Climate Matters: New evidence suggests massive rock avalanche triggered Nepal-Tibet flood

When a wall of rock, ice, and water tore through the Nepal-Tibet border region, the devastation was immediate and staggering. But the science behind why it happened is still coming into focus.

Dr. Mylène Jacquemart, a senior researcher at ETH Zürich who studies glaciology, walked through what we do know and what we don't know about the factors that likely lined up to cause this disaster.

The first assumption for many watching the disaster unfold was a glacial lake outburst flood, or GLOF. GLOFs can occur when a lake dammed by ice or debris suddenly bursts, sending a wall of water downstream. But satellite imagery ruled that out.

"We know that that's not the case, because satellite images from before the disaster show us that there's just no lakes along that drainage that could be at the start of this," Jacquemart said.

Instead, imagery now points to a potential rock avalanche, where a chunk of a mountainside gave way and carried a glacier with it.

"There is a glacier involved, but it was really a very, very large rock avalanche that had some glacier on top of it," Jacquemart explained. "So it was part of a mountain. The glacier was on top of that, and both the rock and the glacier fell down together."

As that unstable mass of rock and ice raced downhill, it essentially swallowed the rivers in its path, transforming into a torrential flood as it went.

"That material very quickly became very mobile as it traveled down the valleys and the rivers," she said. "Along this trajectory, it incorporated additional sediment, but also a lot of water, because it was moving much more quickly than the rivers or the water was flowing in any of the rivers. And so it would just include all of the water that was in those rivers, which is likely what made it so massive and fast."

For a rockslide of this scale to happen, the underlying structure of the rock has to crack in specific points.

"The reason why that rock package was so large is to be found in the geology," she said. "The fractures in the rock need to align in such a way that we can have such a large chunk of rock."

Jacquemart says that process likely took shape over longer timescales, well before any single trigger sets it off. She also pointed to other factors affecting the ground at high altitudes.

"We know that it's high enough to have been affected by permafrost," she said. "What that means is that a large part of that rock would have been frozen year-round."

As permafrost thaws, it can change the internal structure of a rock wall, building up pressure in ways that weaken its structural integrity over time.

"There's various effects that can influence the stability of rock walls that are in permafrost... If those warm and we have water infiltrating into those cracks, it can create really high water pressures in the rock. And that can certainly be a triggering effect for large rock avalanches." Jacquemart said.

The timing also lines up with the region's monsoon season, and some of that heavy precipitation falls as snow at higher elevations, which melts quickly.

"If we look at the satellite imagery from the past days and weeks, it's the monsoon season," she said. "So there is a lot of precip in some cases that has fallen as snow. We also then see that that snow disappears again really quickly. So it provides a lot of meltwater."

Despite all these contributing factors, Jacquemart is careful not to draw a direct line to a single trigger just yet.

"Meltwater can have that effect, as can warming temperatures that do degrade permafrost. So those are options," she said. "But it's really not something at this point that we have any data on that would allow us to make that connection at this point."

Ground shaking was recorded around the time of the event, but that ended up being the result of the landslide, not the cause.

"There was really strong ground shaking that was recorded. But that was the ground shaking caused by the rock avalanche itself," she said.

Even without a confirmed single trigger, the conditions that make events like this possible are becoming more common as the climate warms.

"All the factors that contribute to large slope instabilities and cascading hazards like this, they're all evolving for the worse," Jacquemart said. "We have more meltwater available, whether it's that we have more lakes that are forming where glaciers used to be. Whether that's temperatures directly affecting permafrost. If we look at the global picture, they're all getting worse."

Scientists are still working to untangle exactly how much each factor contributed to this disaster, but the broader pattern is one climate researchers are watching closely across mountain regions worldwide.

"I think it's just worth remembering just how horrifically large this event has been," Jacquemart said. "For me, that's been really shocking to see just how much devastation this has caused, and what it has done to infrastructure, specifically with regard to hydropower infrastructure that's so important for the region."