Glacier mass balance
A glacier's mass balance or surface mass balance —the difference between accumulation and ablation —is crucial to the survival of the glacier. 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 to 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.
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 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 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 Glacier 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.
Definitions
Accumulation
The different processes by which a glacier can gain mass are collectively known as accumulation. Snowfall is the most obvious form of accumulation. Avalanches, particularly in steep mountain environments, can also add mass to a glacier. Other methods include deposition of wind-blown snow; the freezing of liquid water, including rainwater and meltwater; deposition of frost in various forms; and the expansion of a floating area of ice by the freezing of additional ice to it. Snowfall is the predominant form of accumulation overall, but in specific situations other processes may be more important; for example, avalanches can be much more important than snowfall in small cirque basins.Accumulation can be measured at a single point on the glacier, or for any area of the glacier. The units of accumulation are meters: 1 meter accumulation means that the additional mass of ice for that area, if turned to water, would increase the depth of the glacier by 1 meter.
Ablation
Ablation is the reverse of accumulation: it includes all the processes by which a glacier can lose mass. The main ablation process for most glaciers that are entirely land-based is melting; the heat that causes melting can come from sunlight, or ambient air, or from rain falling on the glacier, or from geothermal heat below the glacier bed. Sublimation of ice to vapor is an important ablation mechanism for glaciers in arid environments, high altitudes, and very cold environments, and can account for all the surface ice loss in some cases, such as the Taylor Glacier in the Transantarctic Mountains. Sublimation consumes a great deal of energy, compared to melting, so high levels of sublimation have the effect of reducing overall ablation.Snow can also be eroded from glaciers by wind, and avalanches can remove snow and ice; these can be important in some glaciers. Calving, in which ice detaches from the snout of a glacier that terminates in water, forming icebergs, is a significant form of ablation for many glaciers.
As with accumulation, ablation can be measured at a single point on the glacier, or for any area of the glacier, and the units are meters.
Rates, mass flux, and balance year
Glaciers typically accumulate mass during part of the year, and lose mass the rest of the year; these are the "accumulation season" and "ablation season" respectively. This definition means that the accumulation rate is greater than the ablation rate during the accumulation season, and during the ablation season the reverse is true. A "balance year" is defined as the time between two consecutive minima in the glaciers mass—that is, from the start of one accumulation season through to the start of the next. The snow surface at these minima, where snow begins to accumulate again at the start of each accumulation season, is identifiable in the stratigraphy of the snow, so using balance years to measure glacier mass balance is known as the stratigraphic method. The alternative is to use a fixed calendar date, but this requires a field visit to the glacier each year on that date, and so it is not always possible to strictly adhere to the exact dates for the fixed year method.Mass balance
The mass balance of a glacier is the net change in its mass over a balance year or fixed year. If accumulation exceeds ablation for a given year, the mass balance is positive; if the reverse is true, the mass balance is negative. These terms can be applied to a particular point on the glacier to give the "specific mass balance" for that point; or to the entire glacier or any smaller area.For many glaciers, accumulation is concentrated in winter, and ablation in the summer; these are referred to as "winter-accumulation" glaciers. For some glaciers, the local climate leads to accumulation and ablation both occurring in the same season. These are known as "summer-accumulation" glaciers; examples are found in the Himalayas and Tibet. The layers that make winter-accumulation glaciers easy to monitor via the stratigraphic method are not usable, so fixed date monitoring is preferable.
Equilibrium line
For winter-accumulation glaciers, the specific mass balance is usually positive for the upper part of the glacier—in other words, the accumulation area of the glacier is the upper part of its surface. The line dividing the accumulation area from the ablation area—the lower part of the glacier—is called the equilibrium line; it is the line at which the specific net balance is zero. The altitude of the equilibrium line, abbreviated as ELA, is a key indicator of the health of the glacier; and since the ELA is usually easier to measure than the overall mass balance of the glacier it is often taken as a proxy for the mass balance.Symbols
The most frequently used standard variables in mass-balance research are:- a – ablation
- c – accumulation
- b – mass balance
- ρ – density
- h – glacier thickness
- S – area
- V – volume
- AAR – accumulation-area ratio
- ELA – equilibrium-line altitude
Measurement methods
Mass balance
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 it 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 and density. The snowpack's mass balance is the product of density and depth. Regardless of depth measurement technique 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 on temperate alpine glaciers 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 season. Most stakes must be replaced each year or even midway through the summer.Image:Glaciercrevasse.jpg|thumb|left|250px|Measuring snowpack in a crevasse on the Easton Glacier, North Cascades, USA, the two dimensional nature of the annual layers is apparent
Image:Snowpitglacier.jpg|thumb|right|250px|Measuring snowpack on the Taku Glacier in Alaska, this is a slow and inefficient process, but is very accurate