A glacier is essentially a river of ice. Glaciers form at high latitudes and altitudes where vast quantities of fallen snow accumulate over time and compress into thick masses of ice that flow downhill under their own weight. Glaciers, along with ice caps and ice sheets, cover about 10% of Earth's land mass—mostly in Greenland and Antarctica. Mountain, or alpine, glaciers outside the poles hold only a tiny fraction of this ice, but produce meltwater essential for ecosystem health and for human use in populated areas. Glaciers are sensitive indicators of climate change because their size and mass change relatively quickly in response to long-term climate variations (such as changing temperature, precipitation, and cloud cover) over the course of decades, rather than to short-term weather patterns.
Most of Earth's alpine and non-polar glaciers, which tend to be smaller and more unstable, have shrunken rapidly over the last century. This widespread glacier retreat is most likely the result of climbing global temperatures brought on by human emissions of greenhouse gases such as carbon dioxide (CO2) combined with a natural warming phase. Changes in humidity, precipitation, cloud cover, and albedo (reflectance) that accompany global warming may themselves have a greater impact on glacier retreat in some tropical mountains. If human greenhouse-gas emissions go unchecked and current warming trends continue, alpine glaciers in many places will likely disappear well before the end of the twenty-first century.
Historical Background and Scientific Foundations
Global ice cover and glacial extent have varied widely over the past 750,000 years, advancing during numerous ice ages, and retreating during intermediate warming periods called interglacials. During the last Ice Age, which ended about 10,000 years ago, glaciers, ice caps, and ice sheets covered 32% of Earth's land mass and 30% of its oceans.
Humans began tracking the length and extent of certain glaciers as early as 1534 in European mountain ranges, but thorough, internationally coordinated glacier monitoring did not start until 1894, when the
International Glacier Commission was established in Zurich, Switzerland. Now, long-term data exist for glaciers on every continent. These data generally show that glaciers reached their most recent maximum during the Little Ice Age, a short and relatively mild cooling period that ended sometime in the nineteenth century. General retreat of glacier tongues started in earnest after 1800, and accelerated across the globe after 1850 and through the twentieth century, with a slight lull during the 1970s before escalating rates of loss began in the 1980s and 1990s.
Records for changes in glacier mass balance are not nearly as complete, as they are generally restricted to relatively easy-access glaciers and stretch back only to the mid-twentieth century. Mass balance refers to the equilibrium between a glacier's accumulation of snow and ice through precipitation and its ablation, or loss, of snow and ice by melting and sublimation. Over time, this measure shows whether a glacier's mass is growing (positive), shrinking (negative), or staying the same (zero). Mathematical extrapolations from the data that do exist on mass balance indicate that the global rates of glacier mass loss are increasing rapidly, roughly doubling during 1990/1991–2003/ 2004 over the mass loss rates from 1960/1961–1989/ 1990.
Generally, these glacier fluctuations show a strong statistical correlation with air temperatures over the last century, though other factors, such as changing cloud cover, snowfall, humidity, and climbing sea surface temperature, play roles of varying importance in different locations.
The short, steep, and shallow glaciers in tropical mountain ranges are particularly sensitive to climate change, responding over a scale of a few years. In the tropical Andes of Peru, for example, 10 monitored glaciers retreated 1,935-6,266 ft (590-1,910 m) between 1939 and 1994. The Chacaltaya glacier in Bolivia, meanwhile, lost about 60% of its ice volume between 1940 and 1983, and will likely disappear altogether within a decade.
This trend toward glacier retreat and eventual disappearance in the tropical Andes, which has accelerated markedly since the late 1970s and early 1980s, corresponds with a 0.16-0.27°F (0.09-0.15°C) increase in temperature per decade in the area from 1950 to 1994, with most of the warming taking place after the mid-1970s. But related climate changes, especially increasing humidity and changes in precipitation and cloud cover, may play a larger direct role than temperature in the shrinkage of tropical glaciers. A documented rise in tropical sea surface temperatures, which has forced tropical freeze levels to higher altitudes, is also an important factor in tropical, and potentially worldwide, glacier retreat.
Glaciers in mid- and high-latitude regions of the Northern Hemisphere, such as the northwestern United States, southwestern Canada, and Alaska, have lost mass quickly over the last century due to accelerated summer melting, despite some increases in precipitation. Losses have also been extreme in the European Alps, where glaciers have shrunk 30–40%, and New Zealand, which has lost 20–32% of its glacierized area over the past century.
In Patagonia, massive ice fields and their glaciers have lost about 0.77–3.2 cubic mi (3.2–13.5 cubic km) of ice per year from 1968 to 2000. Patagonia's changes likely stem in part from warmer, drier conditions, but accelerated glacial flow, resulting in the accelerated loss of large chunks of ice at glacier termini through a process known as calving, also plays a role.
Despite trends toward widespread glacier retreat, there are isolated exceptions. Some glaciers in Pakistan's Karakorum have thickened and advanced due to enhanced precipitation, though glaciers in Asian mountain ranges are generally retreating. The tropical Andes also enjoyed periods of glacial advance in the early 1970s and again in 1999 corresponding to cold La Niña conditions in the tropical Pacific. New Zealand glaciers thickened and advanced from the late 1970s through the 1990s, as did some Scandinavian glaciers during the 1990s, potentially due to temporarily enhanced precipitation. But glaciers in both areas have resumed retreat since 2000.
Impacts and Issues
If human greenhouse-gas emissions continue to rise unchecked and current warming trends continue, climate models predict the disappearance of many glaciers before the end of the twenty-first century. In the tropical Andes, for example, many glaciers may vanish within 50 years at current rates of retreat. Meanwhile, Montana's Glacier National Park has lost all but 10 of its 27 original glaciers over the past century, and the extent of those remaining has also been vastly reduced. Under a doubling of CO2 emissions, the park's remaining glaciers will have completely vanished by 2030.
The glaciers and famous snows of Mount Kilimanjaro in Kenya, Africa, will also likely disappear by mid-century for the first time in over 11,000 years if a long-term dry spell, likely part of the climate change picture, persists. However, some studies in the European Alps suggest that many alpine glacier retreat scenarios have been largely overestimated.
The shrinkage and loss of alpine glaciers, which provide important water sources in much of the world, will have direct and severe implications for humans and ecosystems alike. Because they are perennial, glaciers provide an important base flow for streams and rivers beyond more unreliable and now dwindling snowpack melt, sustaining agriculture and drinking water supplies through seasonal variations in many mountain communities from the Andes to the Himalayas. The reduction of the Zongo Glacier in Bolivia, for example, is already creating freshwater supply problems for downstream human communities. The loss of glaciers will also represent a major aesthetic loss for humans who flock to places like Glacier National Park for the scenery, as well as a potential blow to recreational activities such as high-altitude mountaineering.
High-latitude and high-altitude ecosystems will also be affected. Glacier melt provides crucial soil moisture and ensures input of cold water to decrease stream temperatures in glacial basins during warmer seasons, maintaining certain plant communities and cold-adapted aquatic invertebrates and fish. The retreat of glaciers in these areas will thus likely force changes in local vegetation and stream ecology, as well as alter stream sedimentation and open up new ground for plant colonization.
WORDS TO KNOW
ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.
CALVING: Process of iceberg formation when huge chunks of ice break free from glaciers, ice shelves, or ice sheets due to stress, pressure, or the forces of waves and tides.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
ICE AGE: Period of glacial advance.
ICE CAP: Ice mass located over one of the poles of a planet not otherwise covered with ice. In our solar system, only Mars and Earth have polar ice caps. Earth's north polar ice cap has two parts, a skin of floating ice over the actual pole and the Greenland ice cap, which does not overlay the pole. Earth's south polar ice cap is the Antarctic ice sheet.
ICE SHEET: Glacial ice that covers at least 19,500 square mi (50,000 square km) of land and that flows in all directions, covering and obscuring the landscape below it.
SUBLIMATION: Transformation of a solid to the gaseous state without passing through the liquid state.
Widespread melting of glaciers will also play a part in climate change-induced sea level rise. Sea level has risen about 5-9 in (13-23 cm) worldwide over the past century, with mountain glacier melt (excluding the glaciers of Greenland's ice sheet) contributing an estimated 1 in (2.7 cm) of this rise between 1865 and 1990. Melting Alaskan glaciers are among the biggest contributors, accounting for about 30% of mountain glacier inputs to sea level rise. The Arctic, the high mountains of Asia, and Patagonian glaciers also play relatively hefty roles. However, these contributions are miniscule compared to the potential sea level rise that could result from the melting of the vast Antarctic and Greenland ice sheets.
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