Glaciers worldwide are shrinking. The Hagedorngletscher in the Swiss Alps that gave its name to Alpine ski runs in the 1950s no longer exists. The Quelccaya Ice Cap in Peru, the largest tropical ice mass in the world, has retreated by more than 30 percent since 1980. The Glacier National Park in Montana had approximately 150 glaciers in 1850; it has fewer than 30 today, and most of those will be gone by 2030. The Greenland ice sheet is losing approximately 270 gigatonnes per year. Antarctica is losing approximately 150 gigatonnes per year. The total ice loss from mountain glaciers worldwide is approximately 300 gigatonnes per year. Glacial retreat — the continued shrinking of the planet's glaciers as warming temperatures cause more ice to melt than is replaced by snow accumulation — is one of the most visible and consequential aspects of contemporary climate change. It is happening on every continent that has ice (which is to say, every continent), and it has major implications for sea level rise, water resources, ecosystem function, and human communities that depend on glacial water.
How Glaciers Retreat
Glaciers have a "mass balance" — the difference between accumulation (snow that becomes ice) and ablation (ice that becomes water or evaporates). When accumulation exceeds ablation, the glacier grows. When ablation exceeds accumulation, the glacier shrinks. The shrinkage is what we observe as retreat: the lower (terminus) end of the glacier moves up-valley over time as the ice melts faster than it can be replaced. A retreating glacier becomes thinner overall and shorter at its terminus. Eventually, it can become small enough that the ice no longer flows at all — it becomes essentially a static ice patch rather than an active glacier. With continued retreat, even small remnants disappear, leaving the previously glaciated valleys ice-free.
Global Patterns
The retreat is happening on every continent with glaciers. The Alps have lost approximately 50 percent of their glacial mass since the 1850s, with the loss accelerating over the past 30 years. The European Alps in 2003 saw the warmest summer ever recorded, leading to massive glacier mass loss; subsequent decades have seen continued accelerating loss. The Greater Himalaya/Karakoram regions are losing ice on average, though with substantial regional variations — most ranges are losing, but the Karakoram in northern Pakistan has been relatively stable or slightly gaining. The Andes are losing rapidly, with tropical Andean glaciers particularly threatened. Alaska's glaciers (which include some of the largest mountain glaciers outside the polar ice sheets) are losing rapidly. Patagonia's glaciers are losing dramatically — the Northern and Southern Patagonian Icefields together lose about 24 cubic kilometres of ice per year. New Zealand's glaciers have lost approximately one-third of their ice volume since 1977. Almost no major glacierised region on Earth is currently in mass balance.
The Rate of Retreat
Individual glacier termini have been retreating at rates from a few metres per year (slow retreat) to several hundred metres per year (rapid retreat). The Kangerdlugssuaq Glacier in eastern Greenland retreated approximately 8 kilometres in just five years from 2002 to 2007 — about 1.6 km per year. The Jakobshavn Isbræ in western Greenland, one of the most-monitored glaciers in the world, has retreated by approximately 40 kilometres since 1850, with most of that retreat occurring since 2000. The Pine Island Glacier in West Antarctica has retreated about 12 kilometres since the 1990s. The Vatnajökull ice cap in Iceland has lost approximately 16 percent of its volume since 1890. Each individual rate is dramatic; the global pattern of retreat is the most rapid in at least 2,000 years (some estimates suggest in at least 12,000 years).
The Causes
The cause of contemporary glacial retreat is essentially the warming climate, with multiple feedback loops accelerating the response. Higher temperatures produce more melting both at the surface (during the warm season) and at the base (where warmer water encounters ice). Warmer winters reduce snow accumulation in some regions, even as precipitation totals may increase. The increased melt water lubricates glacier beds, accelerating ice flow into the ocean. Reduced sea ice cover affects the marine-terminating glaciers in Greenland and Antarctica, allowing warmer ocean water to reach the ice margins. Soot and dust deposits on glacier surfaces (from forest fires, industrial pollution, and other sources) reduce the albedo of the ice and accelerate melt. The combination of these factors produces an overall warming-driven mass loss that is now substantially exceeding what is being added by snowfall.
Tropical Glaciers
Tropical glaciers — those near the equator at high altitude in regions like the Andes, the high mountains of East Africa, and the Indonesian archipelago — are particularly vulnerable to warming. They sit at high altitudes where the temperatures are near freezing year-round and where small temperature changes can dramatically affect the ratio of melt to snowfall. The Quelccaya Ice Cap in Peru has lost approximately 30 percent of its area since 1980. The Khumbu Icefall on Mount Everest has retreated substantially. The glaciers of Mount Kilimanjaro have lost about 85 percent of their 1912 ice cover, with the remaining ice projected to be gone within decades. The glaciers of Mount Kenya have similar prognoses. The disappearance of tropical glaciers is particularly significant because these are often the only sources of dry-season water for the cities and agriculture below them — entire river systems are at risk of becoming seasonal as their glacier sources disappear.
Mountain Water Towers
Mountain glaciers serve as natural reservoirs, storing winter precipitation as ice and releasing it slowly as meltwater during the dry summer months. Hundreds of millions of people depend on this glacial meltwater for drinking water, irrigation, hydroelectric power, and various other needs. The major rivers of South Asia (the Indus, the Ganges, the Brahmaputra, the Yangtze, the Mekong, and others) all draw substantially on Himalayan and Tibetan glacier melt during the dry season. The cities and agricultural regions of the western Andes (Lima, La Paz, Quito, and many others) depend on glacier meltwater. The western US relies on Sierra Nevada and Rocky Mountain snowpack, much of which is held by alpine glaciers. The continued retreat of these glaciers will reduce the long-term water supplies available to these regions, even as short-term meltwater increases due to the rapid melting. The transition from "glacier-supplied" to "glacier-free" rivers will be one of the major water resource challenges of the coming century.
Sea Level Rise
Glacial retreat contributes substantially to sea level rise. The total contribution from mountain glaciers and ice caps is currently about 0.5-0.7 mm per year — roughly 20-30 percent of total observed sea level rise. The total contribution from Greenland is about 0.7-0.8 mm per year. From Antarctica, about 0.4-0.5 mm per year. The combined contribution from melting land ice is therefore about 1.6-2.0 mm per year, comparable to the contribution from thermal expansion of the warming oceans. The total contribution from melting glaciers and ice caps to ultimate sea level rise depends on the long-term climate trajectory but could be 30-50 cm or more over the next century, with substantially more possible if major Antarctic ice losses materialise. The implications for coastal communities, low-lying islands, and global infrastructure are substantial.
Ecological Consequences
Glacial retreat has substantial ecological consequences. The ice itself is habitat for various microorganisms and a few specialised macroorganisms (ice worms, snow algae, glacier midges). The meltwater streams below glaciers support distinctive cold-water ecosystems, including specialised insects, fish, and microbes that depend on the cold, oxygen-rich, mineral-laden meltwater. As glaciers retreat, these ecosystems contract and ultimately disappear. The deglaciated terrain is gradually colonised by plants and animals from lower elevations — a process called primary succession that can take decades to centuries. The species shifts are dramatic in some cases: high-elevation specialists may be replaced by lowland species, with substantial changes in community composition. Many of the world's endemic high-mountain species are at risk as their habitats shift upward and shrink.
Cultural and Symbolic Impacts
Beyond the ecological and resource implications, glacial retreat has cultural and symbolic implications. Glaciers are central features of many mountain cultures and identities. The disappearance of named glaciers in places like the Alps, the Andes, and the Tetons is producing a kind of grief in the affected communities. Mountain tourism is being substantially affected — Glacier National Park in Montana, named for features that may no longer exist in 30 years; the various Alpine ski resorts that depend on stable winter snowpack; the climbing approaches in the Himalaya that depended on glaciated routes that no longer exist as ice. Indigenous communities in some glaciated regions have cosmological and spiritual relationships with glaciers, and the loss of those glaciers is being recognised as a cultural and spiritual loss in addition to a physical one.
The Continued Trajectory
Glaciers are projected to continue retreating throughout the 21st century under virtually every climate scenario. Even rapid climate action would not stop the retreat — the warming already locked in by past greenhouse gas emissions will continue to drive glacial loss for decades. The pace of retreat will depend on future emissions, but the direction is clear. Most mountain glaciers in the Alps, the Rockies, and many tropical mountains are projected to be substantially or entirely gone by 2100 under typical scenarios. The Himalayan and Karakoram glaciers will likely persist longer due to their size and elevation, but most are projected to be substantially reduced. The Greenland and Antarctic ice sheets will continue losing mass, with the question being the rate and the ultimate stability of the West Antarctic Ice Sheet. The cumulative impact will be among the largest signatures of climate change on Earth — the visible, measurable, persistent loss of the cryosphere on every continent. Glacial retreat is one of the clearest, most directly observable indicators of contemporary climate change, and it will continue to be one of the major ongoing geographical changes for as long as climate change continues.