Mass and Height Change
The map and graph show the gain in the mass of ice when there is precipitation, and how much of this mass is lost when snow and ice melt and when icebergs break off from the ice sheet’s major outlet glaciers. The difference in these mass changes over a glaciological year (September-August) is called the total mass balance of the Greenland Ice Sheet.
The map shows the latest changes in mass derived from data from the GRACE satellites.
The graph illustrates the month-by-month development in changes of mass measured in gigatonnes, Gt (1 Gt is 1 billion tonnes or 1 km3 of water). The left axis on the graph shows how this ice mass loss corresponds to sea level rise contribution. 100 Gt corresponds to 0.28 mm global sea level rise).
This data shows that most of the loss of ice occurs along the edge of the ice sheet, where independent observations also indicate that the ice is thinning, that the glacier fronts are retreating in fjords and on land, and that there is a greater degree of melting from the surface of the ice.
High on the central region of the ice sheet, however, the GRACE satellites show that there is a small increase in the mass of the ice. Other measurements suggest that this is due to a small increase in precipitation/snowfall.
All changes are given relative to June 2006.
Based on this data, it can be seen that during the period 2003-2011 the Greenland Ice Sheet has lost 234 km3 of water per year, corresponding to an annual contribution to the mean increase in sea level of 0.65 mm (Barletta et al. (2013).
Shown above are the monthly changes in ice mass of 8 large drainage basin systems into which the Greenland Ice Sheet can be divided. It can be seen that there are considerable regional differences in the size and timing of the changes of mass in the different basins.
The changes in mass here are calculated on the basis of data from the GRACE satellites using the method described in Barletta et al. (2013).
The graphs illustrate the month-by-month development in changes of mass measured in gigatonnes, Gt (1 Gt is 1 billion tonnes or 1 km3 of water. 100 Gt correspond to 0.28 mm global sea level rise.
The map shows the degree to which the Greenland Ice Sheet has become either thicker or thinner during the five year period from 1 January 2012 until 31 December 2016. It is evident that large parts of the edge of the ice have become several metres thinner each year.
The thinning is enhanced at the largest outlet glaciers, for example Jakobshavn Isbræ in Western and Helheim glacier in Eastern Greenland. It is also apparent that central areas of the Ice Sheet appear to have become slightly thicker, as in the southcentral.
8 different regions in Greenland
The 8 drainage basins have been defined by researchers at NASA and were originally made up of 19 basins. However, it is not possible to separate the small basins from each other when using data from the GRACE satellites, and the 19 basins are therefore combined into 8 larger drainage basin systems.
The original drainage basin definition can be seen at NASA.
It should also be noted that it is not possible to distinguish the signal that comes from the edge of the ice sheet from that which comes from the surrounding small glaciers and ice caps. The changes in mass that are shown on the graphs therefore include the signals from both.
GRACE satellites measure gravitational force
The figures are based on data from the GRACE satellites (jointly operated by NASA and Deutsches Zentrum für Luft- und Raumfahrt, DLR). The gravitational force changes when the amount of ice changes, and this can be measured from the GRACE satellites. Data from the satellites is processed at different centres, which provide monthly models of the Earth’s gravitational field. The method that is used here to derive changes in the amount of ice on the basis of changes in the gravitational force has been developed by researchers at DTU Space and other institutions, and is described in detail in Barletta et al. (2013). The raw GRACE satellite data is carefully processed and validated prior to being released, which explains the delay of several months until the data becomes available.
More about the GRACE mission
GRACE (Gravity Recovery And Climate Experiment) is a joint NASA-DLR satellite mission. The GRACE mission consists of twin satellites that were launched in March 2002. They both orbit the Earth at an altitude of around 500 km. The two satellites are separated by a distance of around 220 km. This distance depends on gravity and can be measured very precisely. This information is used to generate monthly, global models of the Earth’s gravitational field.
The GRACE mission has been very successful, and after more than 15 years of data collection, the mission was finished in October 2017. This was finally due to the battery on one of the satellites dropping out, but the orbit of both satellites now in any case is so low that they will burn up in the atmosphere in December 2017 or January 2018. The anticipated lifetime of the mission was only 5 years.
There will now be a gap in gravity data before the GRACE follow-on mission (GRACE-FO) can deliver data to continue the time series of mass changes of the Greenland Ice Sheet. GRACE-FO is projected to be launched in early 2018.
More about the method
As mentioned above, there are several centres that generate the monthly GRACE gravitational fields which are used to estimate changes to ice mass. The data used here is produced by the Center for Space Research, University of Texas.
The GRACE satellites measure the total changes in ice mass – at the present time. A number of these changes occur, however, due to the fact that the mass of the Earth is constantly changing as a sort of delayed consequence of previous changes in the size of the Ice Sheet (similar to the way in which a sofa slowly regains its shape when you stand up). This is known as Glacial Isostatic Adjustment. It should be noted that the changes in mass illustrated here have not been corrected for the changes in mass due to Glacial Isostatic Adjustment.
Radar-based elevation measurements
Changes in height are based on data from ESA’s CryoSat-2 radar satellite. CryoSat-2 has continuously measured the height of Greenland since July 2010. The novel altimeter on-board CryoSat-2 enables us for the first time to map the coastal regions of the Greenland ice sheet with radar altimetry. The coastal regions are the fastest changing regions, and therefore the elevation change recorded by CryoSat-2 is the most accurate to date. The method used to calculate these changes in height has been developed at DTU Space and is explained in detail here: Simonsen and Sørensen (2017)
CryoSat-2 is a so-called radar altimeter mission. The satellite emits a radar signal, which is reflected by the surface of the Earth and picked up by the satellite. The height of the surface can thus be determined from the time it takes for the signal to travel to the Earth and back again.
Using radar altimeters can be somewhat complicated due to the fact that at certain places the signals can penetrate the uppermost layer of snow. This means that the signal is reflected from somewhere down in the snow and not from the physical surface. The signal’s “travel time” is therefore slightly longer. However, this will not occur in areas with regular melting, as is the case at the edge of the ice.