In Nepal, the water level in the Trishuli River rose nine meters in less than thirty minutes early on a Wednesday morning in late August. In the time it takes to drink a cup of tea, that’s about the height of a three-story building. Power plants, homes, bridges, and roads were all erased. At least 469 people had died and more than 1,500 were missing in Nepal and Tibet by the time the water started to recede.
At first, people thought it was an earthquake. Nepal’s foreign minister told parliament that early reports suggested that a landslide was caused by earthquakes. There was, in fact, what the US Geological Survey thought was a magnitude 4.4 earthquake on the border between Nepal and China. But by that evening, the agency had completely changed its opinion. A tectonic earthquake did not happen. They had measured the force of a glacier collapsing, which is when ice and rock hit the valley floor so hard that it made its own seismic signal. The event was recorded at magnitude 5.2 after it was reclassified.
Daniel Shugar, a geomorphologist at the University of Calgary, looked at Planet Labs satellite images and gave a clear account of what he saw. A piece of ice about 0.2 square kilometers in size had broken off from the lower part of a glacier that was about 5,200 meters high and fell about 1,200 meters to the valley floor below. It was “crashed” that he used. As it went down, it picked up rocks and other debris along the way. It then hit the Lhende River in Tibet, temporarily blocking it, and then sent a huge wave further downstream. That flood fed into the Bhote Koshi River, went through Nepal around 9 a.m., and then poured into the Trishuli.
When you look at the order of events, it seems like the disaster happened almost mechanically, with one failure leading to the next with brutal efficiency. Shugar also saw on the satellite images that many glaciers in the valley that had snow on them the day before were now just ice. It was clear what it meant: it had been warm. Unlikely to be warm. It was warm enough overnight to remove the snow from the tops of several glaciers.
Shugar said that construction in the area may have also played a role, but that line of inquiry is still open. A glaciologist at Monash University in Melbourne named Prof. Andrew Mackintosh brought up something even scarier: the permafrost. For him, frozen ground is like “glue” that holds steep mountain slopes together. That glue gets less stiff as the temperature rises. Slopes that have been stable for hundreds of years start to move. There’s nothing exciting about it like a flood. It’s slow and quiet, and you might not notice it until it’s gone.
The Himalayas are warming up about twice as fast as the rest of the world. When you think about what that number means for a place like Nepal, India, China, Bhutan, and Pakistan, where glaciers control the rivers, it makes sense. In 2023, the UN said that in just over thirty years, Nepal’s mountains had lost almost a third of their ice. According to a different study, the Hindu Kush Himalayan glaciers melt 65 percent faster between 2011 and 2020 than they did the decade before. These are not guesses. They are numbers that show how much has already happened.
It’s tough not to think about Uttarakhand. Around 200 people were killed or went missing in 2021 when a piece of a Himalayan glacier broke off in an Indian state. Later, researchers talked about how the event caused “an extraordinarily large and mobile debris flow.” Nepal just went through a structurally similar event, but it’s still not clear if it was just the glacier that gave way or if a whole section of mountainside, including the glacier and rock, fell apart at the same time.
Mackintosh was careful about what he said. He said that scientists have “very high confidence” that climate change and the melting of glaciers are linked. It is harder to say if a single collapse event can be directly linked to a warming climate. There haven’t been many large glacier collapses that can be used to draw firm statistical lines. It doesn’t make the events less scary that we don’t know what will happen. It actually makes them harder to get ready for.
Nepal has more than 2,000 glacial lakes, and the UN says that 21 of them could be dangerous. Glacial lake outburst floods have already killed more than 12,000 people around the world. These floods happen in a different way than what happened here, but they are still part of the same bigger picture. More meltwater is made when the world warms up. The water needs to go somewhere. It sits behind a natural dam until it doesn’t anymore.
That Wednesday morning’s events were not a “freak event,” which is what some disasters are called, as if nature had done something strange. In a real sense, the mountain was doing something that it can do more and more because of climate change. The ice sheet broke. The river got bigger. And the valley below didn’t have any warnings that were worth the name.
