The catastrophic Tibet-Nepal flood, which has left hundreds dead and thousands missing, has drawn renewed attention to the risks of living in mountain regions with glaciers, as all indications suggest that the disaster was triggered by the collapse of one of these ice masses. The phenomenon is known as a GLOF, or glacial lake outburst flood, a sudden flood caused by the breaching of a glacial lake.
Climate change and rising temperatures are fueling these events: such floods have become six times more frequent over the past century. Major mountain ranges such as the Himalayas, the Andes and the Alps are among the areas most exposed to these disasters. An estimated 15 million people worldwide live at risk from glacial lake floods, half of them in Peru, India, China and Pakistan. Monitoring glaciers and glacial lakes is no easy task, especially in developing countries, but it saves lives: last year, a Swiss village was evacuated two days before the collapse of another glacier.
“Glaciers have been retreating since 1950, and since 1990 that retreat has accelerated. The last four years are among the five with the greatest losses on record, while over the first 25 years of the 21st century the rate of retreat has doubled compared with the previous 25 years,” explains Juan Ignacio López, a glacier expert at the Pyrenean Institute of Ecology (IPE-CSIC). “The Himalayas are no exception; it is one of the global hotspots for ice loss,” he warns. Given the retreat is occurring worldwide, “it is clear that it is not due to local factors but to the temperature increase driven by human-caused climate change.”
This retreat has many effects on the climate system and ecosystems, but it is also contributing to the growth of glacial lakes, which store meltwater. Since the 1990s, the number and surface area of these lakes have increased by 50%. “The water is not held back by a solid dam like those built by humans, but by moraines [ridges of ice and sediment] or blocks of ice, which are highly unstable. When they become destabilized for any reason, they can trigger a sudden flood, or GLOF,” says Jesús Revuelto, who studies the phenomenon at IPE-CSIC. Combined with growing population pressure in mountain regions, this creates risks for millions of people living in those areas.
The first global map of GLOF risk, published in Nature Communications in 2023, shows that the regions most vulnerable to this type of flooding are Central Asia, particularly China, Nepal, India and Pakistan, which share the Himalayas and other mountain ranges; the Andes, including Peru, Bolivia, Chile and Argentina; and the Alps, spanning Switzerland, Italy, France and Austria. Other risk areas include northwestern North America, Iceland and Scandinavia, although the potential impacts there are considerably lower.
Around 15 million people live in these regions, according to the study. Half of them are concentrated in mountain areas of Peru, India, Pakistan and China. López sums it up this way: “More than half of the world’s population exposed to glacier-related risks lives in the Himalaya and other high Asian mountains. Another very challenging area is the Andes, especially in Peru, where there have been many accidents in the 20th century, but precisely for that reason, much has been invested in measures to reduce risk to people and mortality has fallen.”
From the beginning of the 20th century to the 2010s, the annual average number of documented GLOFs increased nearly sixfold, rising from 7.6 per year in the first decade of the century to 43.7 in the 2010s. Revuelto explains: “On one hand, the world population has grown, and there’s a greater chance of having videos of these events, which sometimes occur in remote places. On the other hand, climate change is raising temperatures, causing more ice to melt from the glaciers that flow into these lakes, which can destabilize them, break the natural dams, and trigger major landslides or flash floods.”
More than 3,100 glacial floods
The historical GLOF database compiled by the University of Vienna has identified 3,151 glacial lake outburst floods between the years 850 and 2022 across 27 countries. Most have occurred in North America (833), Central Asia (569), the Alps (443) and the Andes (337). It should be noted that in some of the world’s most remote mountain regions, such as the Andes and the Himalayas, GLOFs were harder to document before the advent of satellite imagery. In fact, it is still unclear which glacial lake burst after the glacier collapse in Tibet.
Because many of these events occur in isolated areas, they often cause no casualties. Even so, the study found that 44 glacial floods claimed at least 3,636 lives, not including events from the past four years. One of the deadliest occurred in Peru in 1941, when a chunk of glacier fell into a lake, triggering a wave that breached a natural dam and unleashed a flood that devastated the city of Huaraz, killing more than 1,800 people. On Thursday, Peru’s National Institute for Research on Glaciers and Mountain Ecosystems warned that at least 58 lakes in the Andes are at “very high risk of overflowing.”
How can future disasters be prevented? Revuelto points to an example from Europe: “Switzerland did it last year in Blatten. They monitored the glacier and slopes with sensors, seismic equipment… They found major displacements and evacuated the village two days before a huge flood.”
In the Alps, resources are available to closely monitor glaciers and other unstable ice masses. In developing countries such as Nepal, India and Pakistan, however, the challenge is far greater. “There are more potentially dangerous areas and steeper slopes, so it is difficult to monitor all the mountains. But at the very least, we should have identified and mapped the areas that are susceptible to this type of landslide, as well as the valleys where they might end up,” he concludes.
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