On 23 July 1983 Ian Whittaker and I were inching our way up the Bonatti Pillar, a legendary Alpine climb up 2,000ft of golden granite on the south-west face of Les Drus, high above Chamonix in France.

Walter Bonatti had made the first ascent of this route alone over five days in 1955. It is a legendary mountaineering story, perhaps one of the greatest exploits in the history of Alpinism, to rank alongside the first ascents of the north faces of the Eiger, the Matterhorn and the Grandes Jorasses.

We all need heroes. Walter Bonatti was the hero of heroes; a man way ahead of his time whose mountaineering prowess was awe-inspiring. I repeated the routes he put up with a sense of reverence. I have followed in the footsteps of so many of my heroes and there were times on their routes when I half expected to see them pass me by dressed in the clothes and the equipment of their time, climbing steadily with grim, hard, unsmiling expressions. I knew that they would not notice me.

Only Bonatti has survived. The rest are all gone, leaving the faint glow of their brilliance on the routes they pioneered. Yet the icy world in which Bonatti played his high-risk games is changing with frightening rapidity. The mountains are melting, and it is not only mountaineers who will suffer the effects. The long-term outlook for the Alpine nations - and those in which the other great ranges lie - is bleak.

The Dru is an extraordinary pinnacle of rock. It sports an icy north face (one of the six classic Alpine north faces), a 3,000ft west face of smooth vertical walls and overhangs, and the spectacular south-west Bonatti Pillar. Few mountains have such a variety of magnificent lines on them or look so beautiful. The Dru crusted with a winter lace-work of ice and gilded in the golden pink of Alpine glow is one of the most striking sights in the Alps.

The Bonatti Pillar itself rises in a series of steep, leaning columns seamed with fissures and bristling with overhangs. It rears up 2,000ft towards the massive capping overhangs just below the summit.

By late afternoon we had reached the Red Walls - 300ft of blank granite split by a hairline crack that bristled with old, rusting pitons. We were tempted to bivouac on a series of terraces at the top of the Red Walls but confidence got the better of us and we decided to try to get past the huge roofs and reach the summit in a day.

As darkness began to close around us we found ourselves in increasingly blank and forbidding territory. The dark shadow of the roofs blackened the early night sky above and tendrils of mist began swirling up from the depths of the icy couloir glinting thousands of feet below.

I began to follow the ropes draped down the corner, clutching in the darkness at unseen holds and shouting for Ian to give me a tight rope. After about 40 feet, the vertical corner seemed to pinch out into a smooth wall. Groping to my left, my fingers slipped into a sharp-edged crack and, with help from Ian, I struggled up until I saw the dark shadows of his legs hanging above me. He was sitting on a narrow ledge.

I clipped myself to a handrail rope that Ian had fixed above the ledge. The handrail had been tied to an old ring piton and stretched across to the far end of the ledge, where he had tied it to a small flake of protruding granite.

Once ensconced inside my bivouac bag I settled myself down on the comforting solidity of the ledge. Seconds later there was a heart-stopping downward lurch accompanied by the thunderous sound of tons of granite plunging into the abyss. I heard a cry of alarm and pain above the roar of falling rock. My arms were outside the bivouac bag as I fell and I flailed them blindly trying to grab something. It must have taken only a fraction of a second but it seemed to last forever.

We bounced on the springy stretch of rope. The handrail had held. I swung gently on the rope with my arms pinned to my sides. I had held the fall on my armpits and for a confused moment I desperately tried to remember whether I had clipped myself to the handrail.

In the sudden darkness, with the sounds of falling rock echoing up from the depths, I was momentarily disorientated. Where was Ian? I remembered that sudden yelp during the fall. Had he gone with it?

"By 'eck!" I heard Ian's broad Lancastrian voice beside me. I poked my head out from my bag and glanced at Ian. His head lolled on to his shoulder and his torch reflected a sodium yellow light off the surrounding rock walls. There was blood on his neck.

We hung side by side on the tightly stretched rope and swore. With the help of our torches we were horrified to find that our ropes had gone. We looked at each other and giggled nervously. Two thousand feet up and no ropes! The handrail shifted suddenly, causing us both to squeak with fright, hearts hammering at the thought of falling again.

I turned and shone my torch on the handrail. It looked odd. I twisted round, grabbed the rope. It shifted again and the peg moved. I lowered myself gingerly back on to the rope.

"Oh God," I whispered.

"What?"

"The peg's buggered. It's coming out."

"Christ! Where's the gear? Let's put something in."

"It's gone. The hardware, boots, everything. It went with the ledge."

Ian was silent. I looked at the flake where the handrail had been tied off. Tiny pebbles and dust trickled from its sheared-off base. Both attachment points could go at any moment. If either went, we would fall into the abyss.

"I think we had better stay very, very still."

"Aye," Ian muttered.

We hung there helplessly for 12 hours until at last a helicopter came into view and we were winched to safety.

Two weeks later, while working as a plongeur in the Montenvers Hotel, I saw an even bigger rock fall on Les Drus - a fall that altered the shape of the summit and spewed helicopter-sized blocks down the north face, creating a 1,000ft high dust cloud.

So what? After being swept 2,000 feet down the north-east face of Les Courtes in 1981 and then having my bed disappear on the Dru in 1983 I am keenly aware that mountains have always been falling down, usually, it would seem, with me attached to them. It happens. The Cairngorms were once Himalayan in scale. Frost, wind and water have ground them down to their present lowly heights.

However, 20 years later it would seem that perhaps Les Drus are falling down rather faster than they should. In 1997 more than 1,500 cubic metres of rock fell into the valley below, destroying classic alpine routes such as the Thomas Gross and the Destivelle routes as well as some pitches of the Bonatti Pillar.

This was nothing compared with the collapse on 29 June this summer, when the west face of Petit Dru suffered yet another enormous rock fall. A fortnight earlier, two climbers on the Quartz Ledge escape route from the top of the north face had been alarmed to discover that a gaping crack had split open along the length of the ledge. It was the first sign that the Bonatti Pillar in its entirety was soon to disappear, alongside the famous Harlin Route on the west face and large chunks of the American Direct.

The collapse occurred above the previous 1997 fall. Fifty years of iconic climbs had disappeared without trace. More surprisingly still, no one was killed. Climbers have been advised to steer clear.

Such warnings are becoming ominously familiar in the Alps nowadays. Two years ago Victor Saunders, one of Britain's leading climbers, and his companion, Craig Higgins, had reached a point halfway up the Matterhorn's Hornli ridge when their climb turned into a nightmare.

"An enormous avalanche hurtled down the mountain's east face," said Saunders. "I have never seen so much rock falling at one time." An almost continuous rain of boulders ricocheted past them as they cowered under an overhang. Within an hour an even bigger rock avalanche was thundering down the north face, obliterating the classic 1931 Schmidt route that I had climbed in 1980. This was swiftly followed by the thunder and dust cloud of yet another vast rock fall. In one of mountaineering's biggest mass rescues, more than 70 climbers had to be hoisted from the slopes of the Matterhorn.

A ban on climbing the mountain was instigated for the first time in history as rock falls battered its broken flanks. It seemed to the survivors that the very Alps had started falling apart.

In the summer of 2003 one of the world's most iconic climbs, the 1938 route on the Eiger's north face, became yet another victim of climate change. Climbers were shocked to find that there was barely any ice left on the route. The huge second ice field, the third ice field and the White Spider had melted away and now consisted of rubble-strewn rock slopes dusted by blackened snow and pocked by forlorn patches of ancient grey ice. The heat wave of last year, reported to have been the hottest Alpine summer in 200 years, seemed to have finished off this venerable climb. It may be that it is only ever climbable during the winter months, when some semblance of névé ice has reformed.

A local guide, Hans Ueli, has reported enormous rock falls. One such fall woke him at five in the morning and, upon looking out of his window, he saw that the lower half of the 6,000ft high face was obscured by an enormous cloud of dust.

Climbs the length and breadth of the Alps have suffered similar collapses. On Fiescherwand there was no snow ice at all on the entire four-mile wide north face. The north face of Les Droites near Chamonix, recently only climbable in the winter, now even in the coldest months presents an insurmountable 600m barrier of smooth, bare rock slabs where once there had been pristine ice fields.

Ironically, only a few days before the Bonatti Pillar disintegrated, a man regarded by some as a half-witted religious bigot announced at the G8 summit in Gleneagles that as far as he was concerned America did not regard global warming as important nor pressing. Leastways that is how I interpreted President George Bush's words.

Scientists now believe global warming is melting the Alps. The ice that for thousands of years had filled the deep cracks at the summit of the Dru has started to melt. The glue holding this rock tower together is leaking away.

More seriously, the crust of permafrost that binds the whole mountain range together is beginning to melt. The foundations of buildings, roads, mines, tunnels, cable-car stations and their supporting pylons are entirely dependent on the frozen solidity of this permafrost. As it steadily melts, the whole infrastructure of Alpine tourism is at risk, as well as a great many lives.

All the most famous ski resorts in Europe are situated in valleys overlooked by mountains held together by permafrost. The high altitude permafrost zones lie on steep slopes above these settlements, roads, railways and valleys. Massive slope failures and landslides leading to blocked rivers, dammed lakes and catastrophic flooding will be especially pronounced in the Alps, which has such steep topography and high population levels.

Already climatologists have predicted the complete failure of the Scottish ski industry due to lack of snow within 20 years and the Alpine ski industry within 50 years. Many Alpine ski resorts would already be out of business but for the snow machines.

Because the best Alpine ski fields and lift systems are above the crucial permafrost altitude of 8,202 feet, it could spell the end of the ski industry as we know it, let alone the more esoteric world of mountaineering. When you consider that one sixth of Austria's gross domestic product comes from Alpine tourism, the effects of permafrost meltdown could be far more wide-ranging than just screwing up our winter sports holidays.

Climatologists, geologists and civil engineers from all over the world are making disturbingly similar reports. Glaciers in Antarctica are thinning twice as fast as they were a decade ago and this may destabilise the west Antarctic ice sheet, which, if melted, contains enough ice to raise sea levels by as much as five metres. A gigantic slab, the Larsen B ice shelf, has already fallen off its eastern side.

Ablation rates of glaciers are speeding up all over the world. Retreating glaciers in the Peruvian Andes are adding huge amounts of melt water to already overburdened mountain lakes, greatly increasing the risk of dam collapses and alluvion avalanches. There are passes in the Cordillera Real in Bolivia that just 20 years ago were glaciated, yet now are rocky moraine fields.

Only two weeks ago it was announced that Kilimanjaro in Tanzania would lose its year-round mantle of snow within 10 years. One-third of Kilimanjaro's ice field has disappeared in the past 12 years.

In Iceland ice cores have shown that temperatures are at their highest since the arrival of the Vikings. The past two years have been the hottest since records began in 1822. At this rate of melting, all the ice will be gone in 200 years.

In the Arctic, a region of sea ice the size of France and Germany has melted away in the past 30 years and there are fears that the inflow of fresh water could possibly lead to the shutdown of the Gulf Stream, which bathes Europe in warm water. This would plunge Britain into winters that would be the equivalent of those in northern Canada. It wouldn't save the ski industry, not unless you like skiing in conditions of 40C below.

Boreholes sunk to monitor ice temperatures in Switzerland, Austria, the Dolomites, the German Alps, the Sierra Nevada and the Abisko mountains in Swedenn have all recorded temperature increases of between 0.5 and 1C during the past 15 years.

The ground temperature in the Alps has risen considerably over the past decade. As air temperatures have increased, the effects below ground are being magnified fivefold. A test borehole dug in Murtel in southern Switzerland has revealed that sub-surface soils have warmed by more than 1C since 1990. Increasing evaporation caused by warmer summers is also triggering thicker falls of winter snow, which insulate the soil and keep it warm. All in all it is not looking good.

Spotting the early signs of the imminent collapse of buildings and valleys may be possible. Mountains collapsing around your ears are a dead giveaway. Noticing that cable stations and other buildings have developed cracks should also be easy. But by then the horse has well and truly bolted. The abrupt disintegration of the Matterhorn, the Dru and the desertification of the north face of the Eiger may mean that some classic routes can no longer be climbed, but they are also the harbinger of far more gloomy events.

Is this global warming? I don't know. It might just be a normal climatic cycle. Somehow, unlike President Bush, I don't think so. It may not be the day after tomorrow but it certainly looks as if it is all because of the day before yesterday.

Joe Simpson is a climber and author of 'Touching The Void'