After Wednesday’s flood on the Nepal-Tibet border, one of the scientists asked to explain it pointed not at rainfall but at frozen ground.

Alton Byers, a faculty research scientist at the Institute of Arctic and Alpine Research at the University of Colorado Boulder, told NBC News that permafrost in the alpine zone is thawing because of climate change, destabilising mountain slopes and making landslides, icefalls and avalanches a growing risk to communities downstream. As slopes crumble, he said, rock and ice pour into steep river valleys where they can block rivers or set off large debris flows. He described an acceleration of landslides alongside warming trends.

What alpine permafrost is

Permafrost is ground that stays below freezing for at least two consecutive years. Most people associate it with Siberia, Alaska and northern Canada, but it also exists at high altitude in mountain ranges much closer to the equator, including the Himalaya, the Andes and the Alps.

In mountains, permafrost does something specific: it functions as cement. Ice fills the cracks and joints in the rock face and binds loose material to the slope. A near-vertical wall that looks like solid rock may be partly held together by ice that has been frozen for centuries.

Warm that ice by a couple of degrees and the binding weakens. The rock itself has not changed. What has changed is the thing holding it in place.

The evidence base

This is not a new hypothesis, and the strongest evidence comes from the Alps, where the rock faces are heavily monitored and the records are long.

A study of 209 rockfalls in the Mont Blanc massif between 2007 and 2015 found permafrost conditions, or temperatures very close to freezing, consistently present for every event where the failure surface was deeper than six metres. Surface temperatures at those sites were significantly elevated for up to two months beforehand. Shallower failures showed no such association, which the authors took as evidence that permafrost processes were driving the deeper ones specifically.

Separately, research published in Nature Climate Change in 2020 documented rapid worldwide growth in the number and size of glacial lakes since 1990 — the other half of the same warming story, and the reason the hazard is changing on both fronts at once.

What this does and does not establish

Being precise here matters, because it is easy to overstate in either direction.

What is well supported: high-altitude permafrost is warming; warming permafrost reduces the stability of steep rock walls; and the frequency of deep-seated rockfall in monitored ranges has risen in step with it.

What is not established: that any individual collapse was caused by climate change. Slopes fail for many reasons — earthquakes, ordinary weathering, water pressure in joints, the simple fact that mountains erode. Attribution science can describe how a warming climate shifts the odds across a population of events. It cannot yet tell you why one particular slope let go on one particular Wednesday.

The Himalaya is also less densely instrumented than the Alps. The physical mechanism is expected to be the same, but the local monitoring record supporting it is thinner.

Why it matters for what gets built

The practical consequence is about infrastructure planning, and it is uncomfortable for the countries most affected.

Nepal has staked a great deal of its development strategy on hydropower, which by its nature is built in steep river valleys fed by glaciers. Several such projects were destroyed on Wednesday. The Karakoram Highway between China and Pakistan runs through comparable terrain, as does the Trans-Alaska pipeline, and researchers have flagged all of them as exposed to outburst floods.

If the hazard is increasing rather than static, then structures designed against a twentieth-century flood record are designed against the wrong number. That is an argument for revising design standards and hazard maps, not for abandoning mountain development — but it is a real cost, and it falls on countries that did least to cause the warming.

Sources

• NBC News, with comment from Alton Byers of INSTAAR, University of Colorado Boulder, 26 August 2026
• Legay, Magnin and Ravanel, “Rock temperature prior to failure”, Permafrost and Periglacial Processes, 2021
• Shugar et al., “Rapid worldwide growth of glacial lakes since 1990”, Nature Climate Change, 2020