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From the summit of the Eggishorn in the Bernese Alps of south-central Switzerland, close to 3,000 m above sea level, you can look down through the thin air across an enormous expanse of frozen water. Almost the entire length of the Aletsch glacier unfurls below you, partly shrouded in the snow that feeds it, the rest revealed as a stunning instrument of nature's pure power.

The largest glacier in the Alps starts in Konkordiaplatz, an immense snow bowl in which four ice streams meet and meld. Only below the bowl, where the winter snows have melted, can you see the surface of the glacier as it begins its curving, muscular descent down the valley. The mountainsides have been shaped and scarred by these rivers of ice as they carved their course over tens of thousands of years, right down to the edge of the Aletsch Forest. There, stone pines and larches live to heroic ages in the glacial temperatures; some are believed to be 600 to 700 years old. That's roughly the same amount of time it takes for a single molecule of water to make the journey from a snowflake on the yoke of the Jungfrau mountain to the Massa, the stream flowing from the glacier's snout.

Everything about the Aletsch is on an epic scale. During three days of hiking, crevice dangling and climbing, glacierologist Sylvain Coutterand, mountain guide Christophe Profit, photographer Pascal Tournaire and I were struck by the Aletsch's grandeur. But we were also alarmed by the sobering evidence of how the last century's warmer temperatures have eaten away at this giant.

The shrinkage was apparent as we trekked down from the Eggishorn to Märjelensee, a glacial lake in a valley on the left bank of the glacier. In the 19th century, the glacier's edge loomed over the lake's surface like a frozen wall. Today, it's an easy walk down from the lake shore to the glacier's otherworldly surface.

Up close, the Aletsch, a play of whites and grays from a distance, reveals all kinds of smaller secrets. The words ice and snow are woefully inadequate to describe the infinite variety of its forms. Sometimes you crunch across a surface like a gravel driveway, or scramble over muddy crushed ice, or slosh through the kind of mess you'd find in a street gutter in late February. Patches of white, damp snow fill some of the crevices — some just centimeters wide, others several meters across. The danger that a snow bridge may give way and send us plunging into the frozen depths of a crevice means that we take frequent detours so we can seek solid ice on both edges before stepping across.

Even in this wild place, civilization has left its traces. Profit finds half-buried in the surface the decayed wooden shaft and iron point of an old ice ax, which must have been left behind by an explorer of an earlier generation. The remnants of a blue balloon lodge on the cusp of a crevice, attached to a card signed by teenager Moritz Hossli, who released it two years ago in Giswil, over the mountains 45 km to the north. Several times every day, fighter jets from the Swiss air force come roaring over this uninhabited valley, one pair so low we gasp and duck. It seems a strange way to pay tribute to the first Alpine region to have been declared, in 2001, a unesco World Heritage Site alongside the likes of the Serengeti and Yellowstone national parks, and the Galápagos Islands.

The Aletsch is familiar territory to everyone else in my group. Coutterand, a researcher at the University of Savoy, first came here in 1986 and has made a lifelong study of how the rocks reveal its changing fortunes. Profit has been on the glacier dozens of times, mostly in winter for ski tours, and Tournaire was here last summer. In the late afternoon, after we've been ascending the glacier for four hours or so, we stash some of our weighty gear — crampons, helmets, ropes and Profit's homemade sausage — near a giant face of collapsing earth at the glacier's edge. Then we begin an arduous climb up the slope on its left bank. Here the glacier's retreat has left a highly unstable mountain of dirt and rock; every other step sets loose a minor rock fall until we finally reach the higher ground of an older, firmer moraine. We have started too late in the day, at least at my pace, to make the snowbound summit we originally set our sights on.

By the time we return to our cache, the sky is darkening and a cruel wind has begun blowing in from the north. None of that strikes my colleagues as even remotely disconcerting. We start the long slog up the glacier toward the Konkordiahütte, a refuge on the edge of Konkordiaplatz, marching through fields of boulders strewn like marbles along the edge of the Aletsch, and passing dramatic caverns and ramparts formed by the constant motion of the ice, which in the shallower portion just south of Konkordia has been measured to flow some 205 m per year. Once night falls and only our headlamps light the way, I wonder if it might have been prudent to rope up so that someone could haul me back out if I were to take a tumble into one of those alluring crevices, which can go as deep as 100 m. Beyond that depth, Coutterand assures me, the sheer mass of the ice closes all gaps. Cold comfort indeed.

The headwind becomes even fiercer and icier in the dark, and it is almost midnight when my headlamp reveals a metal ladder that I figure will lead us to the refuge. But as I climb, my lamp reflects off a latticework of metal staircases, bolted to the cliff face and ascending more than 150 m up to the refuge. The first refuge built here in 1877 was only 50 m above the surface of the glacier, and as recently as the 1960s the ice still reached halfway up to the refuge.

One consequence of this, trivial in the grand scheme of things, is that it is a long haul to the warm fire and longed-for cache of Swiss beer that await us. The environmental consequences of the shrinkage, though, are incalculable. The Aletsch is 23 km long and as much as 900 m thick. It contains an estimated 27 billion tons of water — enough to provide everyone on earth with a liter a day for six years. Yet for all its immensity, it is a fragile wonder. If current trends continue, warns Hanspeter Holzhauser, a glacierologist who has been visiting and studying the Aletsch glacier for 25 years, it could disappear entirely in the geological instant of a half-millennium or so.

The next morning dawns cold, but today we're heading down the glacier rather than climbing up it. At midday we find a crevice about a meter wide that merits deeper exploration. Profit secures a pair of ropes to four ice screws anchored in the glacier. With crampons and a pair of ice axes, I follow Tournaire into another world. Beneath its gritty surface, the glacier reveals its true essence: a pure ice so diamond-hard that it's difficult to plant an ice ax firmly in it. The walls of the crevice are a magical, luminous blue. The light penetrates deeply into the ice walls, revealing tiny frozen droplets of history, fleas and grains of sand subsumed into the glacier during its achingly slow descent.

Along the lower reaches of the glacier's left bank the next day, we take a walk through the centuries by exploring the Aletsch Forest, a 410-hectare microclimate harboring deer and chamois, foxes and ermine. The gnarled stone pines on the high slopes of the ancient moraine are the oldest trees in Switzerland. Lower down, you can see how the forest is encroaching on surfaces laid bare by the glacier's retreat. Light green larches, among the pioneers of the slope, become smaller and smaller as they approach the glacier's receding edge. The lowest slopes are utterly denuded and highly unstable.

The glacier's fate determines all else in the valley. In 1653, the peasants who grazed their cattle on the high meadow of Ussere Aletschji on the right bank of the glacier sought divine intervention to stave off its incessant progress. Two Jesuit priests, Fathers Charpentier and Thomas, came up from Brig in the Rhone Valley to lead the villagers in a procession to the glacier's edge. They sprinkled the great Aletsch with holy water in an attempt to exorcise its diabolical intent. A cross erected in 1818 to mark a later attempt at exorcism still stands on the hillside, far above the glacier's current surface.

All glaciers are mutable, points out Coutterand. "The Aletsch is a relic of the Ice Age," he says. Toward the end of the Ice Age, some 20,000 years ago, the Aletsch was a small part of a much larger glacier that extended all the way down the Rhone Valley, to the current site of Geneva. For millenniums, the soaring Alpine peaks lay submerged in a sea of snow, poking through only here and there.

As temperatures rose, the glaciers receded, but the heavy precipitation in the high Bernese Alps and the protected, elevated valley leading down from them preserved the Aletsch. During the so-called Roman Optimum, a warm-weather period that began around the start of our era and lasted 400 years, the glacier was smaller than it is today, as it was again in the early Middle Ages around 1,000 A.D.

The Aletsch began to expand again when a period known as the Little Ice Age set in around 1350. As the glacier swelled, it mowed down trees and smothered alpine grazing ranges that had been in use for centuries; the valley below would be inundated by flash floods when the waters of the Märjelensee suddenly drained through a crevice. Little wonder that the local population regarded the glacier with awe and terror, viewing its depredations as the work of a mythical and monstrous horned ibex known as Rollibock. Even now, when the booming of settling ice echoes through the mountains, it's easy to understand the unease it inspired. Geysers formed by runoff have been known to spout 100 m into the air.

By 1856, the Aletsch had attained its maximum height since the Ice Age; since then it has rapidly dwindled, losing 3 km of length and an estimated 330 m of altitude at its snout, by Holzhauser's reckoning. That shrinkage is clearly a result of warmer temperatures. It is harder to say to what extent human activity has caused that warming. Holzhauser has researched the glacier's rise and fall through the millenniums, so is cautious about drawing conclusions based only on the last 140 years. But along with virtually every other glacier expert, he is deeply worried. "No one can yet say with scientific certainty that the glacier is shrinking because of a greenhouse effect caused by humans," says Holzhauser. "But the fact is that in the last 100 years the glacier has shrunk faster than ever before. And in the last 10 years it has been reducing at a phenomenal rate of up to 50 m per year."

That pace could quicken still further. Holzhauser and other glacierologists have calculated that the current rate correlates not to today's temperatures but to weather conditions that prevailed 30 to 40 years ago. Over the last three decades, according to the Swiss government's Consultative Organ for Climate Change, temperatures in Switzerland have increased three times as much as the global average. "The Aletsch is so big it reacts slowly," says Holzhauser. "But what we're seeing already augurs a tragedy."

When you're walking through the icy world of the Aletsch glacier, dwarfed by its sheer mass, it is hard to see it as fragile and vulnerable. Yet under the climate onslaught of the last century, it is both. If the entire industrialized world were to commit itself to efforts to counter the warming trend, perhaps its diminution could be reversed. But it may well take another ice age to save the Aletsch. Maybe Rollibock is waiting for nothing less.

• JAMES GRAFF
• Europe's largest glacier been retreating for nearly 150 years. Today, its icy heights and rare flora and fauna are part of a vanishing landscape