Hyderabad: The devastating floods that struck Nepal and neighbouring Tibet this week have brought renewed attention to a dangerous phenomenon in the Himalayas: glacial lake outburst floods, or GLOFs.

But an important distinction is needed in the case of the August 26 disaster.

Initial scientific assessments indicate that the flooding was triggered by a glacier collapse and an ice-and-rock avalanche, which generated a massive debris flow and flash flood. Authorities have subsequently warned about newly formed or debris-dammed lakes that could pose a risk of further flooding.

So, what exactly is a GLOF, and why is it becoming an increasing concern in the Himalayas?

Understanding GLOF

The Intergovernmental Panel on Climate Change (IPCC) defines a glacial lake outburst flood as ‘a sudden release of water from a glacier lake’.

The lake may be held back by glacier ice, a moraine dam or may contain water stored within, beneath or on a glacier.

In simple terms, a GLOF occurs when water that has accumulated around a glacier is suddenly released.

Glaciers naturally melt and retreat, leaving depressions where meltwater can collect. Some lakes form directly in front of glaciers and are held back by piles of rock and sediment known as moraines. Others may be contained by ice.

These natural barriers can be unstable. If they fail, a large volume of water can rush downstream through steep mountain valleys.

The resulting flood is not necessarily just water. It can pick up rocks, mud, boulders, trees and other debris, turning the flood into a highly destructive torrent.

How does a glacial lake suddenly burst?

There is no single trigger.

A landslide or rockfall can fall into a lake, displacing water and causing it to overtop its natural dam. An avalanche of snow, ice and rock can have a similar effect. Water can also seep through a moraine and progressively weaken it until the barrier collapses.

Earthquakes and unusually intense rainfall can further destabilise mountain slopes and natural dams.

This makes GLOFs particularly dangerous because the flood can begin with little warning at a location far upstream from the communities ultimately affected.

Why are scientists concerned about the Himalayas?

The Himalayas are warming, and glaciers across the Hindu Kush Himalaya are losing ice. As glaciers retreat, new lakes can develop and existing lakes can expand.

The United Nations Development Programme (UNDP) has noted that rising temperatures are contributing to glacial retreat and lake expansion in Nepal, increasing GLOF risk.

The IPCC has also assessed that glacier retreat is creating new glacial lakes in high-mountain Asia and increasing exposure to hazards associated with unstable lakes and surrounding slopes.

This does not mean that every GLOF, glacier collapse or Himalayan flood can be directly attributed to climate change. Individual disasters can involve a combination of geological conditions, rainfall, earthquakes, slope instability and glacier dynamics.

What climate change does is alter the background conditions in which these hazards occur.

Nepal has a history of GLOFs

For Nepal, this is not a new hazard.

A joint assessment by ICIMOD and UNDP identified 47 potentially dangerous glacial lakes in the Koshi, Gandaki and Karnali river basins across Nepal, Tibet and India. The assessment found 21 such lakes in Nepal, 25 in China and one in India.

The organisations described GLOFs as a prominent water-induced hazard in Nepal and other mountainous parts of the Hindu Kush Himalaya.

Nepal has also experienced numerous GLOF events in the past. The 2020 assessment recorded 26 GLOF events in Nepal since 1977, including events originating outside the country that had impacts downstream.

Nepal disaster shows why terminology matters

The current disaster also demonstrates why not every glacier-related flood should automatically be called a GLOF.

The initial flooding on August 26 appears to have resulted from a cascade beginning with a glacier and rock collapse, followed by an avalanche/debris flow into the river system.

Media organisations reported that the event dramatically altered river channels and caused widespread destruction downstream.

At the same time, debris from such an event can block a river and create a temporary lake. If that barrier later fails, it can produce another type of outburst flood.

Can GLOFs be prevented?

They cannot always be prevented, but their impacts can be reduced.

Scientists use satellite imagery, field surveys and hydrological monitoring to identify lakes that could pose a threat. High-risk lakes can be monitored, and engineering measures can sometimes reduce water levels or strengthen drainage.

Early-warning systems are particularly important.

UNDP says Nepal is working on measures including monitoring, early-warning systems and structural and non-structural interventions at high-risk glacial lakes.

The challenge extends beyond individual lakes. Mountain communities, roads, bridges, hydropower projects and settlements may lie far downstream from the original source of the hazard.