The following graph shows the mass balance of Vernagtferner summed up since the start of regular observations in 1964.
The mass balance of the glacier is determined on the basis of measurements of snow accumulation and ice ablation during spring and autumn inspections. The data are reported to the World Glacier Monitoring Service and published in the Global Glacier Change Bulletin along with data from more than 120 other glaciers. The Vernagtferner is one of currently about 40 reference glaciers whose long and reliable time series form the basis of an observation network supporting the United Nations Framework Convention on Climate Change.
In principle, the mass balance of a glacier can be compared to maintaining a current account: The total amount of the glacier’s snow and ice at the beginning of the financial year represents the account’s current “credit” balance. The accumulation corresponds to the sum of the total annual income and is counted positively. The ablation, on the other hand, corresponds to the sum of the expenses and has a minus sign. The sign (plus or minus) of the balance between debits and credits determines whether the glacier has gained or lost mass. Accordingly, changes to the total amount are analogous to the new account balance. However, there is no overdraft facility with a glacier. Once the balance reaches zero, the glacier has disappeared.
Along the lines of the Warming Stripes (see also here), which show temperature changes since the start of meteorological observations, mass balance stripes for Vernagtferner starting in 1964 are shown here. The stripes are meant to visualize the changes at first glance - without any numbers. Blue stands for mass increase, while red means mass loss. The colour scale is given by the extreme year 2003.
The following table shows the main parameters of the mass budget of the Vernagtferner for the budget years 1964/1965 to 2019/2020. The mass balance column is color-coded: here, too, blue shades correspond to a positive mass balance (mass increase) and red shades correspond to a negative mass balance (mass loss).
|Year||Area [km2]||ELA||AAR||Balance [mm]||Data||Map|
In addition to the main parameters of the mass budget, the table contains the total area of the glacier on which the analysis is based, the equilibrium-line altitude (ELA), the accumulation area ratio (AAR) and the specific net mass balance in mm water equivalent. In column Data the numbers for the different glacier parts. A legend (in German) can be found here. The analyses were carried out by Oskar Reinwarth, Ludwig Braun and Markus Weber. From the 2003/04 budget year onwards, Christoph Mayer has carried out the analyses.
To illustrate the development over the observation period, the specific mass balance values (balance column) are colour-coded. Negative values (red colours) mean a loss of mass, positive values (highlighted in shades of blue) mean a mass gain of the glacier. It can be seen that the glacier - after a weak growth period in the mid-1970s - has been steadily losing mass since the early 1980s. In addition, there has been a clear increase in mass budget years with extreme mass losses (more than 1 m of water equivalent across the entire glacier) over the last decade.
The extent of the glacier area is an important reference value for assessing the mass balance. However, its change is not necessarily linked to the mass changes directly. Since ice masses are also transported dynamically, an increase or decrease of the ice mass does not have a direct effect on the area. Nevertheless, changes in the glacier boundaries occur over time, especially in peripheral areas.
The extent of the glacier area indicated in the table is not, however, always the current area at the end of the budget year, but the area at the time of cartographic mapping. It is this latter area that forms the basis of the analysis. This so-called contour plot cannot be updated every year, as aerial images must be available for this purpose.
The equilibrium line altitude (ELA) is a hypothetical variable determined by the height distribution of the specific net balance. It is the altitude at which the mass balance in the budget year in question is balanced or changes its sign. This contour line divides the glacier into two areas: above are the areas with a net gain in mass, below are the areas with mass loss. The mean height of the equilibrium line at the Vernagtferner is 3066 m, which corresponds to the altitude range in which the glacier area has the largest extent. If the current equilibrium line is above the long-term mean value, the total mass balance of the glacier is negative. If it is lower, mass growth can be observed. As this is a hypothetical parameter, the ELA can even be located above the ridge in extreme cases (e. g. 1993/94 and 2002/2003). During these years, the glacier loses mass over its entire elevation range.
AAR stands for accumulation area ratio and indicates the ratio of the accumulation area to the total area as a percentage. As a rule of thumb, the mass budget of a glacier is balanced when the ratio of the size of the accumulation area to that of the ablation area is about 2:1. In this case, the AAR is 66%. For higher values, an increase in mass is expected, for lower values, a decrease in mass.