Increasing risk of Glacial Lake Outburst Floods (GLOFs)

By Caroline Taylor

What is a glacial lake outburst flood?

Glacial Lake Outburst Floods (GLOFs) refer to the sudden release of meltwater and sediment from a dammed lake1. Across the globe there are many types of glacial lake dams, including moraine-dams, ice-dams, bedrock-dams, and landslide-dams 2, all of which can produce GLOFs.  

Dig Tsho in the Langmoche valley, Khumbu Himal, Nepal. This glacial lake breached catastrophically in 1985 due to an ice avalanche-induced wave and wiped out a recently finished hydroelectric plant as well as killing several people downstream (With permissions from Matt Westoby).

Why are Glacial Lake Outburst Floods a concern?

GLOFs tend to be sudden and rapid events, causing catastrophic flooding that leads to major geomorphic and socioeconomic impacts downstream 3,4. Some of the largest floods in history have been caused by GLOFs.

Historically, GLOFs are known to have directly caused at least 7 deaths in Iceland, 393 deaths in the European Alps, 5745 in South America and 6300 in central Asia 5. In addition, they have destroying roads, bridges, and hydroelectric developments and resulted in numerous secondary life losses 3.

Today, due to socioeconomic expansion into high mountain regions 6,7, more people are living near dangerous glacial lakes and GLOFs pose a greater risk to downstream communities than ever before.

How do Glacial Lake Outburst Floods occur?

Not all glacial lakes will produce GLOFs. Whether a glacial lake produces a GLOF depends on a range of factors. These include the properties of the dam and glacial lake, as well as characteristics of the surrounding topography and meteorological factors 8.

Specific factors include: the activity of the glacier itself (is it advancing, retreating, or stagnating?); the depth and volume of the glacial lake, inputs from surrounding topography (including avalanching, rock falls and landslides, rainfall); the composition of the dam (is it made up of ice, glacial deposits, a mixture? How tightly or loosely packed is it? Is it vegetated?) and downstream factors such as channel morphology (Steep or shallow? Narrow or wide?). All these individual factors come together to determine whether a GLOF will occur – it can get quite complex!

Diagram showing how a glacial lake outburst flood may be triggered
Diagram showing how a GLOF might be triggered. Clockwise from top left; (a) key glacier parameters; 1) supra- and englacial drainage, 2) crevassed snout above lake, 3) snout steepness, 4) distance between glacier and lake, 5) glacier area, 6) glacier activity (advance, retreat etc.), 7) reaction to climate change, 8) stagnant ice at glacier snout. (b) key lake parameters; 1) lake depth, 2) lake volume, 3) lake area, 4) lake area change and 5) dam freeboard. (c) key surrounding parameters; 1) calving from hanging glaciers, 2) mass movements and/or ice, snow, and rock avalanches, 3) seismic activity, 4) hydro-meteorological setting, 5) permafrost degradation, 6) compound risk. (d) key moraine parameters; 1) slope of lateral moraine, 2) presence of stabilising vegetation, 3) width/height radio, 4) piping and/or seepage, 5) dam top-width, 6) dam type, 7) presence of buried ice, 8) dam composition. (e) Key downstream parameters; 1) land-use, 2) infrastructure, 3) river channel morphology, 4) evidence of past GLOF/debris-flows/flash-flood. (Figure adapted from Westoby et al., 2014).

How is Glacial Lake Outburst Flood risk determined?

GLOF risk is usually defined as a function of hazard, exposure, and vulnerability9:

Hazard isa measure of probability (how likely an event is to occur) and magnitude (the likely size of an event). Here the focus is on the glacial lake itself, alongside external triggers that might cause an outburst!

Exposure refers tothe amount of people, infrastructure, and other human resources likely to be impacted by an event. Think anything that could have a social, economic, political, or environmental impact if it was damaged by flooding… there is a lot!

Vulnerability concerns the factors that increase how the impact of an event is felt and dealt with by an individual or community. Think about anything that could hinder your ability to react and recover to flooding. This could be age, gender, education level, corruption within governments etc. Vulnerability is often the most difficult factor to quantify given it is often subjective.

Taken together, these factors can be used to determine GLOF risk across a variety of scales and is useful for highlighting where might need added support for mitigating the hazard, protecting or reenforcing exposed people and infrastructures or providing support to those more vulnerable parties within the community.

venn diagram of glacial lake outburst flood risk from hazard, vulnerability and exposure
Venn diagram showing Glacial Lake Outburst Flood risk; risk is a combination of a) the physical hazard which includes potential outburst magnitude, b) the downstream exposure, and c) the social vulnerability.

What does GLOF risk look like today?

In 2020, glacial lakes were present in 31 countries across the global, with 15 million people found to be at risk of GLOF impacts.

The most at-risk were found in High Mountain Asia (HMA), where a total of 9.3 million people are exposed to 2211 glacial lakes covering an area 1256.09 km2. In comparison, approximately 800,000 people are at risk of GLOFs in the Pacific North-West (PNW).

More than half of the globally exposed population are found in just four countries: India, Pakistan, Peru, and China, with the Khyber Pakhtunkhwa basin in Pakistan the most at-risk of GLOF impacts! Most people globally live at least 30 km away from a glacial lake, however in High Mountain Asia, 1 million live within just 10km!7.

(It’s not all bad news; in some countries, such as Greenland and Norway, so few people live close to glacial lakes that GLOF risk is almost zero!)

How has GLOF risk changed?

Increased GLOF hazard

Over the last 30 years as atmospheric temperatures have warmed, the number, area, and volume of glacial lakes around the world has grown as glaciers melt and retreat 10,11 increasing the potential magnitude of outburst. This means the likely volume of flooding would be greater, flood runout distances longer, and flood inundation depths and extent bigger. Also, as permafrost melts and warmer temperatures bring wetter weathers, the number of mass movement events (rock falls, landslides, avalanches) that can cause GLOFs has grown, increasing the probability of outburst. As a result, GLOF hazard has increased.

GLOF Hazard

Increased GLOF exposure

The number of people and infrastructure near to glacial lakes has increased in many regions globally as tourism, hydroelectric power, and agriculture expands 6.  In 2020, 13 million people were exposed to GLOFs in just Pakistan! 7. However in other places, such as Nepal, people are moving away from glacial lakes in favour of the city life.

Globally though, GLOF exposure has increased.

GLOF exposure
The number of people exposed to GLOFs across the globe ranges from less than 6000 in Greenland to 13 million in Pakistan!

Decreased GLOF Vulnerability – but more could be done

Efforts to improve the socioeconomic status of people across the globe through the likes of the Sustainable Development Goals has resulted in decreased GLOF vulnerability over the last 20 years 12,13 that’s great news! However, there is still room for improvement, and in many regions, vulnerability remains high, and the impact of a GLOF would still be large.

Where vulnerability is high, overall risk is greater as the ability of people to cope with disaster and recover is reduced. Here shows two locations, Bolivia, and Canada, that have the same number of people exposed to GLOFs yet very different vulnerability levels. Where vulnerability is higher (Bolivia) overall risk ends up being 3x higher than Canada!

What does this mean for GLOF Risk?

Over the past 20 years, GLOF risk has increased in almost all countries where glacial lakes are found.

Despite a decline in vulnerability, increasing exposure and hazard means that since 2000, the number of people at direct risk of GLOF impacts has increased by 3.2 million (27%), with risk in India and Pakistan increasing the most. Even in the Pacific North West, where risk was declining, has seen an increase since 2015 14

Even though risk in HMA is the highest right now, it is increasing rapidly across the Andes, and could surpass High Mountain Asia soon if steps are not taken to reduce hazard or exposure.

What does the future of GLOF risk look like?

Continued increases in Glacial Lake Outburst Flood hazard

If atmospheric temperatures continue to rise, glaciers will continue to lose mass and permafrost in high mountain regions will melt. As a result, the number and size of glacial lakes and frequency of mass movements events is likely to increase, increasing GLOF hazard.

Importantly, as glaciers continue to shrink, new glacial lakes will form where there currently aren’t any, and people that previously were unaffected by GLOFs will become at-risk from them. This spatial shift in risk is concerning, but we have a unique opportunity to prepare communities for this future risk before it fully emerges, allowing people to live alongside this ever-growing risk of GLOFs.   

glacial lake in Bhutan
Immediately below the glacial lake at the foot of Jomolhari in Bhutan. Compared to its size Bhutan hosts a significant number of glacial lakes that pose a threat to downstream populations, and previous GLOFs have caused significant damage (With permissions from Rachel Carr).

Further reading

More detail on current GLOF risk: Danger! Glacial lake outburst floods ahead! – TheScienceBreaker

Royal Geographical Society Geovisualisation series: Mapping glacial lake outburst flood exposure – RGS

About the Author

Caroline Taylor

This article was contributed by Caroline Taylor from Newcastle University. I am a PhD candidate at Newcastle University, specialising in glacial hazards and risk, specifically glacial lake outburst floods. My research investigates the risk of GLOFs from the global to basin scale, integrating aspects of hazard, exposure, and vulnerability. I am particularly interested in how GLOF risk has evolved over time, how it might evolve in the future and how we might begin to mitigate GLOF risk for communities living downstream.


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