Jökull - 01.12.1970, Blaðsíða 61

m
Fig. 10. Areal-elevation curve of Breidamerk-
urjökull, where its three main branches are
shown separately.
Mynd 10. Línurit yfir hœðarskiptingu Breiða-
merkurjökuls.
the recession
OF VATNAJÖKULL
It can be considered highly probable that
the recession of Breidamerkurjökull is well re-
presentative for the glacial shrinkage on other
parts of Vatnajökull. The measured recession
of Breidamerkurjökull has therefore been used
as a basis for the computation of the total
recession of Vatnajökull during the period
1894—1968. The results of these computations
are represented in Table 4, where this period
is subdivided into two successive intervals 1894—
1932 and 1932—1968, and the recession for each
period estimated to be proportional to the
retreat of Breidamerkurjökull. By assuming the
same glacial thinning in each altitude interval
as for Breidamerkurjökull, the total shrinkage
of Vatnajökull has been 268 km3 (Table 3).
The areal-elevation curve of Vatnajökull was
obtained from Bauer’s measurements (1955).
If on the other hand the average thinning of
Vatnajökull as a whole was assumed to be
equal to that of Breidamerkurjökull (Table 2),
the calculated total recession amounted to as
much as 350 km3. The two approaches thus
differ considerably. Both these results are
shown in Table 4 where the latter is given in
brackets. A probable reason for this difference
is the fact that Breidamerkurjökull covers that
part of Vatnajökull, where the precipitation is
at its maximum. The average elevation of the
firnline is for that reason considerably lower
there than on Vatnajökull. Fig. 10 reveales a
very great difference in the areal-elevation of
the three main glaciers forming Breidamerkur-
jökull. To some degree this difference may be
caused by difference in the elevation of their
subsurface. Yet difference in precipitation is a
more decisive factor because it is undoubtedly
far the least on the accumulation area of
Nordlingalaegdarjökull, which lies farther away
from the coast and furthermore in a rain-
shadow. It is difficult to evaluate which of
these differing results is the more reliable be-
cause of the interplay of many meteorological
factors. Both methods of calculation should be
given some credit. Thus the actual shrinkage
of Vatnajökull in all likelihood amounts to
somewhere between 268 and 350 km3 of ice.
DISCUSSION
According to Bauer’s (1955) calculations the
total volume of Vatnajökull is 3520 km3 of ice.
The recession is thus 8—10%. To waste the
Kingdom of Vatnajökull with the same rate of
melting to that after 1930 would take 500—
600 years. The annual average thinning (Table
4) of Vatnajökull during the period 1932—1963
was 60—80 cm/year in water equivalent. The
thinning of Langjökull was about 60 cm/year
for the period 1922—1966 while on Hofsjökull
the thinning was somewhere between 25—60
cm/year in the years 1938—1945, but the maps
of the latter are unreliable (Sigbjarnarson
JÖKULL. 20. ÁR 59