More Information
A website with PTAAGMB results reported from 9 different glaciers, 5 of which are compared with available manual measurements, can be seen at www.ptaagmb.com.
PTAAGMB (Precipitation-Temperature-Area-Altitude Glacier Mass Balance)
The PTAAGMB Model is used for calculating a glacier's mass balance, the primary indicator of its health, and plots the changes to its mass balance over time to predict its future.
Developed in the mid-1990s by glaciologist Wendell Tangborn, the PTAAGMB model provides an easy and reliable alternative to the challenging task of manually measuring glaciers using snow pits and ablation stakes.
The PTAAGMB model only requires data from the precipitation and temperature (PT) observations from nearby low-altitude weather stations and the glacier's area-altitude (AA) distribution.
Glaciers are ultra-sensitive to minute changes in the climate and respond by changing their size and by advancing or retreating. The mass balance, or the difference between snow accumulation and snow and ice ablation, is crucial to glacier health and its survival. The Columbia Glacier in Alaska is a large tidewater glacier that began a drastic retreat in the 1970s due to climate fluctuations and began discharging large quantities of icebergs into Prince William Sound. These icebergs were responsible for a massive oil spill in 1989 when an oil tanker captain tried to avoid them and went aground. [1]
The key to the PTAAGMB model is the glacier’s area-altitude distribution, which is simply the glacier’s surface area as a function of elevation. The AA profile is a unique feature of a glacier that has been shaped by thousands of years of erosion of the bedrock underlying the glacier. Thus, the area altitude distribution has embedded within it the past climate history that has formed the glacier.
The PTAAGMB model uses daily values of such balance variables as snowline altitude, zero balance altitude, glacier balance, balance flux and the accumulation area ratio are correlated throughout the ablation season using two-degree polynomial regressions to obtain the lowest fitting error. When the minimum average error (or maximum R2) is attained, the generated balances and other variables are considered to be real. A simplex optimization technique is used to determine the optimal coefficient values that are used in algorithms to convert meteorological observations to snow accumulation and snow and ice ablation. [2]
The PTAAGMB model has been used successfully on a number of glaciers in various parts of the world: in the United States, the Alaskan glaciers Bering, Gulkana, Lemon Creek, Mendenhall, Wolverine and Wrangell Range; in Washington State, on the South Cascade Glacier; in Europe, the Austrian glaciers Hintereisferner, Kesselwanferner and Vernagt Ferner.
The mass balance and runoff of Langtang Glacier in Nepal was determined with the PTAAGMB model using daily meteorological observations observed at Kathmandu. This is the only Himalayan glacier for which mass balance and runoff have been calculated. [3]
Another feature of the PTAAGMB model is the capability to estimate glacier thickness from ice flow velocity and mass balance measurements. [4] The average thickness of South Cascade Glacier was found to be 83 meters in 1965, based on flow velocity and balance measurements. Borehole depth measurements of the glacier made later approximately agree with this estimate.
The mass balance, runoff and surges of Bering Glacier were calculated with the PTAAGMB model using weather observations at Cordova and Yakutat, Alaska. Ice volume loss measured with the PTAAGMB model agrees within 0.8% of the loss measured with the geodetic method. Runoff from Bering Glacier (derived from simulated ablation and rain) correlates with four of the glacier surges that have occurred since 1951. [5]
Comparison of glacier mass balance by glaciological, hydrological and mapping methods revealed that glaciers internally store a significant amount of liquid water. [6] Stored water in glaciers is now considered the key to understanding the disintegration of Antarctic and Greenland Ice Caps.
A website with PTAAGMB results reported from 9 different glaciers, 5 of which are compared with available manual measurements, can be seen at www.ptaagmb.com.
A glacier is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. Glaciers slowly deform and flow under stresses induced by their weight, creating crevasses, seracs, and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such as cirques and moraines. Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.
Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away.
The cryosphere is an all-encompassing term for those portions of Earth's surface where water is in solid form, including sea ice, lake ice, river ice, snow cover, glaciers, ice caps, ice sheets, and frozen ground. Thus, there is a wide overlap with the hydrosphere. The cryosphere is an integral part of the global climate system with important linkages and feedbacks generated through its influence on surface energy and moisture fluxes, clouds, precipitation, hydrology, atmospheric and oceanic circulation. Through these feedback processes, the cryosphere plays a significant role in the global climate and in climate model response to global changes. The term deglaciation describes the retreat of cryospheric features. Cryology is the study of cryospheres.
Glaciology is the scientific study of glaciers, or more generally ice and natural phenomena that involve ice.
The Ross Ice Shelf is the largest ice shelf of Antarctica. It is several hundred metres thick. The nearly vertical ice front to the open sea is more than 600 kilometres (370 mi) long, and between 15 and 50 metres high above the water surface. Ninety percent of the floating ice, however, is below the water surface.
An ice sheet, also known as a continental glacier, is a mass of glacial ice that covers surrounding terrain and is greater than 50,000 km2 (19,000 sq mi). The only current ice sheets are in Antarctica and Greenland; during the last glacial period at Last Glacial Maximum (LGM) the Laurentide Ice Sheet covered much of North America, the Weichselian ice sheet covered northern Europe and the Patagonian Ice Sheet covered southern South America.
Ablation is removal or destruction of material from an object by vaporization, chipping, or other erosive processes. Examples of ablative materials are described below, and include spacecraft material for ascent and atmospheric reentry, ice and snow in glaciology, biological tissues in medicine and passive fire protection materials.
The climatic snow line is the boundary between a snow-covered and snow-free surface. The actual snow line may adjust seasonally, and be either significantly higher in elevation, or lower. The permanent snow line is the level above which snow will lie all year.
The Greenland ice sheet is a vast body of ice covering 1,710,000 square kilometres (660,000 sq mi), roughly 79% of the surface of Greenland.
Crucial to the survival of a glacier is its mass balance or surface mass balance (SMB), the difference between accumulation and ablation. Climate change may cause variations in both temperature and snowfall, causing changes in the surface mass balance. Changes in mass balance control a glacier's long-term behavior and are the most sensitive climate indicators on a glacier. From 1980–2012 the mean cumulative mass loss of glaciers reporting mass balance to the World Glacier Monitoring Service is −16 m. This includes 23 consecutive years of negative mass balances.
Glacier ice accumulation occurs through accumulation of snow and other frozen precipitation, as well as through other means including rime ice, avalanching from hanging glaciers on cliffs and mountainsides above, and re-freezing of glacier meltwater as superimposed ice. Accumulation is one element in the glacier mass balance formula, with ablation counteracting. With successive years in which accumulation exceeds ablation, then a glacier will experience positive mass balance, and its terminus will advance.
The retreat of glaciers since 1850 affects the availability of fresh water for irrigation and domestic use, mountain recreation, animals and plants that depend on glacier-melt, and, in the longer term, the level of the oceans. Studied by glaciologists, the temporal coincidence of glacier retreat with the measured increase of atmospheric greenhouse gases is often cited as an evidentiary underpinning of global warming. Mid-latitude mountain ranges such as the Himalayas, Rockies, Alps, Cascades, and the southern Andes, as well as isolated tropical summits such as Mount Kilimanjaro in Africa, are showing some of the largest proportionate glacial losses.
Ablation zone or ablation area refers to the low-altitude area of a glacier or ice sheet below firn with a net loss in ice mass due to melting, sublimation, evaporation, ice calving, aeolian processes like blowing snow, avalanche, and any other ablation. The equilibrium line altitude (ELA) or snow line separates the ablation zone from the higher-altitude accumulation zone. The ablation zone often contains meltwater features such as supraglacial lakes, englacial streams, and subglacial lakes. Sediments dropped in the ablation zone forming small mounds or hillocks are called kames. Kame and kettle hole topography is useful in identifying an ablation zone of a glacier. The seasonally melting glacier deposits much sediment at its fringes in the ablation area. Ablation constitutes a key part of the glacier mass balance.
The Whitney Glacier is a glacier situated on Mount Shasta, in the U.S. state of California. The Whitney Glacier is the longest glacier and the only valley glacier in California. In area and volume, it ranks second in the state behind the nearby Hotlum Glacier. In 1986, the glacier was measured to be 126 ft (38 m) deep and over three km in length. The glacier starts on Mount Shasta's Misery Hill at 13,700 ft (4,200 m) and flows northwestward down to the saddle between Mount Shasta and Shastina, where uneven ground causes a major icefall at 11,800 ft (3,600 m). It then flows down the valley between the two peaks, reaching its terminus at 9,500 to 9,800 ft.
The tidewater glacier cycle is the typically centuries-long behavior of tidewater glaciers that consists of recurring periods of advance alternating with rapid retreat and punctuated by periods of stability. During portions of its cycle, a tidewater glacier is relatively insensitive to climate change.
The International Association of Cryospheric Sciences, or IACS, is the eighth association of the International Union of Geodesy and Geophysics (IUGG). It was launched by the IUGG Council on 2007-07-04, developing from the International Commission of Snow and Ice of the International Association of Hydrological Sciences (IAHS) via the transitional Union Commission for the Cryospheric Sciences (UCCS).
Mark Dyurgerov was an internationally known glaciologist and Fellow of the Institute of Arctic and Alpine Research (INSTAAR) of the University of Colorado at Boulder. He was born in Moscow, Russia; both of his parents were engineers, and his mother was also a Russian poet.
Gulkana Glacier is a glacier that flows from the ice fields of the south flank of the eastern Alaska Range. It is accessible by gravel roads from the Richardson Highway near mile post 197 at the Richardson Monument, just two miles north of Summit Lake and 12 miles north of Paxson and the junction with Denali Highway. Closer to the glacier, a suspension bridge allows pedestrians to cross over Phelan Creek. Wildlife includes moose and bears. The peak of a hill just southwest of Gulkana Glacier has a post labeled PEWE 1975. The Arctic Man competition takes place near Gulkana Glacier every spring.
Wendell Tangborn is an American glaciologist specializing in glacier mass balance measurements since 1960, chiefly within the states of Washington and Alaska of the Pacific Northwest. His streamflow forecasting model implements a split-sample calibration approach which greatly improves its accuracy, and eliminates the need for manual measurements such as snow surveys. This model has been utilized by hydroelectric utilities for over thirty years to forecast Columbia River runoff. His PTAA glacier balance model requires only routine weather observations and has been successfully applied to calculate the mass balance of glaciers throughout the world, including the Himalayas.
Guðfinna Aðalgeirsdóttir is professor in Geophysics at the Faculty of Earth Sciences, University of Iceland.