf instead. The incidence, however, is higher.

In July 2015, the Lemthang Tsho GLOF breached in north-western Bhutan after two days of incessant rainfall. The impact of the flooding was minimal, but experts pointed to the vulnerability of glaciers in the region soon after the event. A more alarming example is perhaps of Pakistan, where at least 30 GLOF incidents were reported in the mountainous region of Gilgit-Baltistan and Chitral in a two-month period last year, which the country’s meteorological agency attributed to “unprecedented hot weather”.

How big a threat is it?

A global assessment published in February this year found that three million Indians live in areas where a GLOF can happen at any time. Along with Pakistanis — both countries in the Hindu Kush Himalayan region — the population forms a third of the total number of people globally that face this risk.

In all, globally, the at-risk population is estimated at about 90 million. India, Pakistan, Peru and China account for over 50% of the globally exposed population.

Of this, "one million people living within 10km downstream of a glacial lake, where any early warning time is likely to be low and uncertainty in GLOF magnitude high", the study, ‘Glacial lake outburst floods threaten millions globally’ noted.

A separate study, ‘Glacial Lake Outburst Flood Hazard, Downstream Impact, and Risk Over the Indian Himalayas’, identified 329 glacial lakes bigger than 0.05 km sq in the Indian Himalayas. Of these, 23 were considered very high risk and 50 as high-risk lakes.

Of the 23 critical glacial lakes, the study published in 2020 said, 17 were in the state of Sikkim.

And, a total of 67 of the 329 lakes had at least one hydropower system along their flow path – an added risk factor, as the developments in Sikkim showed this week. The Teesta Urja project created an obstruction in the river, and involved a settlement to come up — that town was among the hardest hit.

Why it happened in Sikkim

The South Lhonak Lake was under threat observation for some years now. A report of the Sikkim Forest and Environment Department found that the lake’s area had increased in the past five years. “Lhonak has grown nearly 1.5 times and South Lhonak nearly 2.5 times of their initial size in 1989…,” it said.

But, the immediate trigger was likely the around 39mm rain that north Sikkim received between Tuesday morning and Wednesday morning, which possibly triggered an avalanche, that set off the GLOF.

The role of climate change

While not directly attributable to climate crisis, the phenomenon of GLOF becomes more likely as the planet heats, glaciers continue to melt, and weather patterns change. Periods of torrential rain over glacial lakes aggravate the situation, putting more pressure on the water bodies, causing them to overflow.

“Both the frequency and severity of such events are going to increase exponentially in the future. Himalayan ecosystem is the most fragile in the world and any disruption in the way we are managing these resources will have a problematic outcome for the people of the region. GLOF is an outcome of warming of the region and this has been a major risk for the region. Once it is formed, you never know what triggers its outburst. Sikkim is an example of this only,” Anjal Prakash, Clinical Associate Professor- Research and Research Director, Bharti Institute of Public Policy, Indian School of Business and IPCC Author, said in a statement.