Permafrost is sub-surface soil that remains at freezing temperature or below for two years or more.

When it thaws -- something now happening at an increased rate as the planet warms -- it can cause rockfalls or landslides.

Experts believe this likely played a role in the destruction of Blatten village last month, with rockfalls on top of Switzerland's Birch glacier increasing the weight load and causing it to collapse down the Lotschental valley in spectacular fashion.

According to a new study by the Swiss Academy of Sciences (SCNAT), the permafrost in the Swiss Alps has never been as warm as it was in the hydrological year 2024, which began on October 1, 2023 and ran until September 30.

"Over the past 10 years (2014-2025), permafrost temperatures have generally increased at the 23 study sites, with changes exceeding more than 0.8 degrees Celsius at a depth of 10 metres, leading to new temperature records in 2024," SCNAT said.

The increase in permafrost temperatures was reinforced in 2024 by early snowfall at high altitudes in autumn 2023, with the early snow cover trapping heat in the ground.

In Switzerland, permafrost is found under about five percent of the total surface area, generally in scree and rock above 2,500 metres.

- Warming -

The hydrological years 2022, 2023, and 2024 are among the five warmest years recorded in Switzerland since measurements began in 1864, with air temperatures 1.4C to 1.9C higher than the 1991-2020 average.

Switzerland is particularly affected by global warming.

The current average near-ground air temperature is already about 2.9C higher than the 1871-1900 pre-industrial average -- compared to a global average of 1.3C, according to MeteoSwiss, the Federal Office of Meteorology and Climatology.

The results from 25 years of coordinated monitoring in the Swiss Alps show that permafrost has warmed significantly, while ground ice content has decreased.

"These changes will continue in the coming years and decades. The warming of permafrost observed in the top 10 metres will penetrate further to greater depths," said SCNAT.

- Hazards -

At all observed sites, the depth of the active layer -- the surface layer of soil that thaws in summer and refreezes in winter -- has increased by up to several metres over the past two decades, reaching unprecedented levels in 2024.

The most significant changes were observed at Schilthorn in the Bernese Alps, where the active layer increased from less than five metres in 2000 to more than 13 metres in 2023 -- and ultimately failed to refreeze during the winter of 2024.

Permafrost warming is hugely important for planning mountain infrastructure and villages, and for monitoring natural hazards.

More than a third of the Swiss Alpine Club's mountain huts could be unstable in the future due to thawing permafrost, according to a study published last year.