Jökull - 01.12.1979, Side 79

The northern outlets of Vatnajökull are passive
compared to the southern ones. The accumulation
as well as the glacier slopes are lower in the north
where the outlets terminate in broad fronts at
higher levels. A similar difference in activity is ob-
served between the northern and southern outlets
of the other ice caps such as Mýrdalsjökull, Hofs-
jökull and Langjökull.
In the maritime Icelandic climate glacier mass
balance is very sensitive to variations in air tem-
perature. A decrease in the average air temperature
greatly increases the amount of precipitation which
falls as snow, especially during the autumn and
spring. Further, ablation is reduced at all ele-
vations. A drop in the annual air temperature by
1°C may lower the firn iine by as much as 200 m.
Any lowering of the firn line will greatly increase
the accumulation areas of the ice caps in Iceland.
GLACIER FLUCTUATIONS IN POST-
GLACIAL TIME
At the maximum of the last glaciation Iceland
was covered with glaciers which stretched out to
the shelf and the neighbouring islands. Some few
nunataks may have reached above the ice cover in
the Tröllaskagi area, in the NW Peninsula and in
eastem Iceland. The ice divide lay south of the
present water divide from the Torfajökull area
northeast towards Kverkfjöll (Fig. 1). The
maximum thickness of the ice cap was of the order
of 1000 m. Separate ice caps lay on the NW Penin-
sula and covered Snæfellsnes.
Two stages of readvance are known during the
recession of the main ice sheet at the end of the
Pleistocene. First, the Álftanes-stage 12500—12000
years B.P. (Older Dryas) when the ice had retreated
onto the present western and northern coast.
Second, the Búdi-stage 11000—10000 years B.P.
(Younger Dryas) after the ice had retreated rapidly
up to the highland in southwestern and northern
Iceland during the Alleröd-interstadial. The glacier
started to retreat very rapidly again about 10000
years B.P. and 8000 B.P. all ice had disappeared
from the area northeast of Torfajökull where the ice
thickness was at maximum during the Pleistocene.
From about 8000 to 2500 years B.P. the climate in
Iceland was considerably warmer and dryer than at
present. The average temperature is believed to
have been 2°C higher than in the period 1920—
1960. During this Climatic Optimum the remains
of the Pleistocene ice presumably disappeared
leaving only small ice caps on the highest moun-
tains like Öræfajökull.
Glaciers in Iceland have experienced two main
periods of growth during Neoglaciation. First dur-
ing the climatic deterioration around 500 years
B.C. at the onset of Subatlantic time. The climate
became colder and presumably precipitation in-
creased. Glaciers expanded from the highest
mountains. Some steep Alpine glaciers reacted
quickly and attained their post-Wiirm maximum.
The outermost moraines fronting Kviárjökull and
Svinafellsjökull (outlets from Öræfajökull) are
probably from this period. Other glaciers expanded
over the highland plateau and built up the present
ice caps in Iceland. In this way Vatnajökull was
formed by outlets from the mountains Öræfajökull,
Grimsfjall, Bárdarbunga, Kverkfjöll, Esjufjöll and
Breidabunga. The newly formed ice caps did not
reach their maximal extent at this time, but first
after the “Little Ice Age”.
The second major glacier advance is historically
recorded in Iceland. At the time of the settlement of
Iceland (874 A.D.) and quite up to the thirteenth
century the climate was similar to that of the warm
period from 1920 to 1960. At that time the glaciers
were smaller than they are today. Some of the
largest outlets from Vatnajökull such as Breida-
merkurjökull and Tungnárjökull were up to 10 km
shorter than at present (Fig. 3). In the fourteenth
century the climate gradually became colder but
the most drastic deterioration in climate set in
during the “Little Ice Age” from 1600 to 1920.
Then the average air temperature was probably 3
to 4°C lower than during the Climatic Optimum
after the Pleistocene. The most extensive glacier
advance culiminated for the steeper glaciers in the
1750’s and in 1850 to 1890 for the broad lobes from
the plateau ice caps (Fig. 4). Breidamerkurjökull
reached its maximum extent in 1894 only 256 m
short of the sea-shore. During the “Little Ice Age”
cultivated land and farms were over-run by ice at
Breidamerkursandur and in the neighbourhood of
Drangajökull. Ice caps formed on the Gláma
plateau in the NW Peninsula and on the mountain
Ok in the South.
A general recession of the glaciers set in during
the 1890’s and became quite rapid after 1930, but
began to slow down during the 1960’s (Fig. 5).
Since 1890 the largest outlets of Vatnajökull have
retreated as much as 2—3 km and the volume of
the ice cap has decreased by the order of 5 to 10%.
The reduction of the smaller outlet glaciers has
JÖKULL 29. ÁR 77