The Road to COP26

Glacial lake outburst floods: How a lesser-known climate phenomenon threatens from above

Glacial lake
outburst floods:
How a lesser-known climate phenomenon threatens from above
As the planet warms and glaciers melt, the growing risk of a natural hazard lurks quietly in the mountain tops.

They strike rapidly, with little or no warning. They sweep down into valleys with torrents of water. And they stretch far, sometimes hitting communities hundreds of kilometers from where they started.

In and around the world's highest mountains, glacial lake outburst floods pose a worsening threat to an increasing number of people.

Glaciers are extremely sensitive to climate change, and as hot air melts ice and pushes glaciers into retreat, huge new lakes are appearing in high, often precarious pockets within mountain ranges. Many are poised to burst their banks.

The scientific journal Nature this year published a paper that found the melting of the world's glaciers has almost doubled in speed over the last two decades. Remarkably, glacial melt in mountain ranges now contributes more to rising sea levels than either the Greenland or Antarctic ice sheets do.

Ahead of a critically important diplomatic summit this month, known as COP26, the world’s leading climate scientists have warned this threat is only likely to intensify in the absence of urgent, rapid and large-scale reductions in greenhouse gas emissions.


The danger and beauty of glaciers is not something that is lost on those studying ice masses day after day, with experts noting how glaciers can both reflect the past and reveal the future.

“The first time going to a glacier and actually being on it is otherworldly,” Fiona Tweed, professor of physical geography at the U.K.’s Staffordshire University, told CNBC via telephone. “Standing on it, sometimes you can hear creaking and groaning noises. It is almost like it is alive because it is moving.”

“When I was doing fieldwork in Iceland years ago, I was lucky enough to get access to an area between the glacier and the bedrock and actually lie down on my back and put my hands on the glacier above me. I could feel it move very fractionally — at a snail’s pace — but it was completely awe-inspiring. I have got no other word for it,” Tweed said.

“Just being up close and personal with this thing which you think you know, having studied it — but it’s very different when you’re there. I’m not a religious person but it was almost like a spiritual experience getting up close and personal with a glacier for the first time.”

This sense of enduring fascination among glaciologists tends to be coupled with a deep sadness at a rapidly disappearing frozen world.

“As a scientist, it is mostly impressive to see how fast these changes are going on,” Matthias Huss, director of the Swiss glacier monitoring network Glamos, told CNBC. “But it also makes me sad of course, as a person.”

“I also love the mountains besides my job, and it is a sad thing to see these glaciers going — especially for the very small ones that are really disappearing right now,” Huss said.

I’m not a religious person but it was almost like a spiritual experience getting up close and personal with a glacier for the first time.
Fiona Tweed, professor of physical geography, Staffordshire University

As glaciers melt and retreat, lakes akin to natural water reservoirs form behind bedrock, ice or moraine dams. These lakes can be situated on top, in front, besides, underneath or even within a glacier.

An outburst flood refers to an event in which the water dammed by a glacier is suddenly released. In places that aren’t volcanic, the conditions required to trigger flooding depend on the nature of the lake itself.

The drainage of bedrock-dammed and ice-dammed lakes, for example, does not tend to result in dam destruction. To trigger an outburst flood in these cases, an event such as a landslide falling into the water may be required to displace the lake over the lip of the dam.

Moraine-dammed lakes, however, are typically unstable because they are composed of material such as sand, pebbles, rocks and ice residue and can be narrow and sharp-crested. Water can seep through the unconsolidated debris, and when this becomes saturated and heavy, it can fail — particularly if the level of lake water is rising.

Many other secondary events, such as an avalanche, a flood from a lake upstream and intense rainfall, can also trigger outburst floods, making them notoriously difficult to predict. It is when these glacial lakes burst that the flooding poses a major risk to life and infrastructure downstream.

Outburst floods can last for hours or days and peak flows have been recorded as high as 15,000 cubic meter per second. That’s the equivalent of 360 Olympic-size swimming pools roaring down a mountain valley every minute.

The first global analysis of the societal impacts of glacier floods, published in 2016 by glaciologists Jonathan Carrivick and Fiona Tweed, found that over 12,000 people have been killed by outburst floods worldwide.

The research identified South America and Central Asia as the region’s most vulnerable to incur large numbers of deaths, extreme damage to infrastructure and the destruction of homes, farms and roads. Nepal was singled out as among the countries likely to have the greatest national-level economic consequences of glacier flood impacts.

In the cases of Nepal and Switzerland, two countries facing the rising threat of glacier related hazards, outburst floods can both illustrate the devastating impact of the climate crisis at the local level and highlight the differing possibilities for high-income and low-income countries to react.

Take a hike through the Himalayas and Swiss Alps with world-leading glaciologists to learn more about why these events are among the world’s most feared natural disasters, whether they can be predicted and what can be done to mitigate the risk.


Nepal, a landlocked country of roughly 29 million wedged between India and China, is mainly situated along the southern slopes of the Himalayan mountain ranges. Highly vulnerable to climate-related threats and one of the poorest and slowest growing economies in Asia, recent studies suggest Nepal faces losing 2.2% of annual GDP as a result of climate change by 2050.

The South Asian country has borne witness to 18 recorded glacial lake outburst floods since 1977, while researchers have since located the glacial sites for 11.

Click on the list of events in the map below to take a tour.

Nepal's GLOF events since 1980
  • 1980: Nagma Pokhari
  • 1985: Dig Tsho
  • 1991: Chubung
  • 1998: Sabai Tsho
  • 2003: Kabache
  • 2004: Kabache
  • 2015: Lhotse Glacier
  • 2016: Lhotse Glacier
  • 2016: Unnamed Lake
  • 2017: Langmale
  • 2021: Unnamed Lake

The most striking and high profile of these events took place on Aug. 4, 1985 in eastern Nepal, just one day’s trekking from the world’s highest mountain, Mount Everest.

“That day was very sunny, no rain at all,” Rijan Bhakta Kayastha, a Nepalese glaciologist at Kathmandu University, told CNBC via telephone. “But, in the downstream, there was a huge flood.”

A large rock and ice avalanche in the high Himalayas crashed into the Dig Tsho glacial lake, generating a surge wave approximately 5 meters high and overtopping the moraine dam. The lake, a body of water measuring 1,500 by 300 meters and with a depth of 18 meters, drained almost completely over a four-hour period. The deluge destroyed bridges, homes, agricultural land and the nearly completed Namche Small Hydropower Plant, some 11 kilometers downstream.

Overall, the sudden outbreak discharged an estimated 6 to 10 million cubic meters of water into the valley below, causing over $3 million in damages and disrupting the downstream community of Khumbu for several months. A Sherpa festival at the time meant fewer people were walking the mountain trails, with four or five people thought to have lost their lives.

The Dig Tsho outburst flood raised alarm on an international scale and prompted a flurry of scientific investigations in the following years. Without doubt, experts say this event showed it is not necessarily the largest lakes that are the most dangerous, particularly if populated areas and infrastructure are situated nearby.

“We only have a very small amount of glacial lake outburst flood experience, and with that small one there was a huge impact,” Kayastha said. “But if big glacial lakes outburst, there will be huge damages in the downstream — mainly on the hydropower sectors and infrastructure, and homes will be washed away.”

What’s more, while Nepal’s Thulagi lake was found to have the smallest monetary value of elements exposed to an outburst flood, ICIMOD warned such an event could indirectly affect more than 2 million people — roughly 7% of the population — through infrastructure damage and loss of goods and services. By comparison, an outburst flood at Imja Tsho and Tsho Rolpa could indirectly impact roughly 500,000 people living downstream.

A spokesperson at ICIMOD told CNBC that while these were the best available estimates at the time, the risk of exposure to glacial lake outburst floods in Nepal was likely to be even higher today.

They added the basins downstream of these three lakes have seen a lot more development in the years since the report was first published, including an increase in the number of settlements and hydropower projects.

To experts at environmental non-profit Germanwatch, apart from reducing risks on the local level, “only the reduction of greenhouse gases can curb global warming and thus the retreat of glaciers in the long run.” This is especially important to Nepal, they added in a 2004 study, because the water reserves of more than 500 million people downstream are at stake due to glacial melt.

A comprehensive study published in 2011 by the International Centre for Integrated Mountain Development, a regional government body based in Kathmandu, outlined the estimated exposure of people and infrastructure in Nepal at risk to glacial lake outburst floods.  It conducted a field investigation into three glacial lakes, namely Tsho Rolpa, Thulagi Lake, and Imja Tsho, and analyzed the vulnerability of human life and property for up to 100 kilometers.

All three lakes were formed as a series of meltwater ponds decades ago and are now 2 to 3 kilometers long, with maximum depths close to 100 meters. The risks to power supplies and revenue from an outburst flood at these lakes was described as “substantial.” When accounting for proposals for hydroelectricity projects along the Marsyangdi river, ICIMOD found a total revenue of $8.98 billion could be at risk for the Imja Tsho, $2.4 billion for Tsho Rolpa and $2.2 billion for Thulagi — in the absence of measures to mitigate this threat.

With Nepal’s GDP estimated at $33.6 billion last year, this modeling shows an outburst flood at the Imja Tsho lake — one of the fastest growing in the Himalayas — poses a danger to more than a quarter of Nepal’s annual output.

Upper part of Lower Barun Galcial Lake-Sept. 2021. Photo: Rijan Bhakta Kayastha
Down wasting of glacier ice: Dr. Kayastha and his team on GPR survey for determining ice thickness from Kathmandu University (November 2018). Photo: Rijan Bhakta Kayastha
Tea time on Ponkar Glacier: from left to right (clockwise); Rijan B. Kayastha, Sharad Joshi, Krishna Kusi, Rakesh Kayastha and Sudan Maharjan (September 2018). Photo: Rijan Bhakta Kayastha
We only have a very small amount of glacial lake outburst flood experience, and with that small one there was a huge impact.
Rijan Bhakta Kayastha, glaciologist, Kathmandu University
Only the reduction of greenhouse gases can curb global warming and thus the retreat of glaciers in the long run.
CO2 emissions per capita (2018): 4.4 metric tons
GDP (2020): $747.9 billion
Official Development Aid (2020): Disbursed $3.6 billion
Population (2020):
8.67 million
Surface area: 41,285 km2
CO2 emissions per capita (2018): 0.4 metric tons
GDP: $33.6 billion
Official Development Aid (2019): Received $1.8 million
Population (2020): 29 million
Surface area: 147,181 km2
Sources: World Bank, Nepal Ministry of Finance, Nepal Ministry of Foreign Affairs
Sources: World Bank, Swiss Federal Statistics Office, Swiss State Secretariat for Economic Affairs

Switzerland, a mountainous Central European country renowned for its wealth, is also facing an outsized impact due to the climate emergency. The Swiss government has previously warned the Alpine nation of roughly 8.6 million has incurred a rise in temperatures that is twice as fast as the global average, resulting in the accelerated retreat of glaciers in the Swiss Alps.

And, if global heating continues, it is feared only a fraction of the current glacier cover will be left by the end of the century, reflecting a huge risk on the seasonal availability of drinking water, water for agriculture and power generation.

Unlike Nepal, Switzerland has one of the most intensively studied mountain ranges in the world and does not rely on foreign aid to implement flood prevention measures. The lakes in Switzerland are generally smaller when compared to Nepal and the infrastructure and settlements are situated much closer to glacial lakes. Glaciologists have therefore warned that even small glacial lakes could cause considerable damage to densely populated communities.

“If we look at the data series of past glacial lake outburst events we had in Switzerland, there is no clear trend of increase with climate change. Some new ice dammed lakes will form but some others will disappear,” Huss told CNBC. “But, if we speak about the moraine-dammed lakes, there it is quite clear that with climate change we will have more moraine-dammed lakes that will appear beneath the retreating ice, so obviously an increase in the hazard potential.”

In its history, more than 100 unusual glacier floods have been observed in the Swiss Alps since the beginning of the “Little Ice Age” in the early 14th century. However, an acceleration of glacial retreat has coincided with the formation of new glacial lakes in places where they have never been before.

In the summer of 1968, catastrophic flooding at the glacier Grubengletscher, which flows from the summit of the Fletschhorn mountain, caused heavy damage in the village of Saas-Balen. The outburst flood eroded the equivalent debris to 1,000 detached family houses from the mountainside, with a similar event taking place only two years later when the same lake burst its banks for a second time.

The damages were estimated to have caused costs of around 20 million Swiss francs ($21.5 million). Experts believe the costs for preventative measures, such as monitoring and investigations, would likely be less than 10% the cost of inaction.

Germanwatch has said that while flood prevention measures in Switzerland meant there was now no immediate risk to downstream communities, a continued acceleration of global heating could see large volumes of water collect once again, including at the Grubengletscher glacier.

It is quite clear that with climate change we will have more moraine-dammed lakes that will appear beneath the retreating ice, so obviously an increase in the hazard potential.
Matthias Huss, director of the Swiss glacier monitoring network Glamos
Pizol 2006-2021: Disappearance of a small mountain glacier (Source: M. Huss, VAW-ETH Zürich)

Glaciologists told CNBC that to forecast how and when a glacial lake outburst flood might take place was extremely difficult, noting that to do so accurately would require a multi-disciplinary investigation of the lakes and associated factors in the surrounding environment.

The identification of potentially dangerous glacial lakes and assessing the structural integrity of these dams, or hazard mapping, would be useful to help design risk management and reduction strategies. These measures include the installation of high-altitude weather monitoring systems and early warning systems to help downstream communities prepare for a potential outburst flood.

Other flood experts have suggested alternative measures, such as the partial draining of glacial lakes — an expensive and challenging logistical operation at high-attitude — and the reinforcing of unstable dams with stone and concrete.

We’re not necessarily short of data but the challenge is actually managing and analyzing that data.
Jonathan Carrivick, senior lecturer in geomorphology at Leeds University

Crucially, glaciologists speaking to CNBC were unanimous in their view that data-sharing at an international level was essential. Jonathan Carrivick, a senior lecturer in geomorphology at the U.K.’s Leeds University, said this is because glacial lake outburst floods, as past experience shows, are transboundary. “They do not care about political lines on a map.”

Carrivick suggested that while some countries were willing to be open and transparent about the potential risk of outburst floods, others were likely to withhold that information fearing it may be interpreted as a sign of weakness.

“We’re not necessarily short of data but the challenge is actually managing and analyzing that data. You basically need supercomputers to process [satellite] images in near real time and to detect changes between them, and then of course you still need an expert to interpret those changes,” Carrivick said.

“We are not there yet. We haven’t got the computing power, or the computing coding expertise set up to do it.”

Climate scientists expect a substantial increase in the number of glacial lake outburst flood events in the coming decades, a trend that is estimated to continue well into the next century. It leaves countries and regions known to be highly vulnerable to these climate phenomena with a race against time to invest in disaster preparedness and help to protect life and infrastructure in the mountain valleys below.


Writer: Sam Meredith
Editors: Matt Clinch, Ted Kemp
Design and code: Bryn Bache
Images and video: Getty Images
Sources: ICIMOD, GFDRR, World Bank, European Geosciences Union, Eos science magazine, Springer Nature, Business Standard

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