Current Surface Mass Budget of the Greenland Ice Sheet
Here
you can follow the daily surface mass balance on the Greenland Ice
Sheet. The snow and ice model from one of DMI’s climate models is driven
every six hours with snowfall, sunlight and other parameters from a
research weather model for Greenland, Hirlam-Newsnow. We can thereby
calculate the melting energy, refreezing of melt water and sublimation
(snow that evaporates without melting first). The result of this is a
change in the snow and ice from one day to the next and this change is
shown below. All numbers are in water equivalent, that is, the amount of
water the snow and ice would correspond to if it was melted.
Left: Map of the surface mass
balance today (in mm water equivalent per day). Right: The average
surface mass balance for today’s calendar date over the period
1990-2011.
In the above figure, we show the daily surface mass
balance (on the left) and you can see where it has snowed and melted
(incl. sublimate) on the ice sheet over the last 24 hours. For
comparison, the map to the right shows the average value for the same
calendar date over the period 1990-2011. This historical average is
based on weather from a somewhat different model and the two are
therefore not always strictly comparable.
The surface mass balance is calculated over a year
from September 1st to August 31st (the end of the melt season). The
figure to the right shows the sum of all the daily changes from
September 1st up to today. Next year on September 1st the map will be
reset and we start over. This accumulated map illustrates how much the
surface mass balance has contributed in each point across the ice
sheet.
The figure below shows the total daily contribution
from all points on the ice sheet (top) and the same accumulated from
September 1st to now (bottom). The blue curves show this season’s surface mass
balance in gigatons (Gt: 1 Gt is one billion tons and corresponds to 1
cubic kilometer of water), and for comparison the mean curves from the
historical model run are shown with two standard deviations on either
side. Note that the accumulated curve does not end at 0 at the end of
the year. Over the year, it snows more than it melts, but calving of
icebergs also adds to the total mass budget of the ice sheet. Satellite
observations over the last decade show that the ice sheet is not in
balance. The calving loss is greater than the gain from surface mass
balance, and Greenland is losing mass at the rate of about 200 Gt/yr.
Map of the accumulated surface mass balance (in mm water equivalent) from September 1st to now.
Top: The total daily contribution to the surface mass balance from the entire ice sheet (blue line, Gt/day). Bottom: The accumulated surface mass
balance from September 1st to now (blue line, Gt). The dark grey line
shows the corresponding average number from the climatology along with
two standard deviations on each side (light grey). In the bottom panel,
we also show the season 2011-12 (red) which had very high summer melt in
Greenland.
Ice flow
Due to gravity, ice flows slowly outwards like dough
on a kitchen counter. When snow falls on top of the ice sheet year
after year, the layers below are slowly compressed into ice. In the
central part of the ice sheet, where little if any melt occurs, new
layers will therefore continually be added. The ice does not grow in
height, however, since the extra ice is balanced by the flow away from
the center. Further out towards the coast we find the equilibrium line,
where snowfall and melt are exactly balanced. Below the equilibrium
line, there is more melt than snowfall and here the net mass loss is
countered by the flow coming out from the center of the ice sheet. Here
it is the ice sheet itself which melts.
For an ice sheet that neither grows or shrinks, there is at all points averaged over the year a balance between
- the amount of snow that falls and is compressed to ice
- the amount of snow and ice that melts or evaporates (sublimates) and
- the amount of ice that flows away due to the ice motion
The
two first contributions make up the surface mass balance. For the ice
sheet as a whole, there is a balance between the surface mass balance
and the amount of ice that calves into the ocean as icebergs.
If climate changes, the surface mass balance may
change such that it no longer matches the calving and the ice sheet can
start to gain or lose mass. This is important to keep track of, since
such a mass loss will lead to global sea level rise. As mentioned,
satellites measuring the ice sheet mass have observed a loss of around
200 Gt/year over the last decade.
Greenland Climate Research Centre collaborates with
Danish Climate Centre at DMI on research in both atmospheric impact on
the Greenland Ice Sheet and the ice flow itself and its interaction with
the rest of the climate system.
Three
processes determine whether the ice sheet grows or diminishes.
Accumulation of snow on top increases mass. In time, the snow is
transformed to ice that flows down through the ice sheet and out towards
the margins. Melt in the lower regions of the ice sheet and iceberg
calving from glaciers reduces the mass. If mass loss exceeds mass gain
the ice sheet will shrink. Graphics Diego Winterborg.
For further information please contact climate scientist Peter L. Langen, Danish Climate Centre, iskappe@dmi.dk.
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