The Greenland Ice Sheet develops throughout the year with the changing weather conditions. Precipitation contributes by increasing the mass, whilst warmth induces melting, which makes the ice sheet diminish. The term surface mass balance is used for the isolated gain and melting of the surface of the ice sheet – excluding that which is lost when glaciers calve off ice bergs and melt in contact with warm sea water. Newly fallen snow is very bright and reflects most of the sunlight that hits it. As the snow warms up or gets older, it becomes darker. Dark areas absorb more energy from the sun, which leads to further warming and melting of ice. This is called the albedo effect.
Here we show:
- Left map/ left tab: Daily contribution to the surface mass balance
- Left map/right tab: Accumulated anomaly of the surface mass balance
- Map on the right: Anomaly of the reflectivity of the Greenland Ice Sheet (the albedo).
A description of the three maps is found below. Snow and ice have different densities from water; therefore snow and ice are converted to water equivalent for ease of comparison.
The day-to-day development of the ice sheet
The map “Daily change” illustrates the daily changes in the surface mass balance on the Greenland Ice Sheet. The figures are updated daily and show how much mass, in terms of snow, ice and water, is lost or gained on the surface of the ice sheet.
The circles correspond to the PROMICE weather stations that are used to monitor the melting processes. Mouse over to see today’s observed weather at these sites. Note that the circles have been moved relative to their actual positions so they can be distinguished. On the large version of the figure, they are marked with small dots at their true positions.
The curve below the map shows the total daily contribution from all points on the ice sheet.
The blue curve shows this season’s surface mass balance measured in gigatonnes (1 Gt is one billion tonnes and corresponds to 1 cubic kilometer of water). For comparison, the mean curve from the period 1981-2010 is shown (dark grey). The same calendar day in each of the 30 years (in the period 1981-2010) will have its own value. These differences from year to year are illustrated by the light grey band. For each calendar day, however, the lowest and highest values of the 30 years have been left out.
How is the ice sheet developing compared to the period 1981-2010?
The map ”Accumulated” shows what the complete surface gain and loss has been over the year compared to the period 1981-2010. The animation shows a frame for every 7th day back in time from the previous 1st September. The maps are calculated by adding up all daily contributions from the 1st of September and up to now. Subsequently, the average over the years 1981-2010 has been subtracted from this sum. The accumulated surface mass balance anomaly is therefore an analysis of how the ice has developed throughout the season in comparison to previous years.
The curve under the map illustrates the daily contribution from all points on the ice sheet accumulated from 1st September until now. The blue curve shows this season’s surface mass balance. The red line shows the corresponding curve for the 2011-2012 season, when there was a record high summer melt in Greenland. The dark grey mean curve and the light grey band are calculated in the same manner as for the daily surface mass balance described above.
Note that the mean curve does not end at zero at the end of the year. Over the year, it snows more than it melts, but calving of icebergs is also part of the total mass budget of the ice sheet. If the ice sheet was in balance, the calving should exactly match this net accumulation over the year. Satellite observations over the past decade show, however, that the ice sheet is not in balance. The calving loss is greater than the gain from surface mass balance, and Greenland has been losing mass faster than 200 Gt per year. Especially noteworthy in this regard is the 2012 season, where the surface mass balance was about 300 Gt below normal.
How reflective is the Greenland Ice Sheet?
The map to the right shows how much sunlight is reflected from the Greenland Ice Sheet. Bright areas reflect more sunlight than dark areas and as a consequence dark areas are heated more than bright areas. This phenomenon is known as the albedo effect.
The map is shown as anomalies, which means that the average of the albedo measured in the period 2000-2009 has been subtracted. In this way it can be seen where the ice is brighter and darker than normal.
Red areas indicate where melting and possibly black carbon from wildfire accumulating on the surface darkens the ice. Blue areas indicate where fresh snow or more snow than normal has accumulated. Albedo thus provides a convenient indicator of the competing effects of ice mass gain from snowfall and ice mass loss from melting. Melting ice tends to be darker (has a lower albedo) because melt causes ice crystals to round and if the melting point is reached, liquid water also lowers the snow and ice reflectivity. Any change in reflectivity thereby tends to amplify subsequent changes through a positive feedback loop. Thus, albedo is a very sensitive ice climate indicator.
The map is based on NASA satellite measurements from the MODIS sensor that measures the reflection of sunlight from the surface. The map is updated weekly. These types of measurements cannot be made in the winter season due to lack of sunlight. The animation shows the latest 50 days of available satellite measurements.
About the model behind the ”Daily contributions” and ”Accumulated”
The figures are based partly on observations made by weather stations on the ice sheet and partly on DMI’s research weather model for Greenland, Hirlam-Newsnow, and since 1 July 2017 the HARMONIE-AROME weather model. This data is used in a model which can calculate the total amounts of ice and snow. This model takes snowfall, melting of snow and bare ice, refreezing of melt water as well as snow which evaporates without melting first (sublimation) into account. The model has been updated in 2014 to better account for meltwater refreezing in the snow, and again in 2015 to account for the lower reflectivity of sunlight in bare ice than in snow. Finally, it has been updated again in 2017 with a more advanced representation of percolation and refreezing of meltwater. At the same time, we have extended the reference period to 1981-2010. The update means that the new maps, values and curves will deviate from the previous ones which appear, for instance, in season reports. Everything shown on this site, however, is calculated with this new model, so that all curves and values are comparable.
Data from the weather stations can fail to appear because of problems with instruments or the transmissions via satellite, if the power on the solar-powered battery is low, the weather station is covered in snow or at worst has fallen over.