Jökull - 01.10.1998, Blaðsíða 29

Interpretation of glacier variations in Iceland 1930-1995
Tómas Jóhannesson1 and Oddur Sigurðsson
National Energy Authority, Hydrological service
Grensásvegi 9, IS-108 Reykjavík
Abstract - Glacier variations in Iceland since 1930 show a clear response to variations in climate
during this period. Most non-surging glaciers retreated strongly during the early halfofthe mon-
itoring periodyfollowing the warm climate between 1930 and 1940. A cooling climate after 1940
led to a slowing ofthe retreat and many glaciers started to advance around 1970. A warming of
the climate since about 1985 has led to an increased number of retreating glaciers in recent
years. The variations of non-surging glaciers in Iceland since 1930 appear to be caused primari-
ly by variations in temperature as there are no long term variations in precipitation over this pe-
riod. Variations of surge-type glaciers are dominated by the surge events. Climate variations do,
nevertheless, seem to play a role in observed long term variations ofsome surge-type glaciers al-
though they are more difficult to interpret thanfor the non-surging glaciers.
INTRODU CTION
On the basis of the original sources, observations
of glacier variations in Iceland from 1930 to 1995
have been checked, corrected and computerised by
Sigurðsson (this volume). In addition to measurements
of the advance and retreat of the monitored glacier
tongues, this compilation includes descriptions and lo-
cation maps of the glaciers and graphs of the varia-
tions at the individual measurement locations. The
present paper presents selected records from this new
data set and summarises the advance/retreat records
from non-surging glaciers in light of measured varia-
tions in the climate of Iceland since 1930.
Ice caps and glaciers cover more than 10% of the
area of Iceland and they receive on the order of 20%
of the total precipitation that falls on Iceland. When
the glaciers are close to a steady state, they thus con-
tribute on the order of 20% to river runoff and infiltra-
tion to groundwater aquifers in Iceland. The relative
importance of glacier runoff is greater in highland
areas. Glacier runoff is, therefore, particularly impor-
tant for hydropower purposes, which are of great eco-
nomic value for Iceland.
It is estimated that during the next decades the
mean surface air temperature of the Earth will rise at a
rate between 0.1 and 0.35 °C per decade due to in-
creasing concentration of C02 and other trace gasses
in the atmosphere (Houghton and others, 1996). If rea-
lised, this warming will have pronounced effects on
glaciers and ice caps and lead to large runoff changes
in glaciated areas. Climatic warming on the order of
0.3 °C per decade can temporarily increase glacier
runoff from some Icelandic ice caps and glaciers by
more than 50% and lead to a decrease in volume by
approximately 40% of some glaciers over the next
century (Jóhannesson and others, 1995; Jóhannesson,
1997; Sælthun and others, in press).
The advance and retreat of glaciers is an indicator
of climate changes and reconstructions of past varia-
tions of glaciers from geological evidence or historical
observations are an important tool for deductions
about past climate. Information about many past cli-
mate changes has in part been deduced from records of
1 Present affiliation: Veðurstofa fslands (Icelandic Meteorological Office), Bústaðavegi 9, IS-150 Reykjavík, Iceland
JÖKULL, No. 45, 1998
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