High up in the natural wonder of the French Alps, the climbers who spend their days among the rockfaces and glaciers have come to a grim conclusion: the mountains are falling down around them.

In the Mont Blanc range, a magnet for mountaineers in the summer, many popular routes up or through the peaks have become too dangerous to take because of the risk of falling debris.

"It's going quickly. Ten years ago, I'd have never thought that it would accelerate like this," said Ludovic Ravanel, an academic at the University of Savoie Mont Blanc who has been studying major rockfalls in the area.

"And if you look at the predictions from my climatologist colleagues, for the next 10 to 20 years, it's only going to get worse," he said.

The Mer de Glace glacier during sunset in Chamonix. AFP

In many areas of Western Europe, climate change is happening too slowly to be noticed, although two record-setting heatwaves in June and July this summer as well as water shortages are focusing minds.

Legendary climbs no longer

Worries about the impact of shorter winters and hotter summers are commonplace in the ski businesses and mountain refuges where people depend on adventure sports for their livelihoods.

Many wanted to know if it would re-freeze overnight, making the snow firmer, or whether certain routes were open and safe to pass in the current conditions.

But all of them shared scare stories that they linked to global warming, including a 40-year-old guide from the nearby town of Thonon who was climbing the Aiguille du Peigne in the Chamonix area.

"The rock starting vibrating," he said. "I won't be in a hurry to go back."

A table of trainee guides, athletic young men under 30 aiming for careers in the industry, voiced their worries about the future of their profession.

Aspiring Mountain guide Yann Grava takes a rest near the Mer de Glace glacier.AFP

They said they had seen the change even in their relatively short lifetimes.

"The snow trails are hit-and-miss. In June, you used to be able to go for it. Nowadays it's not always possible and in July, forget about it," one of them, Remi, said.

Disappearing routes

Confirmation of the decay came in a recent study based on a popular mountaineering book published in 1973 by famed climber Gaston Rebuffat called "100 Most Beautiful Routes."

People walk inside the ice cave of the Mer de Glace glacier. AFP

Ravanel and fellow academics analysed the routes in order to measure how they had changed in the more than 45 years since the first appearance of the book, a bible for several generations of mountaineers.

For guides, the unpredictability of the conditions, with unexpected warm spells in winter or late snowfalls, is making a dangerous job even more nerve-wracking.

But some are keen to simply enjoy it while they can.

"I've started to accept quite a few things," admits Yann Grava, 33, who will finish his training to be a guide next year. "On average, a guide used to be able to work for about 15 years, but for me I think it'll be around 10. The mountains are falling."

Agence France-Presse