Greenland Ice Sheet Mass Balance 
Niels Reeh 
Department of Physical Glaciology, Geophysical Institute, University of Copenhagen 
Haraldsgade 6, DK-2200 Copenhagen N, DENMARK 
1. The mass balance equation 
The mass balance equation, which expresses the law of mass conservation for a glacier, 
may be written: 
Vv 
2> = Qp - Qm - QcaLF , (1) 
where V is the glacier volume, t is time, Qp is the annual precipitation, Qy and QcapF are the 
annual volume losses by melting and calving of icebergs, respectively. All volumes are 
expressed in terms of water equivalents. According to equation (1), the total water loss, Qy, = 
Qm + @caLF may be calculated as 
QL = QP -3 (2) 
i.e. as the total precipitation over the glacier minus the time rate of ice-volume increase. 
2. Areal distributions and ice volumes 
The following table shows the distribution of glacierized and ice-free land in Greenland 
(Holtzscherer and Bauer, 1954). 
Area (km2) % 
Greenland ice sheet 1.730.000 79 
Local glaciers 80.000 4 
Ice-free land 380.000 17 
Total 2.190.000 100 
For the Greenland ice sheet, the accumulation and ablation areas constitute 1.440.000 km2 
and 290.000 km2, i.e. 84% and 16% respectively, of the total ice sheet area. The volume of the 
Greenland ice sheet has been estimated at 2.400.000 km3 of water equivalent (Holtzscherer and 
Bauer, 1954), whereas the volume of the local glaciers has been given by Weidick (1975b), as no 
more than 100.000 km3. 
3. Estimates of actual mass balance 
Several estimates have been made for the actual gain, loss and total balance of the 
Greenland Ice Sheet. They have been summarized by Weidick (1984) as follows: 
Accumulation 500 + 100 km3 of w. eqv. 
Melting 295 + 100 km3 of w. eqv. 
Calf-ice 260 + 60 km3 of w. eqv. 
The estimates show, that the Greenland Ice Sheet is believed not to be greatly out of 
balance under the present climatic conditions. This is confirmed by observations of recent 
thickness changes of the ice sheet, which generally show average thinning-rates on the order of 
0.2 m a! in parts of the ablation area of southern and central west Greenland (Bauer et al., 
1968a; Weidick, 1968; Seckel, 1977) and thickening-rates on the order of 0.1 m a™! in central 
Greenland (Seckel, 1977). 
3.1. Actual precipitation-distribution 
The map in Fig. 1 shows the actual distribution of precipitation over Greenland. The map 
is based on accumulation rate data for the ice sheet, obtained by firm stratigraphic methods (pit 
studies, ice corings) and precipitation records from coastal stations. Compared to previous 
compilations (e.g. Mock, 1967 and Radok et al., 1982) the data comprises results obtained by the 
Greenland Ice Sheet Program (Langway et al., 1984) which has supplied new information in par- 
ticular as regards the southern Greenland Ice Sheet (Reeh and Clausen, in prep.). The main 
difference between Fig. 1 and previous maps (e.g. Mock, 1967) is, that the accumulation rate 
over the southwestern slope of the ice sheet between 65N and 69N shown in Fig. 1, is only about 
50% of the corresponding values given by Mock (1967). 
