Glacier mass balance

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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 1970's 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. It 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.

The key symptom of a glacier in disequilibrium is thinning along the entire length of the glacier.^{(Pelto)(Pelto and Harzell)} 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 are most likely to fall into disequilibrium with the 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.

[edit] 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

[edit] 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.
  • 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 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.
  • 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.

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

• • 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.

[edit] 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

[edit] 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.^[1]

[edit] 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.

• Aerial mapping or photogrammetry. Sometimes the earliest data for the glacier surface profiles is from images that are used to make topographical maps and digital elevation models. 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).^{(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.^[2]

[edit] Mass balance research worldwide

[edit] Norway mass balance program

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.^[3]

[edit] 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 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).[1]

[edit] 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.[2]

[edit] United States Geological Survey (USGS)

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 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.[3]

[edit] 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. [4]

[edit] 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. [5]

[edit] Cited references

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Glaciers

* Dyurgerov, M. (M. Meier and R. Armstrong, eds.). Glacier mass balance and regime measurements and analysis, 1945–2003. Institute of Arctic and Alpine Research, University of Colorado. Distributed by National Snow and Ice Data Center, Boulder, CO..

* Mauri S. Pelto (Nichols College). The Disequilibrium of North Cascade, Washington Glaciers 1984–2004. In "Hydrologic Processes". Retrieved on February 14, 2006.

* Pelto, M.S. and Hartzell, P.L. (2004). "Change in longitudinal profile on three North Cascades glaciers during the last 100 years". Hydrologic Processes 18: 1139–1146.

* Mauri S. Pelto, Matt Beedle and Maynard M. Miller. Mass Balance Measurements on the Taku Glacier, Juneau Icefield, Alaska 1946–2005. Juneau Icefield Research Program. Retrieved on February 14, 2006.

* 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". Earth Surface Processes and Landforms 28: 273–298.

[edit] Additional

1. ^ Mauri S. Pelto, Director NCGCP (March 28). Glacier Mass Balance. North Cascade Glacier Climate Project.
2. ^ Andreas Bauder, G. Hilmar Gudmundsson (March 28). Mass Balance Determination using Photogrammetric Methods and Numerical Flow Modeling. Laboratory of Hydrolic, Hydrology and Glaciology.
3. ^ Norwegian Water Resources and Energy Directorate (March 28). Mass balance measurements. Glaciological investigations in Norway.