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:''See also ]'' :''See also ]''
] and ]. The downward trend in the late 1980s is symptomatic of the increased rate and number of retreating glaciers.]] ] and ]. The downward trend in the late 1980s is symptomatic of the increased rate and number of retreating glaciers.]]
] ]


Crucial to the survival of a ] is its '''mass balance''', the difference between ] and ablation (melting and sublimation). ] may cause variations in both temperature and snowfall, causing changes in mass balance. Changes in mass balance control a glacier's long term behavior. It is the most sensitive climate indicator on a glacier. Crucial to the survival of a ] is its '''mass balance''', the difference between ] and ] (melting and sublimation). ] may cause variations in both temperature and snowfall, causing changes in mass balance. Changes in mass balance control a glacier's long term behavior and is the most sensitive climate indicator on a glacier.


A glacier with a sustained negative balance is out of equilibrium and will retreat. A glacier with a sustained positive balance is out of equilibrium and will advance. Glacier retreat results in the loss of the low elevation region of the glacier. Since higher elevations are cooler than lower ones, the disappearance of the lowest portion of the glacier reduces overall ablation, thereby increasing mass balance and potentially reestablishing equilibrium. However, if the mass balance of a significant portion of the accumulation zone of the glacier is negative, it is in disequilibrium with the local climate. Such a glacier will melt away with a continuation of this local climate. A glacier with a sustained negative balance is out of equilibrium and will retreat, while one with a sustained positive balance is out of equilibrium and will advance. Glacier retreat results in the loss of the low elevation region of the glacier. Since higher elevations are cooler than lower ones, the disappearance of the lowest portion of the glacier reduces overall ablation, thereby increasing mass balance and potentially reestablishing equilibrium. However, if the mass balance of a significant portion of the accumulation zone of the glacier is negative, it is in disequilibrium with the local climate. Such a glacier will melt away with a continuation of this local climate.<ref>{{cite web| author=Dyurgerov, M. (M. Meier and R. Armstrong, eds.)| title=Glacier mass balance and regime measurements and analysis, 1945&ndash;2003| work=Institute of Arctic and Alpine Research, University of Colorado. Distributed by National Snow and Ice Data Center, Boulder, CO.| url=http://nsidc.org/data/g10002.html| accessyear=2002 (updated 2005)}}</ref>


The key symptom of a glacier in disequilibrium is thinning along the entire length of the glacier.{{ref_harv|nichols|Pelto|*}}{{ref_harv|nichols2|Pelto and Harzell|*}} For example, ] (pictured below) will likely shrink to half its size, but at a slowing rate of reduction, and stabilize at that size, despite the warmer temperature, over a few decades. However, the Grinnell Glacier (pictured below) will shrink at an increasing rate until it disappears. The difference is that the upper section of Easton Glacier remains healthy and snow-covered, while even the upper section of the Grinnell Glacier is bare, melting and has thinned. Small glaciers with shallow slopes are most likely to fall into disequilibrium with the climate. The key symptom of a glacier in disequilibrium is thinning along the entire length of the glacier.<ref>{{cite web | author=Mauri S. Pelto (Nichols College) | title=The Disequilibrium of North Cascade, Washington Glaciers 1984&ndash;2004 | work=In "Hydrologic Processes" | url=http://www.nichols.edu/departments/glacier/diseqilibrium.html| accessmonthday=February 14 | accessyear=2006}}</ref><ref>{{cite journal | author=Pelto, M.S. and Hartzell, P.L. | title=Change in longitudinal profile on three North Cascades glaciers during the last 100 years | journal=Hydrologic Processes | year=2004 | volume=18 | issue= | pages=1139&ndash;1146 | url=http://www.nichols.edu/departments/glacier/Longitudinal%20Profile.pdf }}</ref> For example, Easton Glacier (pictured below) will likely shrink to half its size, but at a slowing rate of reduction, and stabilize at that size, despite the warmer temperature, over a few decades. However, the ] (pictured below) will shrink at an increasing rate until it disappears. The difference is that the upper section of Easton Glacier remains healthy and snow-covered, while even the upper section of the Grinnell Glacier is bare, melting and has thinned. Small glaciers with shallow slopes such as Grinnell are most likely to fall into disequilibrium if there is a change in the local climate.


In the case of positive mass balance the glacier will continue to advance expanding its low elevation area, resulting in more melting. If this still does not create an equilibrium balance the glacier will continue to advance. The glacier may advance into the ocean at which point iceberg calving losses may bring about equilibrium. In the case of positive mass balance, the glacier will continue to advance expanding its low elevation area, resulting in more melting. If this still does not create an equilibrium balance the glacier will continue to advance. If a glacier is near a large body of water, especially an ocean, the glacier may advance until iceberg calving losses bring about equilibrium.


==Measurement methods== ==Measurement methods==
] ]
] showing recession since 1850 of 1.1&nbsp;km ]]] ] in ] showing recession since 1850 of 1.1&nbsp;km ]]]


===Mass balance=== ===Mass balance===
Mass balance is measured by determining the amount of snow accumulated during winter, and later measuring the amount of snow and ice removed by melting in the summer. The difference between these two parameters is the mass balance. If the amount of snow accumulated during the winter is larger than the amount of melted snow and ice during the summer, the mass balance is positive and the glacier has increased in volume. On the other hand, if the melting of snow and ice during the summer is larger than the supply of snow in the winter, the mass balance is negative and the glacier volume decreases. Mass balance is reported in meters of water equivalent. This represents the average thickness gained (positive balance) or lost (negative balance) from the glacier during that particular year. Mass balance is measured by determining the amount of snow accumulated during winter, and later measuring the amount of snow and ice removed by melting in the summer. The difference between these two parameters is the mass balance. If the amount of snow accumulated during the winter is larger than the amount of melted snow and ice during the summer, the mass balance is positive and the glacier has increased in volume. On the other hand, if the melting of snow and ice during the summer is larger than the supply of snow in the winter, the mass balance is negative and the glacier volume decreases. Mass balance is reported in meters of water equivalent. This represents the average thickness gained (positive balance) or lost (negative balance) from the glacier during that particular year.


To determine mass balance in the accumulation zone, snowpack depth is measured using probing, snowpits or ] stratigraphy. Crevasse stratigraphy makes use of annual layers revealed on the wall of a crevasse. Akin to tree rings, these layers are due to summer dust deposition and other seasonal effects. The advantage of crevasse ] is that is provides a two-dimensional measurement of the snowpack layer, not a point measurement. It is also usable in depths where probing or snowpits are not feasible. In temperate glaciers, the insertion resistance of a probe increases abruptly when its tip reaches ice that was formed the previous year. The probe depth is a measure of the net accumulation above that layer. Snowpits dug through the past winters residual snowpack are used to determine the snowpack depth. Snowpack density is another way used to determine mass balance. In either situation the observed depth is multiplied by the snowpack density to determine the accumulation in water equivalent. It is necessary to measure the density in the spring as snowpack density varies. Measurement of snowpack density completed at the end of the ablation season yield consistent values for a particular area and need not be measured every year. In the ablation zone, ablation measurements are made using stakes inserted vertically into the glacier either at the end of the previous melt season or the beginning of the current one. The length of stake exposed by melting ice is measured at the end of the melt (ablation) season. Most stakes must be replaced each year or even mid-way through the summer.
*To determine mass balance in the accumulation zone, snowpack depth is measured using probing, snowpits or ] stratigraphy.
**Probing: In temperate glaciers, a probe the insertion resistance of a probe increases abruptly when its tip reaches ice that was formed the previous year. The probe depth is a measure of the net accumulation above that layer.
**Snowpits dug through the past winters residual snowpack are used to determine the snowpack depth.
**Crevasse stratigraphy makes use of annual layers revealed on the wall of a crevasse. Akin to tree rings, these layers are due to summer dust deposition and other seasonal effects. The advantage of crevasse ] is that is provides a two-dimensional measurement of the snowpack layer, not a point measurement. It is also usable in depths where probing or snowpits are not feasible.
**Snowpack Density. In either situation the observed depth is multiplied by the snowpack density to determine the accumulation in water equivalent. It is necessary to measure the density in the spring as snowpack density varies. Measurement of snowpack density completed at the end of the ablation season yield consistent values for a particular area and need not be measured every year.


] ]
] ]
**In the ablation zone ablation measurements are made using stakes inserted vertically into the glacier either at the end of the previous melt season or the beginning of the current one. The length of stake exposed by melting ice is measured at the end of the melt (ablation) season. Most stakes must be replaced each year or even mid-way through the summer.


===Net balance=== ===Net balance===
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===Annual balance=== ===Annual balance===
Is the mass balance measured between specific dates. The mass balance is measured on the fixed date each year, again sometime near the start of October.<ref>{{cite web | url=http://www.nichols.edu/departments/glacier/mb.htm | title=Glacier Mass Balance | work=North Cascade Glacier Climate Project | author=Mauri S. Pelto, Director NCGCP | date=March 28 | year=2006}}</ref> Annual balance is the mass balance measured between specific dates. The mass balance is measured on the fixed date each year, again sometime near the start of October in the mid northern latitudes.<ref>{{cite web | url=http://www.nichols.edu/departments/glacier/mb.htm | title=Glacier Mass Balance | work=North Cascade Glacier Climate Project | author=Mauri S. Pelto, Director NCGCP | date=March 28 | year=2006}}</ref>


===Geodetic methods=== ===Geodetic methods===
Geodetic methods are an indirect method for the determination of mass balance of glacier. Maps of a glacier made at two different points in time can be compared and the difference in glacier thickness observed used to determine the mass balance over a span of years. This is best accomplished today using Differential ]. Sometimes the earliest data for the glacier surface profiles is from images that are used to make ]s and ]s. Aerial mapping or ] is now used to cover larger glaciers and icecaps such found in ] and ], however, because of the problems of establishing accurate ground control points in mountainous terrain, and correlating features in snow and where shading is common, elevation errors are typically not less than 10&nbsp;m (32&nbsp;ft).<ref>{{cite journal | author= David Rippin, Ian Willis, Neil Arnold, Andrew Hodson, John Moore, Jack Kohler and Helgi Bjornsson| title= Changes in Geometry and Subglacial Drainage of Midre Lovénbreen, Svalbard, Determined from Digital Elevation Models | journal= Earth Surface Processes and Landforms| year= 2003| volume= 28| issue=| pages= 273&ndash;298| url= http://www.ulapland.fi/home/hkunta/jmoore/pdfs/Rippin_et_al_2003.pdf}}</ref> Laser altimetry provides a measurement of the elevation of a glacier along a specific path, e.g., the glacier centerline. The difference of two such measurements is the change in thickness, which provides mass balance over the time interval between the measurements. Again a good method over a span of time but not for annual change detection. The value of geodetic programs is providing an independent check of traditional mass balance work, by comparing the cumulative changes over ten or more years.<ref>{{cite web | url=http://www.vaw.ethz.ch/research/glaciology/glacier_mechanics/gz_mass_balance_photogrammetric | title=Mass Balance Determination using Photogrammetric Methods and Numerical Flow Modeling | work=Laboratory of Hydrolic, Hydrology and Glaciology | author=Andreas Bauder, G. Hilmar Gudmundsson | date=March 28 | year=2006}}</ref>
Geodetic methods are an indirect method for the determination of mass balance of glacier. Maps of a glacier made at two different points in time can be compared and the difference in glacier thickness observed used to determine the mass balance over a span of years. This is best accomplished today using Differential ].
*Aerial mapping or ]. Sometimes the earliest data for the glacier surface profiles is from images that are used to make ]s and ]s. Because of the problems of establishing accurate ground control points in mountainous terrain, and correlating features in snow and where shading is common, elevation errors are typically not less than 10&nbsp;m (32&nbsp;ft).{{ref_harv|map|Ribbin, et alia|*}}
*Laser altimetry. This provides a measurement of the elevation of a glacier along a specific path, e.g., the glacier centerline. The difference of two such measurements is the change in thickness, which provides mass balance over the time interval between the measurements. Again a good method over a span of time but not for annual change detection.
The value of geodetic programs is providing an independent check of traditional mass balance work, by comparing the cumulative changes over ten or more years.<ref>{{cite web | url=http://www.vaw.ethz.ch/research/glaciology/glacier_mechanics/gz_mass_balance_photogrammetric | title=Mass Balance Determination using Photogrammetric Methods and Numerical Flow Modeling | work=Laboratory of Hydrolic, Hydrology and Glaciology | author=Andreas Bauder, G. Hilmar Gudmundsson | date=March 28 | year=2006}}</ref>


==Mass balance research worldwide== ==Mass balance research worldwide==
Mass balance studies have been carried out in various countries worldwide, but have mostly conducted in the ] due to there being more mid-latitude glaciers in that hemisphere.


===Alaska===
⚫ ===Norway mass balance program===
The ] near ] has been studied by the Juneau Icefield Research Program since 1946, and is the longest continuous mass balance study of any glacier in ]. Taku is the world's thickest known temperate alpine glacier, and experienced positive mass balance between the years 1946 and 1988, resulting in a huge advance. The glacier has since been in a negative mass balance state, which will may result in a retreat if the current trends continue.<ref>{{cite web| last =Pelto| first =Mauri| authorlink =| coauthors =Matt Beedle, Maynard M. Miller| title =Mass Balance Measurements of the Taku Glacier, Juneau Icefield, Alaska 1946-2005| work =Juneau Icefield Research Program| url =http://www.nichols.edu/departments/glacier/taku.html| accessdate = 2007-01-09 }}</ref>
⚫ Norway maintains the most extensive mass balance program, largely funded by the hydropower industry. Mass balance measurements are currently performed on twelve glaciers in Norway. In southern Norway six of the glaciers have been measured for 42 consecutive years or more, and they constitute a west-east profile reaching from the very maritime Ålfotbreen glacier, close to the western coast, to the very continental Gråsubreen, in the eastern part of Jotunheimen. Storbreen in Jotunheimen has the longest series of all glaciers in Norway with 55 years of measurements, while Engabreen has the longest series (35 years) in northern Norway. The Norwegian program is where the traditional methods of mass balance measurement were largely derived.<ref>{{cite web | url=http://www.nve.no/modules/module_109/publisher_view_product.asp?iEntityId=1640 | title=Mass balance measurements | work= Glaciological investigations in Norway | author=Norwegian Water Resources and Energy Directorate | date=March 28 | year=2006}}</ref>


===Swiss mass balance program=== ===Austrian Glacier Mass Balance===
⚫ The mass balance of Hintereisferner and Kesselwandferner glaciers in ] have been continuously monitored since 1952 and 1965 respectively. Having been continuously measured for 55 years, Hintereisferner has one of the longest periods of continuous study of any glacier in the world, based on measured data and a consistent method of evaluation. Currently this measurement network comprises about 10 snow pits and about 50 ablation stakes distributed across the glacier. In terms of the cumulative specific balances, Hintereisferner experienced a net loss of mass between 1952 and 1964, followed by a period of recovery till 1968. Hintereisferner reached an intermittent minimum in 1976, briefly recovered in 1977 and 1978 and has continuously lost mass in the 30 years since then.<ref>{{cite web| title =Mass balance of Hintereisferner | work =| publisher =Institute for Meteorology and Geophysics, University of Innsbruck, Austria| date =January 20, 2004| url =http://meteo9.uibk.ac.at/IceClim/HEF/mass.html| accessdate = 2007-01-09 }}</ref>
⚫ Temporal changes in the spatial distribution of the mass balance result primarily from changes in accumulation and melt along the surface. As a consequence, variations in mass of glaciers reflect changes in climate and the energy fluxes at the earth's surface. For the two alpine glaciers Gries in the central Alps and Silvretta at the north slope in the eastern Swiss Alps long time series of mass balance measurements have been measured. The distribution of seasonal accumulation and ablation rates are measured in-situ. Traditional field methods are combined with remote sensing techniques to track changes in mass, geometry and the flow behaviour of the two glaciers. These investigations contribute to the Swiss Glacier Monitoring Network and the International network of the World Glacier Monitoring Service (WGMS).

===New Zealand===
Glacier mass balance studies have been ongoing in ] glaciers since 1957. ] has been studied since then by the New Zealand Geological Survey and later by the Ministry of Works, measuring the ice stratigraphy and overall movement. However, even earlier fluctuation patterns were documented on ] and ]s in 1950. Other glaciers on the ] studied include ] since 1968, while on the ], glacier retreat and mass balance research has been conducted on the glaciers on ] since 1955. On Mount Ruapehu, permanent photographic stations allow repeat photography to be used to provide photographic evidence of changes to the glaciers on the mountain over time.<ref>{{cite web| title =Glaciers of New Zealand | work =Satellite Image Atlas of Glaciers of the World| publisher =U.S. Geological Survey| date =| url =http://pubs.usgs.gov/pp/p1386h/nzealand/nzealand.html| accessdate = 2007-01-16 }}</ref>


===North Cascade glacier mass balance program=== ===North Cascade glacier mass balance program===
The North Cascade Glacier Climate Project measures the annual balance of 9 glaciers, more than any other program in North America. These records extend from 1984&ndash;2005 and represent the only set of records documenting the mass balance changes of an entire glacier clad range. To monitor an entire glaciated mountain range in North America, which was listed as a high priority of the National Academy of Sciences in 1983. North Cascade glaciers annual balance has averaged −0.52 m/a from 1984&ndash;2005, a cumulative loss of over 12.5&nbsp;m or 20&ndash;40% of their total volume since 1984 due to negative mass balances. The trend in mass balance is becoming more negative which is fueling more glacier retreat and thinning. The North Cascade Glacier Climate Project measures the annual balance of 9 glaciers, more than any other program in North America. These records extend from 1984&ndash;2005 and represent the only set of records documenting the mass balance changes of an entire glacier clad range. To monitor an entire glaciated mountain range in North America, which was listed as a high priority of the National Academy of Sciences in 1983. North Cascade glaciers annual balance has averaged −0.52 m/a from 1984&ndash;2005, a cumulative loss of over 12.5&nbsp;m or 20&ndash;40% of their total volume since 1984 due to negative mass balances. The trend in mass balance is becoming more negative which is fueling more glacier retreat and thinning.<ref>{{cite web| last =Pelto| first =Mauri| title =Glacier Mass Balance| work =United States Mass Balance Surveys| publisher =| date = November 9, 2006| url =http://www.nichols.edu/departments/glacier/mb.htm| accessdate = 2007-01-09 }}</ref>


===United States Geological Survey (USGS)=== ===Norway mass balance program===
⚫ ] maintains the most extensive mass balance program in the world and is largely funded by the hydropower industry. Mass balance measurements are currently performed on twelve glaciers in Norway. In southern Norway six of the glaciers have been measured for 42 consecutive years or more, and they constitute a west-east profile reaching from the very maritime Ålfotbreen Glacier, close to the western coast, to the very continental Gråsubreen Glacier, in the eastern part of ]. Storbreen Glacier in Jotunheimen has been measured for a longer period of time than any other glacier in Norway, a total of over 55 years, while Engabreen Glacier has the longest series (35 years) in northern Norway. The Norwegian program is where the traditional methods of mass balance measurement were largely derived.<ref>{{cite web | url=http://www.nve.no/modules/module_109/publisher_view_product.asp?iEntityId=1640 | title=Mass balance measurements | work= Glaciological investigations in Norway | author=Norwegian Water Resources and Energy Directorate | date=March 28 | year=2006}}</ref>
⚫ The USGS operates a long-term "benchmark" glacier program to monitor climate, glacier mass balance, glacier motion, and stream runoff. This program has been ongoing since 1965. Three benchmark glaciers exist, Gulkana Glacier in the Alaska Range monitoring begun in 1965, Wolverine Glacier in the Coast Range of Alaska, monitoring began in 1965 and South Cascade Glacier in Washington State where monitoring began in the ] of 1957. This program monitors one glacier in each of these mountain ranges collecting detailed data to understand glacier hydrology and glacier climate interactions.

===Austrian Glacier Mass Balance===
⚫ The mass balance of Hintereisferner and Kesselwandferner have been continuously monitored since 1952 and 1965 respectively. The Hintereisferner is one of the world’s longest directly measured series. The record is based on directly measured data and a consistent method of evaluation. Currently this measurement network comprises about 10 snow pits and about 50 ablation stakes distributed across the glacier. In terms of the cumulative specific balances, the glacier experienced a net loss of mass during the period from 1952 to 1964, followed by a period of recovery till 1968. Hintereisferner reached an intermittent minimum in 1976, briefly recovered in 1977 and 1978 and continuously lost mass since then.


===Sweden Storglaciären=== ===Sweden Storglaciären===
The Tarfala Research Station in the Kebnekaise region of northern Sweden is operated by Stockholm University. It was here that the first mass balance programme was initiated immediately after World War Two, and continues to the present day. This survey was the initiation of the mass balance record of Storglaciären which is the longest in the world. This glacier was the first location for the development of traditional mass balance methods. The Tarfala Research Station in the ] region of northern ] is operated by ]. It was here that the first mass balance program was initiated immediately after ], and continues to the present day. This survey was the initiation of the mass balance record of Storglaciären Glacier, and constitutes the longest continuous study of this type in the world.<ref>{{cite web| title =Storglaciären| work =| publisher =Stockholm University| date =February 9, 2003| url =http://www.glaciology.su.se/ours/sg.html| doi =| accessdate = 2007-01-09 }}</ref>

⚫ ===Swiss mass balance program===
⚫ Temporal changes in the spatial distribution of the mass balance result primarily from changes in accumulation and melt along the surface. As a consequence, variations in the mass of glaciers reflect changes in climate and the energy fluxes at the earth's surface. The ] glaciers Gries in the central ] and Silvretta in the eastern Alps, have been measured for many years. The distribution of seasonal accumulation and ablation rates are measured in-situ. Traditional field methods are combined with remote sensing techniques to track changes in mass, geometry and the flow behaviour of the two glaciers. These investigations contribute to the Swiss Glacier Monitoring Network and the International network of the ] (WGMS).<ref>{{cite web| last =Bauder| first =Andreas| coauthors =Martin Funk | title =Mass Balance Studies on Griesgletscher and Silvrettagletscher| work =The Swiss Glaciers| publisher =Laboratory of Hydraulics, Hydrology and Glaciology, Swiss Federal Institute of Technology| date = March 20, 2006| url =http://www.vaw.ethz.ch/research/glaciology/glacier_change/gz_mass_balance_gries_silvrettaglacier | accessdate = 2007-01-09}}</ref>

===United States Geological Survey (USGS)===
⚫ The USGS operates a long-term "benchmark" glacier monitoring program which is used to examine climate change, glacier mass balance, ], and stream runoff. This program has been ongoing since 1965 and has been examining three glaciers in particular. Gulkana Glacier in the ] and Wolverine Glacier in the ] of ] have both been monitored since 1965, while the South Cascade Glacier in ] State has been continuously monitored since the ] of 1957. This program monitors one glacier in each of these mountain ranges, collecting detailed data to understand glacier hydrology and glacier climate interactions.<ref>{{cite web| title =Benchmark Glaciers| work =Water Resources of Alaska-Glacier and Snow Program| publisher =United States Geological Survey| date =July 9, 2004| url =http://ak.water.usgs.gov/glaciology/| accessdate = 2007-01-09}}</ref>


==Cited references== ==Cited references==
{{Commons|Glacier|Glaciers}} {{Commons|Glacier|Glaciers}}
==References==
{{note_label|Dyurgerov|Dyurgerov|*}}{{cite web
<div class="references-small">
| author=Dyurgerov, M. (M. Meier and R. Armstrong, eds.)
⚫ <references/>
| title=Glacier mass balance and regime measurements and analysis, 1945&ndash;2003
</div>
| work=Institute of Arctic and Alpine Research, University of Colorado. Distributed by National Snow and Ice Data Center, Boulder, CO.
| url=http://nsidc.org/data/g10002.html
| accessyear=2002 (updated 2005)}}


⚫ ]
{{note_label|nichols|Pelto|*}}{{cite web
| author=Mauri S. Pelto (Nichols College)
| title=The Disequilibrium of North Cascade, Washington Glaciers 1984&ndash;2004
| work=In "Hydrologic Processes"
| url=http://www.nichols.edu/departments/glacier/diseqilibrium.html
| accessdate=February 14 | accessyear=2006}}


]
{{note_label|nichols2|Pelto and Harzell|*}}{{cite journal
| author=Pelto, M.S. and Hartzell, P.L.
| title=Change in longitudinal profile on three North Cascades glaciers during the last 100 years
| journal=Hydrologic Processes
| year=2004
| volume=18
| issue=
| pages=1139&ndash;1146
| url=http://www.nichols.edu/departments/glacier/Longitudinal%20Profile.pdf }}

{{note_label|gps|Pelto, Beedle and Miller|*}}{{cite web
| author= Mauri S. Pelto, Matt Beedle and Maynard M. Miller
| title= Mass Balance Measurements on the Taku Glacier, Juneau Icefield, Alaska 1946&ndash;2005
| work=Juneau Icefield Research Program
| url=http://www.nichols.edu/departments/glacier/taku.html
| accessdate=February 14 | accessyear=2006}}

{{note_label|map|Ribbin, et al.|*}}{{cite journal
| author= David Rippin, Ian Willis, Neil Arnold, Andrew Hodson, John Moore, Jack Kohler and Helgi Bjornsson
| title= Changes in Geometry and Subglacial Drainage of Midre Lovénbreen, Svalbard, Determined from Digital Elevation Models
| journal= Earth Surface Processes and Landforms
| year= 2003
| volume= 28
| issue=
| pages= 273&ndash;298
| url= http://www.ulapland.fi/home/hkunta/jmoore/pdfs/Rippin_et_al_2003.pdf}}

=== Additional ===
⚫ <references/>

⚫ ]

Revision as of 05:46, 14 May 2007

See also Retreat of glaciers since 1850
Global glacial mass balance in the last fifty years, reported to the WGMS and NSIDC. The downward trend in the late 1980s is symptomatic of the increased rate and number of retreating glaciers.
Map of mountain glacier mass balance changes since 1970. Thinning in yellow and red, thickening in blue. The 1970s were a decade of more positive mass balance than the 1980-2004 period as seen above.

Crucial to the survival of a glacier is its mass balance, the difference between accumulation and ablation (melting and sublimation). Climate change may cause variations in both temperature and snowfall, causing changes in mass balance. Changes in mass balance control a glacier's long term behavior and is the most sensitive climate indicator on a glacier.

A glacier with a sustained negative balance is out of equilibrium and will retreat, while one with a sustained positive balance is out of equilibrium and will advance. Glacier retreat results in the loss of the low elevation region of the glacier. Since higher elevations are cooler than lower ones, the disappearance of the lowest portion of the glacier reduces overall ablation, thereby increasing mass balance and potentially reestablishing equilibrium. However, if the mass balance of a significant portion of the accumulation zone of the glacier is negative, it is in disequilibrium with the local climate. Such a glacier will melt away with a continuation of this local climate.

The key symptom of a glacier in disequilibrium is thinning along the entire length of the glacier. For example, Easton Glacier (pictured below) will likely shrink to half its size, but at a slowing rate of reduction, and stabilize at that size, despite the warmer temperature, over a few decades. However, the Grinnell Glacier (pictured below) will shrink at an increasing rate until it disappears. The difference is that the upper section of Easton Glacier remains healthy and snow-covered, while even the upper section of the Grinnell Glacier is bare, melting and has thinned. Small glaciers with shallow slopes such as Grinnell are most likely to fall into disequilibrium if there is a change in the local climate.

In the case of positive mass balance, the glacier will continue to advance expanding its low elevation area, resulting in more melting. If this still does not create an equilibrium balance the glacier will continue to advance. If a glacier is near a large body of water, especially an ocean, the glacier may advance until iceberg calving losses bring about equilibrium.

Measurement methods

The Easton Glacier which retreated 255 m from 1990 to 2005 is expected to achieve equilibrium.
Grinnell Glacier in Glacier National Park (US) showing recession since 1850 of 1.1 km USGS

Mass balance

Mass balance is measured by determining the amount of snow accumulated during winter, and later measuring the amount of snow and ice removed by melting in the summer. The difference between these two parameters is the mass balance. If the amount of snow accumulated during the winter is larger than the amount of melted snow and ice during the summer, the mass balance is positive and the glacier has increased in volume. On the other hand, if the melting of snow and ice during the summer is larger than the supply of snow in the winter, the mass balance is negative and the glacier volume decreases. Mass balance is reported in meters of water equivalent. This represents the average thickness gained (positive balance) or lost (negative balance) from the glacier during that particular year.

To determine mass balance in the accumulation zone, snowpack depth is measured using probing, snowpits or crevasse stratigraphy. Crevasse stratigraphy makes use of annual layers revealed on the wall of a crevasse. Akin to tree rings, these layers are due to summer dust deposition and other seasonal effects. The advantage of crevasse stratigraphy is that is provides a two-dimensional measurement of the snowpack layer, not a point measurement. It is also usable in depths where probing or snowpits are not feasible. In temperate glaciers, the insertion resistance of a probe increases abruptly when its tip reaches ice that was formed the previous year. The probe depth is a measure of the net accumulation above that layer. Snowpits dug through the past winters residual snowpack are used to determine the snowpack depth. Snowpack density is another way used to determine mass balance. In either situation the observed depth is multiplied by the snowpack density to determine the accumulation in water equivalent. It is necessary to measure the density in the spring as snowpack density varies. Measurement of snowpack density completed at the end of the ablation season yield consistent values for a particular area and need not be measured every year. In the ablation zone, ablation measurements are made using stakes inserted vertically into the glacier either at the end of the previous melt season or the beginning of the current one. The length of stake exposed by melting ice is measured at the end of the melt (ablation) season. Most stakes must be replaced each year or even mid-way through the summer.

Measuring snowpack in a crevasse on the Easton Glacier, North Cascades, USA, the two dimensional nature of the annual layers is apparent
Measuring snowpack on the Taku Glacier in Alaska, this is a slow inefficient but accurate process

Net balance

Net balance is the mass balance determined between successive mass balance minimums. This is the stratigraphic method focusing on the minima representing a stratigraphic horizon. In the northern mid-latitudes, a glacier's year follows the hydrologic year, starting and ending near the beginning of October. The mass balance minimum is the end of the melt season. The net balance is then the sum of the observed winter balance (bw) normally measured in April or May and summer balance (bs) measured in September or early October.

Measuring snowpack on the Easton Glacier by probing to the previous impenetrable surface, this provides a quick accurate point measurement of snowpack

Annual balance

Annual balance is the mass balance measured between specific dates. The mass balance is measured on the fixed date each year, again sometime near the start of October in the mid northern latitudes.

Geodetic methods

Geodetic methods are an indirect method for the determination of mass balance of glacier. Maps of a glacier made at two different points in time can be compared and the difference in glacier thickness observed used to determine the mass balance over a span of years. This is best accomplished today using Differential Global Positioning System. Sometimes the earliest data for the glacier surface profiles is from images that are used to make topographical maps and digital elevation models. Aerial mapping or photogrammetry is now used to cover larger glaciers and icecaps such found in Antarctica and Greenland, however, because of the problems of establishing accurate ground control points in mountainous terrain, and correlating features in snow and where shading is common, elevation errors are typically not less than 10 m (32 ft). Laser altimetry provides a measurement of the elevation of a glacier along a specific path, e.g., the glacier centerline. The difference of two such measurements is the change in thickness, which provides mass balance over the time interval between the measurements. Again a good method over a span of time but not for annual change detection. The value of geodetic programs is providing an independent check of traditional mass balance work, by comparing the cumulative changes over ten or more years.

Mass balance research worldwide

Mass balance studies have been carried out in various countries worldwide, but have mostly conducted in the Northern Hemisphere due to there being more mid-latitude glaciers in that hemisphere.

Alaska

The Taku Glacier near Juneau, Alaska has been studied by the Juneau Icefield Research Program since 1946, and is the longest continuous mass balance study of any glacier in North America. Taku is the world's thickest known temperate alpine glacier, and experienced positive mass balance between the years 1946 and 1988, resulting in a huge advance. The glacier has since been in a negative mass balance state, which will may result in a retreat if the current trends continue.

Austrian Glacier Mass Balance

The mass balance of Hintereisferner and Kesselwandferner glaciers in Austria have been continuously monitored since 1952 and 1965 respectively. Having been continuously measured for 55 years, Hintereisferner has one of the longest periods of continuous study of any glacier in the world, based on measured data and a consistent method of evaluation. Currently this measurement network comprises about 10 snow pits and about 50 ablation stakes distributed across the glacier. In terms of the cumulative specific balances, Hintereisferner experienced a net loss of mass between 1952 and 1964, followed by a period of recovery till 1968. Hintereisferner reached an intermittent minimum in 1976, briefly recovered in 1977 and 1978 and has continuously lost mass in the 30 years since then.

New Zealand

Glacier mass balance studies have been ongoing in New Zealand glaciers since 1957. Tasman Glacier has been studied since then by the New Zealand Geological Survey and later by the Ministry of Works, measuring the ice stratigraphy and overall movement. However, even earlier fluctuation patterns were documented on Franz Josef and Fox Glaciers in 1950. Other glaciers on the South Island studied include Ivory Glacier since 1968, while on the North Island, glacier retreat and mass balance research has been conducted on the glaciers on Mount Ruapehu since 1955. On Mount Ruapehu, permanent photographic stations allow repeat photography to be used to provide photographic evidence of changes to the glaciers on the mountain over time.

North Cascade glacier mass balance program

The North Cascade Glacier Climate Project measures the annual balance of 9 glaciers, more than any other program in North America. These records extend from 1984–2005 and represent the only set of records documenting the mass balance changes of an entire glacier clad range. To monitor an entire glaciated mountain range in North America, which was listed as a high priority of the National Academy of Sciences in 1983. North Cascade glaciers annual balance has averaged −0.52 m/a from 1984–2005, a cumulative loss of over 12.5 m or 20–40% of their total volume since 1984 due to negative mass balances. The trend in mass balance is becoming more negative which is fueling more glacier retreat and thinning.

Norway mass balance program

Norway maintains the most extensive mass balance program in the world and is largely funded by the hydropower industry. Mass balance measurements are currently performed on twelve glaciers in Norway. In southern Norway six of the glaciers have been measured for 42 consecutive years or more, and they constitute a west-east profile reaching from the very maritime Ålfotbreen Glacier, close to the western coast, to the very continental Gråsubreen Glacier, in the eastern part of Jotunheimen. Storbreen Glacier in Jotunheimen has been measured for a longer period of time than any other glacier in Norway, a total of over 55 years, while Engabreen Glacier has the longest series (35 years) in northern Norway. The Norwegian program is where the traditional methods of mass balance measurement were largely derived.

Sweden Storglaciären

The Tarfala Research Station in the Kebnekaise region of northern Sweden is operated by Stockholm University. It was here that the first mass balance program was initiated immediately after World War Two, and continues to the present day. This survey was the initiation of the mass balance record of Storglaciären Glacier, and constitutes the longest continuous study of this type in the world.

Swiss mass balance program

Temporal changes in the spatial distribution of the mass balance result primarily from changes in accumulation and melt along the surface. As a consequence, variations in the mass of glaciers reflect changes in climate and the energy fluxes at the earth's surface. The Swiss glaciers Gries in the central Alps and Silvretta in the eastern Alps, have been measured for many years. The distribution of seasonal accumulation and ablation rates are measured in-situ. Traditional field methods are combined with remote sensing techniques to track changes in mass, geometry and the flow behaviour of the two glaciers. These investigations contribute to the Swiss Glacier Monitoring Network and the International network of the World Glacier Monitoring Service (WGMS).

United States Geological Survey (USGS)

The USGS operates a long-term "benchmark" glacier monitoring program which is used to examine climate change, glacier mass balance, glacier motion, and stream runoff. This program has been ongoing since 1965 and has been examining three glaciers in particular. Gulkana Glacier in the Alaska Range and Wolverine Glacier in the Coast Ranges of Alaska have both been monitored since 1965, while the South Cascade Glacier in Washington State has been continuously monitored since the International Geophysical Year of 1957. This program monitors one glacier in each of these mountain ranges, collecting detailed data to understand glacier hydrology and glacier climate interactions.

Cited references

References

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  2. Mauri S. Pelto (Nichols College). "The Disequilibrium of North Cascade, Washington Glaciers 1984–2004". In "Hydrologic Processes". {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
  3. Pelto, M.S. and Hartzell, P.L. (2004). "Change in longitudinal profile on three North Cascades glaciers during the last 100 years" (PDF). Hydrologic Processes. 18: 1139–1146.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  4. Mauri S. Pelto, Director NCGCP (March 28). "Glacier Mass Balance". North Cascade Glacier Climate Project. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)
  5. David Rippin, Ian Willis, Neil Arnold, Andrew Hodson, John Moore, Jack Kohler and Helgi Bjornsson (2003). "Changes in Geometry and Subglacial Drainage of Midre Lovénbreen, Svalbard, Determined from Digital Elevation Models" (PDF). Earth Surface Processes and Landforms. 28: 273–298.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  6. Andreas Bauder, G. Hilmar Gudmundsson (March 28). "Mass Balance Determination using Photogrammetric Methods and Numerical Flow Modeling". Laboratory of Hydrolic, Hydrology and Glaciology. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)
  7. Pelto, Mauri. "Mass Balance Measurements of the Taku Glacier, Juneau Icefield, Alaska 1946-2005". Juneau Icefield Research Program. Retrieved 2007-01-09. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  8. "Mass balance of Hintereisferner". Institute for Meteorology and Geophysics, University of Innsbruck, Austria. January 20, 2004. Retrieved 2007-01-09.
  9. "Glaciers of New Zealand". Satellite Image Atlas of Glaciers of the World. U.S. Geological Survey. Retrieved 2007-01-16.
  10. Pelto, Mauri (November 9, 2006). "Glacier Mass Balance". United States Mass Balance Surveys. Retrieved 2007-01-09.
  11. Norwegian Water Resources and Energy Directorate (March 28). "Mass balance measurements". Glaciological investigations in Norway. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)
  12. "Storglaciären". Stockholm University. February 9, 2003. Retrieved 2007-01-09.
  13. Bauder, Andreas (March 20, 2006). "Mass Balance Studies on Griesgletscher and Silvrettagletscher". The Swiss Glaciers. Laboratory of Hydraulics, Hydrology and Glaciology, Swiss Federal Institute of Technology. Retrieved 2007-01-09. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  14. "Benchmark Glaciers". Water Resources of Alaska-Glacier and Snow Program. United States Geological Survey. July 9, 2004. Retrieved 2007-01-09.
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