Jökull - 01.12.1967, Blaðsíða 60

Skaftárjökull glacier, western tongue,
about 10—12 km
Tungnaárjökull glacier, southern part,
about 8—10 km
Tungnaárjökull glacier, northern part,
about 5—6 km
The more perpendicular the flow-direction
of the glaciers is to the trend of mountain
ranges the less is their advance. According to
the above-said the glacier can be supposed to
have increased in thickness for 200—300 m at
Kerlingar during the period from 1600 to 1890.
The advance has been far from being uniform
all this time and probably a greater part of it
took place between 1600 and 1750. As an aver-
age the advance of Skaftárjökull has been
about 40 m/year, while it has presumably been
about 60 m/year from 1600 to 1750. Yet no
records exist as to a more rapid advance of
the Skaftárjökull and Tungnaárjökull glaciers
during that period. On the other hand there
are records available as to a more rapid advance
of the glaciers Skeidarárjökull ancl Breidamerk-
urjökull.
Since about 1890 glaciers have been retrcat-
ing. In the first decades the retreat was slow,
but during the last decades it has been more
rapid. Altogether Skaftárjökull has retreated
for 2 km. The southern part of Tungnaárjökull
has also retreated for 2 km while its northern
part has only retreated little, but beconie much
thinner. Yet the retreat is there about half a
km or more. From 1890 Skaftárjökull has re-
treated for 30 m/year as an average, but since
1946 the annual average retreat has been about
50 m.
For a few years measurements of a longi-
tudinal profile of Tungnaárjökull a short way
from Jökulheimar have been carried out by S.
Pálsson, deputy and S. Freysteinsson civil engi-
neer. The profile covers 3—4 km in length and is
measured perpendicular to the glacier margin.
J. Björnsson, chief engineer, has utilized these
measurements in calculating the correlation
between recession and elevation of glacier (pers.
comm.). This correlation he then used in cal-
culating the recession of Tungnaárjökull in
the future. The shape of the glacier at differ-
ent times was calculated in a computer. Björns-
son obtained the result that Tungnaárjökull
will for the most part have disappeared in the
year 2020 with small remnants of it remaining
up to the year 2050.
These speculations give an indication as to
how fast the process of glacier oscillations can
be and has in all probabilities been since the
settlement period.
Aíany authors have presented qualitative
arguments as to a great increase in the size of
glaciers since the colonization of Iceland, but
is has proved very difficult to assess it quantita-
tively how great this increase was. The quantita-
tive results already obtained as a rule signify
minimum values. The results of our investiga-
tions indicate a great increase in the size of
glaciers. For example it can be mentioned that
if parallel changes to those at Tungnaárjökull
have also occurred at Skeidarárjökull ancl
Breidamerkurjökull, which there is no reason
to cloubt, these glaciers ltave hardly existecl in
the beginning of the cold climatic periocl.
During the climatic depression from 1200 to
1400 glaciers liave undoubtedly increased con-
siderably in size. From the experience of the
cold climatic period it can be concluded that
their advance during this period was about
half of what it was btween 1600 ancl 1890.
During the comparatively short, mild period
from 1400 to 1450 glaciers may be supposed to
have retreatecl, yet somewhat less than in the
present century. Anyway, glaciers were more
extensive in the beginning of the colcl climatic
period about 1450 than they were about 1200
and a difference of some km in the extension
of the outlet glaciers of Vatnajökull is prob-
able. It is also probable that these glaciers
were about 20 km shorter in the settlement
period than during their maximum extension
in the nineteenth century.
As a further argument for a ntuch smaller
Vatnajökull before the cold climatic periocl it
is wortli while to mention tales on cross-country
routes on Vatnajökull, e. g. Nordlingalægd,
formerly frequented by fishermen from north-
ern Iceland. This route is now about 80 km
long on glacier and reaches up to about 1350 m
elevation. Before the cold climatic periocl this
route was probably approximately lialf that
length on glacier with its highest part just
exceeding 1000 m. In view of this there is little
reason to dispute that Nordlingalægd was a
294 JÖKULL 17. ÁR