Reading passage
Glaciers are often described as rivers of ice, but the comparison understates their strangeness. A glacier is formed where more snow falls in winter than melts in summer, so that layer upon layer accumulates over decades and centuries. As the pile deepens, the weight of the overlying snow squeezes out trapped air and recrystallises the flakes below into dense, bluish ice. When the ice becomes roughly fifty metres thick, it begins to behave like a very stiff fluid, deforming under its own weight and flowing downhill at rates ranging from a few centimetres to several metres a day.
This slow motion makes glaciers powerful agents of erosion. Embedded with rock fragments, the base of a glacier grinds the underlying bedrock like coarse sandpaper, while freeze-thaw cycles around its margins pluck away blocks of stone. Over thousands of years, these processes carve the classic signatures of glaciated terrain: U-shaped valleys with steep walls, hanging tributary valleys, and polished rock surfaces scored with long parallel scratches called striations. Much of the scenery now celebrated in mountain national parks, from the Alps to the Rockies, was sculpted by ice that vanished thousands of years ago. Glaciers also transport enormous quantities of debris. Rock material carried within the ice is deposited at the glacier's edge or snout in ridges called moraines, which can dam valleys and create lakes. Some of these natural dams collapse suddenly, releasing destructive floods known to mountain communities on several continents.
Glaciers also function as archives of past climate. Each annual layer of snow preserves a sample of the atmosphere at the moment it fell, including dust, pollen, volcanic ash, and bubbles of ancient air. By drilling cores through the ice sheets of Greenland and Antarctica, scientists have reconstructed a continuous record of temperature and greenhouse gas concentrations reaching back more than eight hundred thousand years. These cores revealed, among other things, that carbon dioxide levels today are higher than at any point in that entire span, and that abrupt warming events in the past have sometimes occurred within a single decade.
For human societies, the most immediate role of glaciers is as water towers. In the dry season, when rain is scarce, meltwater from glaciers sustains rivers used for drinking, irrigation, and hydropower. The great river systems of South Asia, including the Indus and the Ganges, depend heavily on the melting of Himalayan snow and ice, supporting the agriculture of well over a billion people. A modest glacier retreat can initially increase summer flows, but once the ice passes a critical volume, the annual supply becomes erratic and eventually declines. In the Peruvian Andes, entire cities and mining operations depend on this seasonal melt, which is why local water managers now track glacier volume almost as closely as reservoir levels.
That threshold is now approaching in many regions. Since the mid-nineteenth century, the great majority of the world's mountain glaciers have been shrinking, and the pace of loss has accelerated sharply since the 1980s. According to the World Glacier Monitoring Service, recent decades have brought the fastest retreat in its more than one hundred years of observation. Iconic ice fields on Kilimanjaro and in the European Alps have lost a large share of their area, and many small glaciers in the Andes and Central Asia are expected to disappear entirely within the present century.
The consequences of this loss reach the sea. Meltwater from glaciers and ice sheets raises global sea levels, and although mountain glaciers hold far less ice than Greenland or Antarctica, they have contributed disproportionately to recent sea-level rise precisely because they respond so quickly to warming. Rising seas in turn threaten low-lying deltas and coastal cities. Meanwhile, communities downstream face a doubled risk: in the short term, floods from suddenly released meltwater, and in the long term, chronic water scarcity once the frozen reserve is gone.
Adaptation strategies range from the practical to the experimental. Some regions are building reservoirs to capture irregular meltwater, while researchers have tested covering vulnerable ice with reflective sheets to slow summer melting. Such measures can buy time locally, but glaciologists emphasise that only the stabilisation of the global climate can secure the long-term future of the world's remaining ice, and with it the rivers, archives, and landscapes that ice sustains.