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In mountain ranges around the world, glaciers are melting as . Europe's Alps and Pyrenees from 2000 to 2023. These and other icy regions have provided freshwater for people living downstream for centuries - almost glaciers today. But as glaciers melt faster, they also pose potentially lethal risks.

Water from the melting ice often drains into depressions once occupied by the glacier, creating large lakes. Many of these are held in place by precarious ice dams or rock moraines deposited by the glacier over centuries.

Too much or a can break the dam, sending huge volumes of water and debris sweeping down the mountain valleys, wiping out everything in the way.

Today, over 10 million people across the world are vulnerable to glacial lake outburst floods.

In High Mountain Asia alone, these flooding hazards are projected to triple by 2100, especially with continued high emissions.

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— International Cryosphere Climate Initiative (@ICCInet)

These risks and the loss of freshwater supplies are some of the reasons the United Nations declared and March 21 the first . As an and a , we study the impact that ice loss can have on the stability of the surrounding mountain slopes and glacial lakes. We see several reasons for increasing concern.

Erupting ice dams and landslides

Most glacial lakes as a result of warming trends since the 1860s, but their since the 1960s.

Many people living in the Himalayas, Andes, Alps, Rocky Mountains, Iceland and Alaska have of one type or another.

A glacial lake in October 2023 and destroyed a . Residents had little warning. By the time the disaster was over, .

Juneau, Alaska, has been hit by in recent years from a glacial lake dammed by ice on an arm of Mendenhall Glacier. Those floods, , were driven by a melting glacier that slowly filled a basin below it until the basin's ice dam broke.

Scientists investigate flooding from Mendenhall Glacier's Suicide Basin.

Avalanches, rockfalls and slope failures can also trigger glacial lake outburst floods. These are as frozen ground known , robbing mountain landscapes of the cryospheric glue that formerly held them together. These slides can create massive waves when they plummet into a lake. The waves can then rupture the ice dam or moraine, unleashing a flood of water, sediment and debris.

That dangerous mix can rush downstream at (30-100 kph), destroying homes and anything else in its path.

The casualties of such an event can be staggering. caused by a snow and ice avalanche that fell into Laguna Palcacocha, a glacial lake in the Peruvian Andes, overtopped the moraine dam that had contained the lake for decades. The resulting flood destroyed one-third of the downstream city of Huaraz and and .

Teardrop-shaped Lake Palcacocha, shown in this satellite view, has expanded in recent decades. The city of Huaraz, Peru, is just down the valley to the right of the lake.

In the years since, the danger there has only increased. Laguna Palcacocha has . At the same time, the population of Huaraz has risen to over . A glacial lake outburst flood today could living in the water's path.

Governments have responded to this widespread and growing threat by and programs to identify potentially dangerous glacial lakes. Some governments have in the lakes or built flood diversion structures, such as walls of rock-filled wire cages, known as gabions, that divert floodwaters from villages, infrastructure or agricultural fields.

Where the risks can't be managed, communities have been encouraged to use zoning that prohibits building in flood-prone areas. Public education has helped build awareness of the flood risk, but the disasters continue.

View a related video

Flooding from inside and thawing permafrost

The dramatic nature of glacial lake outburst floods captures headlines, but those aren't the only risks. As scientists expand their understanding of how the world's icy regions interact with global warming, they are identifying a number of other phenomena that can lead to similarly disastrous events.

, for instance, originate inside of glaciers, commonly those on steep slopes. Meltwater can collect inside massive systems of ice caves, or conduits. A sudden surge of water from one cave to another, perhaps triggered by the rapid drainage of a surface pond, can set off a chain reaction that bursts out of the ice as a full-fledged flood.

An englacial conduit flood begins in the Himalayas. Elizabeth Byers.

Thawing mountain permafrost can also trigger floods. This permanently frozen mass of rock, ice and soil has been a (6,000 meters) for millennia.

Freezing helps keep mountains together. But as permafrost thaws, even solid rock becomes less stable and is more prone to breaking, while ice and debris are more likely to become detached and turn into destructive and dangerous debris flows. Thawing permafrost has been increasingly implicated in glacial lake outburst floods because of these new sources of potential triggers.

In 2017, nearly of Nepal's 20,935-foot (6,374-meter) Saldim Peak collapsed and fell onto the Langmale glacier below. Heat generated by the friction of rock falling through air melted ice, creating a slurry of rock, debris and sediment that plummeted into Langmale glacial lake below, resulting in a .

These and other forms of glacier-related floods and hazards are being exacerbated by climate change.

Flows of ice and debris from high altitudes and the sudden appearance of meltwater ponds on a glacier's surface are two more examples. Earthquakes can also trigger glacial lake outburst floods. Not only have thousands of lives been lost, but and other structures have also been destroyed.

Impermanent frost. Nepali Times.

A reminder of what's at risk

The International Year of Glaciers' Preservation and World Day for Glaciers are reminders of the risks and also of who is in harm's way.

The global population depends on the cryosphere - the that's covered in ice. But as more glacial lakes form and expand, floods and other risks are rising. A study published in 2024 counted more than 110,000 glacial lakes around the world and determined lives and homes are at risk from glacial lake outburst floods.

The U.N. is encouraging more research into these regions. It also declared 2025 to 2034 the "." Scientists on several continents will be working to understand the risks and find ways to help communities respond to and mitigate the dangers.