How a glacier collapse may have triggered the Nepal-Tibet disaster
While individual glacial events cannot be directly attributed to climate change, experts say rising global temperatures are putting increasing pressure on glaciers.
A massive piece of ice that fell thousands of metres from a glacier into a river below may have triggered the disaster in Nepal and Tibet.
Scientists are still piecing together information from satellite and ground-level data to figure out exactly what happened to cause the flood, which killed at least 353 people and left more than 1,300 missing, including foreigners and pilgrims.
"A large chunk of glacier fell from about 5,200 metres into a valley," said Simon Cox, principal scientist at New Zealand's GNS Science.
It "transformed into this raging mass of debris... where it picks up rocks and sediment and trees and anything in the way and fires it on down through the canyon," he told AFP.
The wall of water first hit a border post, with CCTV footage showing mud and debris engulfing the area as people fled. It then travelled downstream into Nepal, flooding villages and destroying bridges and buildings.
What caused the wall of water?
Early speculation suggested a glacial lake outburst flood (GLOF), which occurs when a barrier holding back glacial meltwater suddenly collapses and releases the water downstream.
"Water does flow within glaciers and flows beneath glaciers," said Adrian McCallum, a glaciologist at the University of the Sunshine Coast.
"But to my mind, the only reason why a large volume of water should all of a sudden flow down a valley is because it has been released," he told AFP.
However, other experts said they found no existing glacial lakes near the site of the collapse. They suggested that water may instead have accumulated beneath the glacier and been released as the ice fell.
Another possibility is that the falling ice temporarily dammed the river, allowing water to build up before being suddenly released.
Summer glacier melt and monsoon rains may also have already raised river levels, while debris and mud picked up along the way further increased the volume of the flow, according to Cox.
Water levels across the flood-affected region rose by as much as nine metres in just 30 minutes following the surge from the glacier collapse, according to researchers at a Kathmandu-based climate research group.
“It was a huge rise, a huge surge of water that happened suddenly,” Saswata Sanyal said.
The glacier collapse was powerful enough to register as a magnitude 5.2 seismic event at 8:37 am (4:52 am CEST) and was initially mistaken for an earthquake.
After further analysis, the US Geological Survey determined that the event was a glacial collapse and debris flow and that no earthquake had occurred.
How satellites helped reconstruct the disaster
With much of the affected area inaccessible, experts are using satellite imagery and available ground measurements to reconstruct the disaster
"We can look at satellite imagery taken before and after the event, and try and see what has changed, essentially looking for what is missing from the mountains in the 'after' imagery," said Lauren Vargo, a glaciologist at New Zealand's Victoria University of Wellington.
Shortly after the seismic activity was recorded, a nearby water monitoring station stopped transmitting, according to Nepali authorities.
Stations downstream then began going offline one by one. Water levels in Kalikhola, near the Tibet border, reached up to 12.3 metres at 2:14 pm (10:29 am CEST), authorities said.
Why warnings may not have been enough
Nepali authorities first received reports of flooding at around 9:00 am local time (5:15 am CEST) and warned communities along the Bhote Khoshi river to "stay on high alert and stay in safe areas".
However, it remains unclear how many people received the warning or how much time they had to respond.
"The reality is, if you are in a community underneath a large glacier and potentially a glacier lake where a GLOF could occur, then to some extent, it's a ticking time bomb," said McCallum.
Even for those who received a warning, options would have been limited, said Hatim Sharif, a hydrologist at the University of Texas San Antonio.
"Running will not help," nor will trying to flee by vehicle, he told AFP, because the mud and water moving at high speed form a lethal combination "like liquid concrete."
The only solution is to climb up a hill, at least six metres, he estimated.
INTERVIEW
Nepal’s deadly glacier collapse exposes climate risks across the Himalayas
A massive glacier collapse that triggered deadly flooding on the Nepal-Tibet border has exposed the growing dangers facing the rapidly warming Himalayas, where scientists say retreating ice and thawing ground are making mountain landscapes increasingly unstable.
Issued on: 27/08/2026 - RFI
Scientists are still investigating what caused Wednesday's collapse and say it is too early to attribute this particular disaster to climate change.
But rising temperatures are already changing glaciers, glacial lakes and permafrost across the region, increasing the risk of destructive events involving ice, rock and water.
The disaster began when a huge mass of ice and rock fell from a glacier high in the Himalayas and temporarily blocked the Lhende Khola river. When the blockage gave way, water, rock and debris surged downstream, devastating communities along the Nepal-Tibet border.
The US Geological Survey initially recorded a magnitude 4.4 earthquake in the area, but later concluded that no earthquake had occurred. Instead, the glacier collapse and debris flow generated the seismic signal, which was later measured as a magnitude 5.2 event.
Hundreds of people have been killed and more than 1,000 remain missing. Water levels on the Trishuli River at Galchhi rose by as much as nine metres in just 30 minutes, according to the International Centre for Integrated Mountain Development, or ICIMOD.
The disaster comes as the Hindu Kush Himalaya region undergoes rapid changes. ICIMOD says glacier ice-loss rates across the mountain range have doubled since 2000, while permafrost is degrading and snow patterns are changing.
Prashant Baral, a cryosphere analyst and permafrost research consultant at ICIMOD in Kathmandu, spoke to RFI about what may have triggered the disaster, how climate change is reshaping the Himalayas, and the risk of another collapse.
RFI: What do scientists know about the causes of the massive landslide and floods on the Nepal-Tibet border?
Prashant Baral: We analysed the satellite images today [Thursday], and we can see that there has been a rock-ice avalanche. The possible trigger could have been a glacier collapse or bedrock failure or climate change-related conditions, but we cannot ascertain that as of now.
What we can say for sure is that there was a huge mass of debris, rock and ice that fell down from the glacier and it temporarily blocked the river channel. And then the dam was breached. And then this led to a deposit of debris downstream, which affected the settlements and infrastructure downstream. All of this happened within a very short period of time.
RFI: Can you give us more information about the amount of material involved?
PB: Regarding deposits, we need to do further investigations to precisely ascertain what volume of deposits came down the valley and then got deposited downstream. But as of now, it seems that there were millions of cubic meters of rock, ice and debris that fell down and blocked the river channel, but then were subsequently mixed with the river water. And then they moved downstream and flooded the settlements and infrastructure.
RFI: Did this phenomenon cause an earthquake, or did an earthquake cause the movement in the mountains?
PB: I cannot say for sure whether the earthquake resulted due to the event or that the earthquake caused the event. But it could have been that due to this massive rock avalanche, an earthquake of small size could have occurred, or vice versa. I cannot ascertain this for now.
RFI: The US Geological Survey has said the collapse of the glacier caused an earthquake.
PB: If you look at the volume of deposits that have come downstream and the volume of the mass movement that has occurred, it is significantly large. And it's quite likely that that could have triggered an earthquake of that magnitude on the Richter scale. But we will know more for sure in the coming days.
Preserving the planet's glaciers is a 'matter of survival', says UN
RFI: Is this big landslide or ice slide a result of climate change in the Himalayas?
PB: Like I said before, we would require detailed investigation of temperature records and other details to ascertain this event or attribute this event to climate change. But we've seen in the past that one thing we can say for sure is that climate change has led to increase in temperature, and this rise in temperature has caused changes in the cryosphere and the components of the cryosphere – such as glaciers, glacial lakes, permafrost. And due to these changes in these cryosphere components, the likelihood of the occurrence of these kind of events is growing. So the frequency and magnitude of these events certainly can be associated with the increase in temperature.
RFI: Is there a risk of another landslide of rocks, debris and ice occurring in the same area?
PB: If we look into the trigger of this event, it occurred due to the failure of the glacier tongue [a floating platform of ice attached to the front of marine-terminating glaciers that extend into the sea]. But whether the bedrock failure led to the collapse of the glacier tongue or vice versa, we understand that the ground is quite unstable for now. So we would certainly need to monitor this area in the immediate future, because as of now, it seems that the mass movement has made the area unstable.
We cannot deny the possibility of occurrence of similar events in the future. Having said that, the magnitude and scale of this event is quite rare in the Himalayan region. Monitoring is extremely necessary to see if the ground is still unstable and if we have to prepare ourselves for another event, even if not of this scale, but of a smaller scale, and yet quite devastating.
Himalayan glaciers melting 65 percent faster than previous decade: study
RFI: There is still an obstruction that could contain rocks, ice or water. Could it collapse and release them?
PB: We haven't looked into the detail of that. But given the amount of rock, ice and debris that has fallen into the valley, I believe if all of that amount has already been released, due to the water channel, then a certain volume of that debris and rock and ice would still be remaining in the river channel and would require monitoring in the coming days.
RFI: Is this kind of climate-related risk threatening people across the Himalayan region?
PB: You can see that. There's quite a contradiction that [on the one hand there's] the increase in temperature, our cryosphere is changing and the grounds are becoming unstable, the glaciers are retreating, and the deglaciated landscapes are quite unstable. And then there’s the formation of these glacial lakes, and their volume is expanding.
But on the contrary, the infrastructure and the settlements are moving up into the mountains.
These two things are occurring together. Climate change can cause changes in the cryosphere and then contribute to the occurrence of these kind of events. If you look into these details, then certainly climate change is threatening the lives and livelihoods of people in the Himalayan region.
RFI: Why are settlements moving up into the mountains?
PB: Settlements for tourism and also infrastructure such as hydropower are being built. And when hydropower is built, then there is also a community there.
This interview by Lucile Gimberg has been edited for clarity.
