The mud tsunami in Nepal has brought the risk from the warming climate to mountain communities into sharp focus.
Thousands of people are still missing, and the death toll is rising. Dozens of bridges have been washed away and vast sections of road obliterated, making rescue operations through the rugged Himalayan terrain even more difficult.
Beyond melting ice and glacial lakes at risk of overflowing, the very ground people have built their lives on is shifting. Scientists have warned for years that a warming climate is destabilising the Himalayas, and there are far-reaching consequences for infrastructure and those who live there. Could some of these places become too dangerous for people to live in?
It's a question causing Umesh Haritashya, a glacier expert from the University of Dayton, to pause.
"It's hard to tell people to move," says Haritashya, who is part of an international investigation to help understand the unfolding disaster.
"We know where the slope instability is, we know where the glacial lakes are, we know the area is warming faster. We need to figure it out to communicate properly to these communities so that they can understand the risk associated with it, and then they can figure it out."
Melting at the 'third pole'
The Himalayas' majestic snow-capped peaks and colourful villages draw millions of tourists to the region every year. But a warming climate is turning the very geography that draws them there into a growing threat.
There are more than 55,000 glaciers in the Himalayas, Karakoram, and Hindu Kush mountain ranges, containing more ice and snow than any region outside the polar regions. It's why the region is often called the "third pole".
The area is warming faster than the global average, causing the glaciers to retreat. Not only does it increase the risk of a glacial collapse, but the melting ice also turns into large glacial lakes, held back by loose sediment and rock.
"I know as a scientist, we all want you to be cautious until our data says that's clear to attribute something to climate change," Haritashya said.
"But climate change is certainly playing a role in that area because we are seeing the frequency of these events occurring in these kinds of valleys."
Haritashya's research has logged more than 1,466 glacial lakes in the region, most of which have formed in response to the rising temperatures since the second half of the 20th century. Because their barriers are unstable, they can fail directly, but avalanches or rockfalls crashing into them are another major danger, generating a huge displacement wave that overtops the dam and triggers a glacial lake outburst flood (GLOF).
'Part liquid, part solid, part soup'
But another problem lies beneath the surface. Large areas of the Himalayas are permafrost. The ground is not just made of soil or rock; it's also moisture that remains frozen solid year-round, as long as temperatures remain low enough.
Ice acts a little like cement inside mountain slopes. As temperatures rise and that ice thaws, previously frozen rock, sediment and moraine material can become unstable, increasing the likelihood of landslides, debris flows and rock-ice avalanches.
"The ice plays the role of the glue between the rocks or between the fragments of the rocks," Haritashya said.
"As the permafrost starts to thaw, these frozen ice pieces start to melt. And once they melt, there is no longer glue that can hold the rocks together."
In a study published this year, researchers from the School of Earth, Ocean and Climate Sciences, Indian Institute of Technology Bhubaneswar, concluded that a warming climate was shifting the 0°C boundary where the permafrost remains solid higher up the mountains.
The decline in snow cover in the Himalayas from January 2022 to January 2025. Snow cover varies from season to season, but there has been a long term decline in the region.
It is too early to determine what triggered the glacier break behind Wednesday's disaster. That glacier, however, sat above 5,000 metres, where temperatures are generally cold enough to keep ice frozen year-round.
What's really concerning, according to the researchers, is what happens when several consequences of warming interact and start a dangerous cascade.
"Climate change is melting the ice much faster, and as the ice melts, the slope decline starts to become open," Haritashya said.
"As they [the slopes] become clear of ice, the stability of the slope changes."
The warmer climate causes glaciers to melt and retreat, leaving dents that turn into glacial lakes. At the same time, warming permafrost destabilises the steep slopes surrounding those lakes. A rock or ice avalanche can then crash into a lake, displacing huge quantities of water, triggering a flood.
Thawing permafrost is already creating a very real billion-dollar infrastructure problem for communities who have built their lives on shifting ground. As the ice within frozen soil melts, the ground loses strength and bearing capacity. It can settle unevenly, slump or collapse. That creates problems for everything built on top of it.
Glaciologist and polar engineer Adrian McCallum at the University of the Sunshine Coast says cold climate countries from North America to Asia are increasingly facing this issue.
"Melting permafrost is a huge issue for infrastructure, for building a house. You used to be able to build it on solid frozen ground, but that frozen ground is no longer solid. It becomes part liquid, part solid, part soup. "
A Chinese study assessing the risk of permafrost thawing in the Qinghai‒Tibet Engineering Corridor found real-life examples where infrastructure had been impacted. Cracked and buckled roads, uneven surfaces, bridges and building foundations losing bearing strength, and damaged pipeline and electricity infrastructure are just some of the consequences when the ground shifts.
"So it is a real problem for managers in China, Tibet, and that area where there is permanent or was permanent permafrost because it may not be; the word permafrost is an inappropriate term because it's not permanent anymore," Dr McCallum said.
In the Kinnaur region, in the western Himalayas of northern India, researchers have identified some degree of permafrost in about a third of the villages. One newly built road towards the India–Tibet border actually crosses several rock glaciers.
From snow to rain
Ruth Gamble is a climate historian and director of La Trobe Asia, who has studied Tibet and Himalayan rivers for more than a decade. She's currently in Kathmandu, near where the landslip occurred, and was set to travel through the border pass that was wiped out by the flash flood in a few days.
She says the warming climate is also changing rain and snowfall patterns that have sustained the region.
"So it's changing the climate cycles in this region because it's fed by two climate cycles. One comes up from the Bay of Bengal and the Arabian Sea as the monsoon at this time of year," she said.
"And then you have a cold cycle that comes in from across Eurasia in the winter that produces all of the snow and ice mainly. But the monsoon is getting more erratic, and the area it's coming from is warmer.
"So that means it's dropping more rain than snow, and it's doing it in big bunches as opposed to softer events. So you get like a cloud lake outburst flood or a supercell event, dumping a whole lake full of water on a place, as opposed to a more dribbly-type monsoon that will soak into the ground."
History shows scale of threat
For Australians, the symptoms of climate change are often synonymous with extreme heat. But when the stability of the ground itself is contingent on staying frozen, it shows just how far-reaching a temperature increase of a few degrees can be.
Dr Gamble says we are seeing the real-time impact of climate change on communities with records and histories that stretch back thousands of years.
"Because I'm a historian, I look all the way back, and I know that there were outburst floods and landslides," she said.
"I was looking at one yesterday that happened in 603 CE; we've got records of ones going back all that time.
"But they were rare; the people saw them as being rare events, and then what's happening now is every year there's more and more."
With the emissions from burning fossil fuels driving these temperature increases yet to peak, the world is set to see more events like this until emissions can be brought down to zero and the planet stops warming.
Dr McCallum says this means the world must focus on how to help these communities adapt.
"Humans are historically a clever species, and we will adapt, but can we adapt at an appropriate or sufficient rate to match the challenges that the climate is throwing at us? Perhaps this is an example of we're not doing that," he said.
"So I think we need to educate, support, enable, empower others to hopefully adapt at a faster […] amidst a rapidly changing climate."
Shreya K.C, a Kathmandu-based climate activist, says Nepal couldn't be expected to pay for a crisis it did little to create.
"Climate justice for Nepal means confronting who created this crisis and who is being forced to pay for it," she says.
Professor Haritashaya says the world owes these communities a responsibility to act.
"It's an issue that transcends the boundaries," Haritashya says.
"We need to think about a trans-boundary solution and not just a solution for Nepal."
