In the 19th century up to a metre of ice was dug each day out of the glacier in southwest Switzerland, close to the border with France, and taken to Paris and Marseille for mixing in the anise-flavoured liqueur adored by the French.

The ice grew back overnight.

These days, Parisian cafe owners get their ice elsewhere.

"Nowadays of course the ice is way, way, way up. It's amazing how much has changed there," said Kev Reynolds, author of a guide to a Chamonix-to-Zermatt walking route, who has made several trips through the valley since the 1980s.

"Vegetation will soon be setting in down there, where a few years ago there was ice."

Switzerland has been particularly hard hit by a warming climate, with ski resorts often short of snow cover and potential water supply problems as sources melt away.

The Trient glacier starts at a height of about 3,300 metres and the end, in the Trient valley, is now at some 1,900 metres. It used to run down almost as far as a refreshment hut at about 1,600 metres.

It is just one of the many signs of the havoc climate change is wreaking on the mountains. I walked over, around or across many of them this summer, including the Chamonix-Zermatt trek from Mont Blanc to the Matterhorn.

Most hikers take about two weeks to complete the trail, which forces a way through some of the highest mountains in Western Europe across ridges and deep valleys, climbing more than 12,000 metres in altitude over the course of the journey.

It skirts glaciers where not long ago technical equipment could have been used to cross the ice. Some sections have been wiped out by rockfall, forcing walkers to take long and often uncomfortable detours over boulder fields.

EVER MORE DANGEROUS

One of the starker examples is a jumbled mass of debris and boulders where the Grand Desert glacier used to stretch below the peak of Rosablanche.

Only a few years ago, the route used to cross the glacier itself at a safe point, with no dangerous crevasses. Now there is no real path but red stripes painted on the rubble carried down by the glacier and left behind in this barren wilderness.

Further on, falling rock has forced a change of route around the dammed Lac de Dix high above the Rhone valley.

The glacier below this path, curving down from the pyramidal Mont Blanc de Cheilon, is two pitifully thin trails of white easily crossed without ropes or specialist equipment, even after a relatively cool summer.

At several parts along the final stretch into Zermatt, signs warn hikers to hurry over exposed sections, now fitted with protective fixed ropes, lest they be hit by falling rock.

Further along, an easy crossing of a glacial torrent has been washed away and replaced with a nerve-racking, 50-metre-long cable bridge dangling over the depths.

Reynolds recently tried to research an alternative route to Zermatt on the other side of the valley, avoiding these rockfall areas, but was forced back.

"It promised to be a terrific thing, but by golly it got so dangerous because it's just falling apart," he said. "It's impossible now, I wouldn't recommend it to anybody."

After two weeks of hard travel, my first full view of the Matterhorn's iconic needle caused a shock, even from far away down the valley.

What used to be a classic north face, sheathed in ice and shadow, is now predominantly rock.