The Great Aletsch Glacier (Grosser Aletschgletscher) is the largest glacier in the Alps, a river of ice about 23 km (14 mi) long (2014) in the eastern Bernese Alps in the Swiss canton of Valais. It covered about 81.7 km² (31.5 sq mi) and held some 15.4 km³ of ice in 2011. Four tributary ice streams meet at Konkordiaplatz, where the ice has been measured at up to 905 m (2,969 ft) thick, and from there the glacier curves south toward the Rhône valley, ending in the meltwater river Massa. It is the centrepiece of the Swiss Alps Jungfrau-Aletsch UNESCO World Heritage Site, inscribed in 2001, and like most glaciers it is retreating: 3.2 km shorter since 1870, 1.3 km of that since 1980. Because a glacier lives by a yearly balance of snow gained and ice lost, this guide follows Aletsch through its water year, from winter snowfall at the Jungfraujoch to late-summer melt in the Massa gorge.
The Four Firns Above Konkordiaplatz
Every water year on Aletsch starts in the high basins where snow outlasts summer and slowly turns to firn and then ice. Apart from the Finsteraarhorn, all the highest summits of the Bernese Alps stand inside the glacier’s drainage basin: the Jungfrau and Mönch on its northern rim, the Gross Fiescherhorn and Gross Wannenhorn to the east, and the Aletschhorn, at 4,193 m (13,757 ft), to the west. Four ice streams collect their snow and meet at Konkordiaplatz.
The Grosser Aletschfirn comes in from the west along the foot of the Aletschhorn, fed by three firns that start at around 3,800 m. The Jungfraufirn, the straight continuation of the main glacier, flows down from the saddle between the Jungfrau and Mönch. The Ewigschneefäld, the “eternal snowfield”, curls down from the east flank of the Mönch and drops over a 25 to 30 per cent slope into the junction. The smallest, the Grüneggfirn, arrives from the east below the Grünegghorn. The Konkordia Hut of the Swiss Alpine Club overlooks the meeting point at 2,850 m. It stands about 150 m above the surface of the ice, and climbers reach it from the glacier by a long flight of stairs or a steep path.
December to March: Snow Piles Up at the Jungfraujoch
Winter is when Aletsch earns its ice. The Jungfraujoch, the glacier saddle at 3,463 m (11,362 ft) between the Jungfrau and Mönch, sits on the upper snows of the Jungfraufirn and is officially the coldest place in Switzerland. Its weather station, at 3,580 m, records 1991–2020 mean temperatures of −12.5 °C in January and −13.3 °C in February, and even the average daily high stays below −8 °C from December to March. Its climate lies on the border between tundra and ice cap. Snow that falls here does not melt; it is buried by the next fall and compacted, adding to the ice that will flow down toward Konkordiaplatz over the coming years.
The mountains split the weather. The Jungfrau-Mönch-Eiger wall forms a barrier between the wet, sub-oceanic north side, where precipitation exceeds 2,200 mm a year and falls mostly in summer, and the drier Valais slopes to the south, which get about 1,000 mm, with more of it in winter. Down at the glacier’s southern edge, winter belongs to skiers. Riederalp, at 1,930 m, and Bettmeralp are car-free villages reached by cable car, with about 438 and 452 residents respectively in 2020, and they share some 30 km of pistes looking across the ice to the Pennine Alps. Belalp, at about 2,100 m on the western side, is a ski village too.
May and June: The Thaw Reaches the Aletsch Forest
The glacier’s season turns in early summer. At the Jungfraujoch the average daily high first climbs above freezing in June, at 1.7 °C, and the station logs about 170 hours of sunshine that month. Lower down, the snow pulls back from the slopes above the ice, and the Aletsch Forest emerges. It is a forest of Swiss stone pine (Pinus cembra) near the tree line, and it grew up on ground the glacier itself laid down: it developed on moraine left behind after the ice reached its last maximum around 1850. On the Riederfurka, at 2,065 m between Riederalp and the glacier, the historic Villa Cassel, once a summer house for guests from politics and finance, is now a centre run by the conservation group Pro Natura, with a permanent exhibition about the site.
The wider World Heritage Site shows how altitude sorts life. About 80 per cent of it is glacier and bare rock, 6 per cent forest, 5.2 per cent alpine meadow and 8 per cent scrub. It holds 1,800 species of vascular plants and 700 mosses, and 529 species of flowering plants and ferns grow above the tree line. Broadleaf forest grows from 900 to 1,300 m on north-facing slopes and about 200 m higher on south-facing ones, and the subalpine belt of pine and spruce reaches up to 2,000 m.
July and August: Melt Season on the Lower Glacier and in the Massa
High summer is when Aletsch spends its ice. August is the warmest month at the Jungfraujoch, with a mean of 0.6 °C and an average high of 3.5 °C, and far below the snowline the lower glacier melts fast. From Konkordiaplatz the ice, about 1.5 km wide, moves south-east at around 180 m a year, then swings south-west past the Eggishorn and Bettmerhorn. Melt thins it from more than 900 m at Konkordiaplatz to about 150 m lower down. Two dark medial moraines, the Kranzbergmoräne in the west and the Trugbergmoräne in the east, run from the junction all the way to the snout, and the lowest part of the glacier is largely buried under rock debris. The toe now lies at about 1,560 m, well below the local tree line.
All that meltwater leaves through the Massa, a river only about 7 km long. It runs through the Massa Gorge, is held in the Gibidum reservoir and used for hydroelectric power, and joins the Rhône near Brig. A suspension bridge has crossed the Massa since 2008, linking hikes on either side of the glacier. On the eastern flank, near Bettmeralp, lies the Märjelensee, a small glacier lake at 2,301 m that formed beside the ice in the 19th century. Summer is also when the glacier draws the most attention: on 18 August 2007, at about 10 °C, the photographer Spencer Tunick posed some 600 volunteers on the ice in a project with Greenpeace to draw attention to glacier loss.
September and October: How Much Ice Was Lost This Year
By September the Jungfraujoch average high is back to 0.5 °C, and by October it is below freezing again. Fresh snow starts to cover the upper glacier, and the year’s account can be settled: how far the snout has moved and how much thickness was lost. The answers have been mostly negative for a long time. The glacier retreated a record 114.6 m (376 ft) in 2006 alone. Between 1880 and 2009 it shortened by 2.8 km, and 800 m of that, about 30 per cent, happened in the last fifth of the period, after 1980. It has also lost more than 300 m of thickness.
Size has slowed the damage. Since the end of the Little Ice Age around 1850, Aletsch has lost about 20 per cent of its ice mass, while other Swiss glaciers have lost up to 50 per cent, because a glacier this large reacts more slowly to a warmer climate. That buffer will not last: projections cited for the glacier suggest that by 2100 it will hold only about one tenth of its 2018 ice mass.
From 18,000 Years Ago to the Glacier of 1860
Each modern water year adds to a much longer record. About 18,000 years ago, in the last glacial period, the ridge between Riederalp and the glacier was buried in ice, and only the summits of the Bettmerhorn, Eggishorn and Fusshörner rose above it. After a major retreat the glacier advanced again about 11,000 years ago, reaching the Rhône valley and spilling over the Riederfurka; its moraines are still visible in the Aletsch Forest. In 1860 it was 3 km longer than today, with its ice surface 200 m higher.
Retreat has since cut the glacier off from its neighbours. The Oberaletsch Glacier joined it until about 1880 and now only feeds the Massa, and the Mittelaletsch lost its connection at the end of the 20th century. The Gibidum reservoir lies roughly where the snout reached in the 19th century, more than 4 km from today’s toe. The physicist John Tyndall, whose glacier studies brought him to the area, built a summer chalet at Belalp in 1877, and after his death in 1893 a memorial was raised at 2,340 m within sight of the ice.
Rhine or Rhône: The Jungfrau Ridge Watershed
Where Aletsch’s water ends up is decided at its very top. The ridge between the Jungfrau and Mönch is one of Europe’s main watersheds. Snow that falls on its north side drains through the Weisse Lütschine to the Aare and the Rhine, and on to the North Sea. Snow that falls on the Jungfraufirn to the south becomes Aletsch ice, then Massa water, then the Rhône, and reaches the Mediterranean. The Jungfrau Railway, open since 1912, tunnels through the Eiger and Mönch to Jungfraujoch station at 3,454 m, the highest in Europe.
The whole basin is protected. The Jungfrau-Aletsch site was the first natural World Heritage Site in the Alps; with its extensions it covers 82,388 ha, 56 per cent in Valais and 44 per cent in Bern, and its 35,000 ha of glacier form the largest continuous ice area in the Alps. For the mountains around it, see our guide to the Alps; for the weather that feeds and melts it across the country, read about the climate of Switzerland; and to see how Aletsch measures up against Antarctica’s and Asia’s giants, browse our list of the largest glaciers in the world.