Jökull - 01.12.1979, Blaðsíða 80

Fig. 4. Variation in the
outlet glaciers of Dranga-
jökull and southern
Vatnajökull, (in meters).
From Thorarinsson 1974.
been much more catastrophic. Hoffellsjökull lost
one third of its volume in the period 1890 to 1936.
On Breidamerkursandur an area of 52 km2 became
deglaciated during the period 1894 to 1968. Dis-
tricts cultivated by farmers in the 12th century are
exposed again. The ice caps on Ok and the Gláma
plateau have disappeared. Glaciers fed from large
accumulation areas which lie well above the firn
Fig. 5. Variation of Sólheimajökull, S-Mýrdals-
jökull. Summer temperatures and winter preci-
pitation at Vík in Mýrdalur.
line have retreated later than outlets which were
more drastically affected by the rise in the firn line.
The glacier recession has had important hydro-
logical effects. The reduction in the volume of
glaciers in Iceland during the first half of the cen-
tury amounts to a specific discharge from glaciers
of the order of 20 1/s km2. For the higher located ice
caps such as Hofsjökull and Langjökull the values
are 10 and 15 1/s km2, respectively. A change
towards equilibrium in glacier mass balance would
reduce the specific discharge from these ice caps by
15—20%, with profound consequences for electri-
cal power production.
The general recession of the glaciers has been
interrupted by local short-lived advances of in-
dividual glaciers. Some of the advances may be
explained as rapid response to local climatic oscil-
lations. This may for example apply to the advance
of three outlet glaciers from Drangajökull during
the period 1933 to 1942. On the other hand, many
outlet glaciers have experienced catastrophic ad-
vances. Periodic surges with return periods of
several decades have been observed in most of the
broad-lobed outlets of Vatnajökull. The surge of
Brúarjökull in 1963 to 1964 involved 40% of the
area of Vatnajökull. The ice front advanced as
much as 8 km at velocities up to 4—5 m/hour. In
1890 Brúarjökull advanced 10 km in a surge and
piled up a 20 m high push moraine. Surges are also
typical for outlets from Hofsjökull and Langjökull.
The general retreat of glaciers has slowed down
in Iceland since the 1960’s. The trend towards
cooler summers since the 1940’s has begun to affect
the glacier snouts (Fig. 5). Some of the most
responsive glacier outlets are already advancing
such as the main southern outlets from Mýrdals-
jökull called Sólheimajökull and Höfdabrekkujök-
ull. About 20% of the area of Mýrdalsjökull lies
between 1000 m and 1200 m elevation and the
lowering of the firn line has greatly increased the
78 JÖKULL 29. ÁR