Glacier mass balance

As mass balance, the difference between mass inflow ( accumulation ) and mass loss ( ablation) is called an ice body in glaciology. The total mass gain or loss of a glacier, an ice cap or ice sheet of a hydrological cycle - in usually one year - is called total mass balance. The specific mass balance is the change in mass of a period with respect to a point of the glacier. In most cases the total mass balance is determined by integration of measured, over the glacier surface distributed specific mass balance data. By dividing the total mass balance of the glacier surface, we obtain the average specific mass balance, which allows a comparison of the behavior of various glaciers. This is mainly the published size, it is usually expressed in millimeters or meters of water equivalent per year and can be considered as " average change in ice thickness ." It is often also referred to as reduced annual mass balance. In case of positive mass balance over several years is advancing a glacier, with negative he withdraws. A glacier is in equilibrium with the air, its mass is balanced.

Most of the accumulation is carried out by snow influenced by Windverfrachtungen and avalanches. The largest mass loss is caused in most glaciers by melting snow, firn and ice from the surface. However, other processes may also be important: at the ice shelves and glaciers calving tide plays a big role, steep hanging glaciers lose a lot of weight by outgoing avalanches, in dry areas, the sublimation lyricisms snow is a non-negligible factor.

To determine the mass balance of a glacier, there are different methods. The oldest and still the basic method is the so-called glaciological method. In this, the change in the surface levels at various points distributed over the glacier is measured. From this, the specific mass balance is determined at this point after estimating the near-surface firn or ice density. Knowledge of the total mass of a glacier is not necessary to determine the mass balance, it is also often not precisely known.

  • 3.1 Direct glaciological method
  • 3.2 Indirect methods based on the glaciological method
  • 3.3 Geodetic method
  • 3.4 Hydrological method
  • 3.5 Model-based methods
  • 3.6 Other Methods

Historical development

The earliest known efforts to identify a mass balance started in 1874 by the Rhone glacier. Farms were the former researches the so-called " glacier College ", which in 1869 by the Swiss Alpine Club (SAC ) and the Swiss Society for Natural Sciences (SNG, today SCNAT) was initiated. The aim of that research was the historical development of the glacier as well as the relationship between changes in the glacier surface and glacier advances to understand. The data collected at that time do not meet today's standards, especially because the density of the firn was not determined in the accumulation area of the glacier. For the period from 1884 to the end of that series of measurements in 1909 the comparability could be produced with today's data by certain assumptions and extrapolations. The average of the mean specific mass balance of this period was -130 mm water equivalent.

Continuous measurements of the specific mass balance at two points of Claridenfirn have been conducted since 1914. Cutting-edge contributions for mass balance measurements in the modern sense, involving the entire glacier, made ​​the Swedish glaciologist Hans Ahlmann (1889-1974) in the 1920s and 1930s. He first led the measurements of each year for another glacier, later they realized the importance of several years, directly comparable data of a glacier. For the Storglaciären in northern Sweden will be determined in 1945 in an uninterrupted sequence mass balance data, the world's longest sequence. Later, the Taku Glacier in Southeast Alaska, the Storbreen in Norway and a growing number of glaciers in the Alps followed.

It was soon recognized that it was necessary to unify the approach of the mass balance determination largely in order to compare the data from different researchers and aggregate can. An early proposal to which was in 1962 by Mark Meier. After some discussion it was created under the auspices of the International Association of Scientific Hydrology ( IASH, today IAHS ) a consensus, the core points were published in the Journal of Glaciology, 1969. This publication prevailed with a few little published later additions as the de - facto standard. In the meantime, this standard are several inconsistencies in the interpretation of some terms originated, also there was a need to better cover the mass balance calculation of the ice sheets, so that the International Association of Cryospheric Sciences ( IACS) has published a document in 2011 with the aim of continuing the remediation.

Basics

Contribution of glacier surface mass balance

Conveniently, play at most glaciers are crucial for the mass balance processes in the area of the most accessible for measurements glacier surface. The key here are snowfall, avalanches, melting, refreezing of water, sublimation and Resublimation and Windverfrachtungen. An important factor is also the mass loss by calving at ending in the aquatic glaciers. While in valley glaciers the majority of the mass loss occurs through runoff in the channel about, for example, in Greenland is responsible for almost 50 % of the loss of ice calving from Auslassgletschern into the sea.

In particular, in polar glaciers but can not be entirely neglected the processes in the glacier inside. For example, while the melt water in the ablation zone of valley glaciers practically unhindered flow, you go in the accumulation of polar ice fields on the assumption that 60 % of the meltwater refreezes. To a significant ablation on the glacier surfaces may cause, which is the case for example in northern Greenland ice sheet volcanism or geothermal sources.

Financial year, summer and winter balance

The period between two annual minima of glacier mass is one of the definitions for the balance sheet or financial year of a glacier. When the glaciers of the mid-latitudes the financial year therefore begins in the fall, at the end of the ablation season. The glacier surface at the beginning of a financial year is subsequently reconstructed in some places on the dirty intermediate layer. A second special time is at the end of the accumulation period, with most glaciers in the spring when the ice thickness is maximum. The data obtained between these times are called winter and summer balance. In this oriented to the layer sequence definition ( Stratigraphic System) are the financial years due to different weather conditions are not always the same length, which affects the comparability of the data. Also, the minimum and maximum, particularly with large glaciers does not occur at all points one at the same time.

Therefore Another definition is a fixed calendar date for the beginning of the financial year and differentiation of winter and summer balance (Fixed -Date System). When the glaciers of the mid-latitudes of the northern hemisphere the financial year begins, leaning even to the hydrological year, usually on October 1, the boundary between winter and summer balance is March 1. If it should not be possible - for example, due to the weather - the measurements actually perform current term, an attempt is made to extrapolate the data of the actual date, befindlicher example, using the data near weather stations. If true about the cycle of Fixed -Date system is maintained, but will not engage in such extrapolation and thus unequal length financial years to be accepted, this is referred to as a floating -data system. If several of these approaches are combined to obtain the data to match multiple definitions, called the combined system. Viewed over longer periods of time, the data of all the systems do not differ significantly.

Note, however, that based on a twice held in measuring the surface modification, as it is at least necessary to distinguish between summer and winter balance, none of the definitions actually the complete accumulation and ablation can be measured - for example, as well as snowfall in the summer months is possible. However, such a distinction between summer and winter balance sheet provides the only practical way to estimate the influence of various environmental factors. There are glaciers, where there is no such seasonal cycle, and no such distinction between winter and summer balance is possible. For example, there is in the air glaciers monsunalem an active phase during which both the majority of the accumulation as well as the ablation occurs.

Terminology

The specific mass balance is the local change in mass of a glacier with respect to a surface and can be expressed in kilograms per square meter ( symbol).

Similar rainfall that are specified as relative water depth to a surface, the specification often takes the form of a Eisdickenänderung. Since the density of glacier ice is not uniform, the density of water () is usually used and representative expressed the specific mass balance in meters of water equivalent.

To explicitly express the time reference, the data is represented in the form of specific mass balance rate (). Here, the specific mass balance is given by integration of the mass balance rate over time.

Mostly, the information provided by the mass balances refer implicitly to the period of a year. Especially when winter () and summer balance ( ) can be determined separately, the annual balance sheet is referred to as net balance.

When using the glaciological method in areas with negative net balance typically expected the other way around, so the net balance measured as annual change and determined from the difference between the winter balance, the balance of the summer.

The total mass balance () is obtained by integrating the specific mass balances over the surface of the glacier (). By the total mass balance is divided by the area of the glacier, we obtain the mean specific balance ().

Height dependence and equilibrium line

The specific mass balance is significantly different at different points of the glacier. For most glaciers, there is a clear separation between a higher accumulation area in which the annual specific net balance is positive everywhere, and a deeper ablation zone in which it is negative. The dividing line at which the mass balance is exactly balanced (ie true ), equilibrium line ( Equilibrium Line altitute, ELA) is called. For most glaciers, the equilibrium line is near the Firngrenze at the end of the summer. An exception are polar glaciers, which occurs in the lower part of the Nährgebiets ice again by freezing melt water, so-called Superimposed Ice.

Another derived from the mass balance of a glacier parameter is the ratio between the accumulation area and total area ( Accumulation Area Ratio AAR). Hot or poor snow years, this ratio is small. In valley glaciers on the assumption that these are located at a ratio of between 55 % and 65 % in equilibrium with the air. When Pasterze the ratio was in four financial years in the period from 2005 to 2010 between 45% and 49%, an outlier in 2008 there were only 16 %.

The so-called Massenbilanzgradient expresses the rate of change of the specific mass balance based on the level. A high Massenbilanzgradient indicates a climate sensitivity of the glacier. Massenbilanzgradient in the range of the equilibrium line is also referred to as an activity index.

But there are also glaciers, which can not be nutrients and ablation zone separate clear: When glaciers in the Antarctic, the accumulation area over the entire glacier may extend, they lose their mass almost exclusively by the calving. Also by avalanches, coastal fog or shading there may be deeper "islands" with positive mass balance.

Methods

There are several methods to determine the mass balance of a glacier. The oldest and still fundamental is the so-called direct glaciological method, in which the changes to the glacier surface are measured on site. All other methods are referred to as "indirect". This is emphasized but usually only if to be collected on the basis of directly given past data using simple and less data in the following years, the mass balance of a glacier is also estimated. In addition, there are other methods, especially the geodetic method in which the glacier to the measurement must not be entered. However, none of the methods for all the glaciers is appropriate and gives for each glacier sufficiently accurate results. In order to estimate the accuracy of the result better, it is therefore advisable to combine several methods.

Direct glaciological method

In the direct glaciological method, the surface changes are determined at representative measuring points as possible and each determines from the specific mass balance. Based on the data obtained by this measurement network can be estimated and used to calculate the mean specific mass balance by interpolating the specific mass balance for the entire glacier surface. Measurement points needed to be both in nutrients as well as in the ablation zone.

To measure the ablation must rods, also called Ablationspegel be drilled so deep into the ice that they do not fall out at the end of the ablation season - this can near the glacier terminus a depth of ten meters be enough. The next exploration of the glacier, the height variation is measured. Assuming an ice density of 900 kilograms per cubic meter, the mass change is calculated therefrom. If it is expected that the ablation is also extend to the area above the Firngrenze, rods must be set and also the density profile are determined in advance in rod near the safe side there.

Also for measuring the accumulation of rods are set. With large amounts of snow it may be impossible to prevent these disappear in the snow - there are different strategies, yet to find such rods again, for example, the attachment of a transmitter or a strong magnet. At the end of accumulation period, the amount of fallen snow must be determined. In the mid-latitude glaciers it prepares usually no difficulty in determining the layer before the start of the accumulation period - it is due to the experience gained during the ablation season dust "dirty" and also harder by frozen melt water than the surrounding layers. In addition, a mark on the rod be helpful in difficult cases dark-colored sawdust in the vicinity of the rod can be scattered. To determine the density of the accumulated snow, in the rod near one shaft is dug usually and snow profile analyzed on the wall of the shaft. For sealing determining a core can be removed, but there is a danger that the snow is compacted at removal, which can lead to an overestimation of the density.

The exact position of the poles is determined during the measurement of the surface modification. The fact that the rods have moved with the ice, is not usually taken into account. The accuracy of the thus determined mass balance can be difficult to assess, especially in glaciers with extensive areas that are difficult to access, such as rift zones. The glaciological method requires a relatively high time and personal effort.

Indirect methods based on the glaciological method

The measurements of the past have shown that the height profile of the specific mass balances of many glaciers over several years across very similar and basically only shifts depending on the weather patterns of the respective year. This allows to limit themselves in subsequent years to a few representative as possible measuring points (index stakes ) and still be able to estimate the mass balance of the entire glacier with sufficient accuracy. Also includes many glaciers a correlation between the mean specific mass balance and the height of the equilibrium line (ELA ) and the ratio of the area of the Nährgebiets of the total area ( AAR). Thus, the specific mass balance based on a determined from the historical data obtained by means of direct glaciological method formula ELA or AAR can be calculated approximately. Attractive thing is that ELA and AAR can be determined on the basis of recorded at the end of the ablation season aerial images and therefore no on-site measurements are needed. However, the method does not work when the Firngrenze is not identical with the equilibrium line again because of melting frozen water. Also one may the last possible date for an acceptable recording not to be missed because of early snowfall may make a determination of the equilibrium line impossible.

Geodetic method

In the geodetic method, the volume change is determined by the height of the glacier model is compared to two specific points in time, often a multi-year period is examined. From the change in volume of the mass change is calculated assuming density. It should be noted that a change in ice thickness at a point by both a mass loss or gain as well alone by the flow of the ice can be caused. The change in volume of an ice column at a point of the glacier is therefore made up of a mass balance attributable to contributions and another caused by the movement of the ice Post:

Here, the contribution of glacier dynamics may well exceed the mass change. This means that, for example, at positions at which a volume increase is measured, the ablation may be greater than the accumulated yet, so there is a negative specific mass balance.

The major contribution of this vertical movement at the glacier surface afford emergence and submergence. These are usually in the accumulation down ( submergence ) and in the ablation zone up ( emergence) directed. These movements are essential to ensure that a balanced befindlicher with the climate glacier maintains its shape by the compensated due to accumulation and ablation increase in volume and decreases. For the glacier as a whole, the vertical movements cancel each other, as long as its overall density does not change.

As long as these vertical movements are not known accurately enough, no determination of the mass balance for parts of the glacier is possible by means of the geodetic method also can not be quantified separately accumulation and ablation. Basis for determining the volume change are accurate topographic maps and for the past decades increasingly digital elevation models obtained by aerial or satellite images, also laser scanning and radar interferometry can be used. Difficulties in this process, the lack of contrast in particular prepare in snowy accumulation area. The estimation of the density of the ice and snow in particular can be very inaccurate, it may also be necessary to plan for corrections for releasing yourself deeper layers glacier. The geodetic method is particularly suitable as a supplement to the glaciological method, in particular to uncover systematic errors.

Hydrological method

From hydrological point of view, the total mass balance of a glacier can be determined by the losses due to runoff and evaporation will be deducted from the sum of the rainfall in the catchment area of the glacier. In addition, but also the changes of the unsaved in the form of glacial ice water play a role, either groundwater or befindliches within the glacier water, the amount at the beginning of the ablation season rises sharply in particular. The actually measured density required for the measurement of precipitation in mountainous regions is hardly attainable in practice. Also, a sufficiently accurate measurement of the water runoff is extremely complex. Therefore, the mass balance calculation is not very accurately by means of the hydrological method - the error rate is often in the order of 100 % - which is why it is usually used only in combination with other methods. In contrast to the glaciological method, however, mass changes are also recorded in the interior and at the bottom of the glacier.

Model-based methods

In this approach, the method for the numerical weather forecast models are similarly used that simulate the relevant for the mass balance of a glacier behavior in interaction with weather and climate. The modeling approaches focus here primarily on the ablation. Here come relatively simple degree day approaches used as well as detailed energy balance models, for example, also take into account the solar radiation, albedo or wind. The choice of method is not least depending on what data is available. The temporal and spatial distribution of rainfall can usually be mapped only roughly. Such models need to be calibrated using data lying near weather stations and otherwise determined mass balance data of the past. Not standing with the climate related movements such as avalanches or glacier surges are a problem.

Other methods

In different ways and the flow of the glacier is included. In this example, the flow of ice is a glacier cross section determined (flux gate). This may be of particular interest for calving glaciers or Auslassgletschern. These data are often combined with other data obtained. Goes even further, the approach to combine the different flow rates of the glacier surface with the data obtained by means of geodetic method (flux divergence ) to it to derive a spatial distribution of the mass balance, which is not possible with the geodetic method. So far, the accuracy of the data is still not sufficient, since the models of glacier dynamics vertical ice movements currently can not adequately represent.

Also gravimetric methods have been used for determining the mass balance of large glaciated areas. Useful data for this can currently only the Gravity Recovery And Climate Experiment ( GRACE ). Whether this method is also applicable for smaller-scale mass balance provisions, is controversial.

Objectives and results

Objective of the mass balance determination of glaciers has always been to be able to better understand and predict the behavior of glaciers, especially in view of glaciers caused by disasters such as glacial lake outburst flood. Furthermore, the development of the mass balance of a glacier is usually a response to a changing climate, which occurs practically without delay. Therefore, there is a significant motivation for the detailed determination of mass balances is to better understand the relationships between climate and the resulting change of the glacier, the glacier dynamics. This allows for a historical glacier behavior to drawing conclusions on the then climate, but on the other it allows in particular a more precise picture of the behavior of glaciers in climate models. It is also of importance of the hydrological aspect, at regional level, with regard to the future supply of drinking water, for other globally in the forecast of the expected sea level rise. Whether the ice sheets of Greenland and Antarctica or the other glaciers and ice caps of Earth will make the greater contribution to sea level rise in the first half of the 21st century, is disputed.

Glaciers and ice caps

Direct measurements of the mass balance have been carried out at about 300 glaciers worldwide and cover roughly the period since the second half of the 20th century from. Of these, the data of about 250 glaciers by the World Glacier Monitoring Service ( WGMS ) were collected as a contribution to the Global Terrestrial Network for Glaciers ( GTN -G) and standardized processed provided. However, the data were collected consistently only 37 glaciers for the period 1980-2010. These so-called " reference glaciers " glaciers are not representative selection of the glacier dar. The total amount of all glaciers with mass balance data certainly delivers a significantly distorted picture of the world. Most lie in the Alps or in Scandinavia, there are some in North America and the high mountains of Central Asia. On the other hand completely under-represented are the glaciers in northern Asia, and South America; the ice sheets of Greenland and Antarctica have to be considered separately anyway. Even under other angles these glaciers selection is unbalanced: for a small glaciers are over-represented, also plays the accessibility of the glacier, logically, a role that also if the weather patterns makes any on-site measurements often enough possible. To what extent based on this data to draw conclusions about the glaciers are still possible world, is controversial. There is general agreement that in underrepresented regions measurement series should be started. Another strategy is to try to derive from cumulative length changes of the glacier mass balances. This is attractive because changes in length are much easier to identify and there are far more historical data. At least the order of magnitude of the mass balance can be estimated in this way.

For the 37 glaciers with gapless directly determined mass balance data 1980-2010 the average yearly mean specific mass balance in the first decade of the 21st century -0.75 meters water equivalent was. This means that the mass loss has doubled since the 1970s. In the 1980s, still showed a third of these glaciers to a positive mass balance, in the first decade of the 21st century, it was only a fifth, suggesting that the glaciers fall completely covers more areas. Some glaciers has been observed that there is an increase in the Massenbilanzgradienten. This is caused by increased ablation in the ablation zone and an opposite, slightly lower increase of accumulation in the accumulation area - due to the somewhat higher temperatures occur at higher altitudes obvious to more precipitation. This makes the glacier for more sensitive to temperature changes.

Greenland and Antarctic ice sheet

The mass balances of the two ice sheets are of great interest, since their behavior is crucial to the rise in sea level. Would they completely melt, this meant an increase of about 65 to 70 meters.

With the exception of the lower-lying coastal areas of the Greenland ice sheet can be found at the polar ice no significant mass loss by melting. The specific mass balance is therefore influenced by the continental climate because the rainfall mainly focus on the areas that are a few hundred kilometers from the sea. This means that the specific mass balance with the distance from the coast decreases. In Antarctica, the annual balance sheet on the coast is typically water equivalent of between 300 and 600 millimeters at the South Pole is less than 100 millimeters. The ice sheets are losing their mass mainly by calving in Antarctica makes this 90 % and in 50% of the Greenland mass loss. In the Antarctic subglacial melting at the base of ice shelves is another factor.

End of the 1990s, the mass balance of the ice sheets was almost unknown. At the beginning of the 21st century could be the measurement uncertainty does not say anything about whether the ice sheets of Greenland and Antarctica increase or decrease. Currently, three different, largely independent methods are used:

  • Mass balance method (Mass Budget Method): Here, the accumulation and ablation at the surface is determined Moreover, the ice flow is determined at the edges of the ice sheet. The determination of the surface balance is calculated by simulation models that are calibrated or verified data based on recovered directly. To determine the outflow at the edges, flow velocity and ice thickness of the ice streams and outlet glaciers using satellite are measured.
  • Geodetic Method ( Altimetry Method): The changes in the height of the surface is determined using laser scanning and radar interferometry by satellites such as ERS I / II, Geosat or ICESat, it is the derived volume and mass change.
  • Gravimetric method ( gravity method): Since April 2002, the gravitational field of the earth and its changes over time measured by the two satellites of the GRACE project. In order to draw conclusions on the mass changes are various other effects such as tides need to be removed.

Corrections due to the postglacial land uplift must be considered in the gravimetric method, to a lesser extent in the geodetic method. You should also remember that the ice of the sea level rise will take effect as soon as it swims. For this purpose, the line must be determined from the ice of the ice shelf or glacier begins to float on the sea, called the grounding line. In the gravimetric method, the floating ice does not count anyway the current ice mass. In the other methods must be estimated and taken into account if these shifts due to the thinning of the ice towards the coast line the course of the grounding line.

All methods have their weaknesses. By combining the process attempting to obtain a more accurate result. A study from 2012 tried to summarize the data of previous measurements and evaluate the latest findings. It is emphasized here that long series of measurements are important so that temporary fluctuations do not affect the validity of the results. For the period 1992-2011 this an average mass balance of about -213 gigatonnes per year was determined. This accounted for the largest part of the Greenland ice sheet with approximately -142 gigatonnes per year, the Antarctic Peninsula and the West Antarctica also showed a negative mass balance, while the East Antarctica showed a positive trend. 360 gigatons correspond approximately to a sea level rise of one millimeter, so the ice sheets have caused, according to this study since 1992 in total about a sea level rise of 11.2 millimeters. The Greenland ice sheet used is predominantly thinner at its edges, which is also due to increased melting processes at the surface. The positive mass balance in East Antarctica could be due to the increased rainfall due to temperature rise, but it could also be a natural fluctuation. In principle, a change in glacier dynamics is observed in the two ice sheets, the flow rates in the peripheral areas and Auslassgletschern have increased, causing more ice is delivered to the oceans.

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