Jökull - 01.12.1967, Page 42

years, which corresponds to 1.5 m3/sec increase
in the discharge of the Kolgríma river. The
average thinning per year of the measured
glacier area was about 110 cm in water equi-
valent.
Conclusions
No empirical conclusion can be drawn from
these observations because they are too un-
precise and do not cover field enough, yet
they give some idea concerning some important
points.
The average annual thinning of Langjökull
was found to be 37 cm during the period 1890—
1966, but 60 cm/year in the years 1922—1966.
This result is in a good accordance witli com-
parable investigations. Thorarinsson (1940) has
compiled the results of measurements of the
thinning rate of some European and Artic
glaciers accomplished in the years 1925—1939.
According to these measurements the average
annual thinning rate on these glaciers proved
to be 37 cm. Liest<J>l (1962) has calculated the
average thinning of the Tverrábreen in Norway
to have been 85 cm/year during the period
1927—1942. Glacier investigations in Norway in
the water budget years 1962/63 and 1963/64
give variation in balance of the glaciers rang-
ing from 200 cm/year as an average thinning
to 95 cm/year as an average thickening (0strem
and Liest/il 1964, Pytti and 0strem 1965).
The preceding calculations of the average
thinning of some Icelandic glaciers seem to be
in a good agreement with comparable invest-
igations in Norway. The comparison, there-
fore, seems to manifest, that the method used
by the present author, is quite usable in cal-
culating the variations in the regime of gla-
ciers, at least concerning inland ices.
The immense shrinkage of glaciers in Ice-
land during the last half a century has causecl
an increase in the discharge of the glacier rivers
almost proportional to the glacier coverecl part
of their drainage area. Briefly stated the in-
crease in the discharge of the glacier streams
exceeds 10%, if 30% or more of their drainage
area is covered with glaciers, but the effect of
the glacial recession on the discharge of the
great rivers, as Thjórsá and Ölfusá, is much
less, causing hardly more than 3—4% increase.
These results are in good agreement with
the increase in air temperature. The average
air temperature in Iceland during the last half
a century has been about l]/20 C higher than
it was in the last century (Bergthorsson 1967),
and by this the firn limit has been raisecl for
about 100 m.
It was not until in the 1920ies that glacier
retreat was generally observed (Eythorson 1930),
which is in good agreement with the increase
in average air temperature. Decrease in winter
accumulation can also cause the same trencl,
but it can hardly be so in this case, because
ameliorating climate is probably followed by
increase in precipitation at least in the south-
ern part of the country.
There has been no halt in the retreat of
the larger glaciers in spite of the fact that the
average temperature has been somewhat lower
in the last three years, but this climatic deme-
lioration is hardly comparable to the climate
of last century. The present condition of the
large glaciers may be far from being in equi-
librium with the warmer climate and it is there-
fore probable that this recent lowering in tem-
perature is not great enough to reverse the
present development of these glaciers (Fig. 9).
Fig. 9: Tlie profile SS shows a longitudinal
section of an inland ice which has attained an
equilibrium with a cold climatic epoch. The
firn limit of equilibrium is in point A. If now
the clirnate becomes warmer, the firn limit is
raised to point B, the glacier trends to reach
an equilibrium in the profile TT with the
firn limit at the point Bi, which lies approxi-
mately at the same elevation, or somewhat
liigher, than point B, because the firn limit is
also dependent on the rate of advance of the
glacier. A long time will elapse before tlie
glacier reaches the state of equilibrium in the
profile TT. During that time the profile ol
the glacier is somewhere between the profiles
SS and TT, whereoí the profile YY is an ex-
276 JÖKULL 17. ÁR