Jökull - 01.01.2019, Side 30

Terminus lakes on the south side of Vatnajökull
ditional ablation due to the presence of the lake may
then be crudely estimated from the lake area times a
heat flux on the order of 80 W m−2. A lake with suf-
ficient supply of ice fragments may therefore melt on
the order of 5–10 mwe a−1 of ice, which is similar to
the rate of ice melt at elevations close to sea level in
the ablation area of S-Vatnajökull. Thus, lakes with
sufficient calving flux can effectively be considered
an extension of the ablation area of the ice cap.
Glaciers adjust to changes in climate through a
feedback between mass balance and glacier extent
such that the extent is reduced during times of neg-
ative mass balance and the glacier tends to approach a
steady state and vice versa during periods of positive
mass balance, when glaciers tend to advance. This
stabilizing feedback is partly decoupled when glacial
lakes or lagoons form in front of retreating termini
because calving of the glacier terminus may fill the
lake with melting ice fragments. Melting of ice in the
∼60 km2 of glacial lakes by termini of S-Vatnajökull
has become a significant component in the mass bal-
ance of the ice cap, and this component may be ex-
pected to become increasingly important in the fu-
ture if the area of the lakes continues to increase at a
rate similar to that in the period 2000–2018. Many of
the outlet glaciers are located in overdeepened troughs
where the lake depth increases with the retreat of the
terminus. This tends to increase the calving of the
ice front and the lake area in a positive feedback loop
even when the glacier surface mass balance does not
become more negative.
All the terminus lakes by S-Vatnajökull are con-
tinuously drained by glacial rivers. None are dammed
up by terminal moraines and, therefore, they are not
a potential source of dangerous outburst floods down
to the lowlands through the typical release mechanism
of jökulhlaups from moraine-dammed lakes, such as
the many newly formed lakes in the Himalaya (Wester
and others, 2019). On the other hand, the location
of the river outlets from the lakes may change and
call for the construction of dykes for the protection
of roads and vegetated areas. However, sedimentation
in the lakes leads to increased erosional power of the
river downstream from the outlets and helps to keep
the rivercourses stable.
A potential flood danger from the terminus lakes
stems from landslides into the lakes that create
tsunami waves with high run-up on lake shores and
sudden jökulhlaups propagating as flash floods down
the glacier rivers from the lakes. This applies es-
pecially to the lakes at the termini of Öræfajökull,
from Morsárjökull eastward to Fjallsjökull, as well as
Heinabergsjökull and Hoffellsjökull, all of which are
popular tourist attractions.
The data collected in this paper are important for
the local community in the districts south of Vatna-
jökull and for the Vatnajökull National Park. They
are useful for the planning and operation of commu-
nication lines (roads, bridges), for land use planning
and public safety. The data call for further analysis
for evaluating the future growth of the terminus lakes.
Such analysis needs to be based on the surface and
bedrock topography of the glaciers and on scenarios
for future changes in the climate.
Acknowledgements
The study was partly financed with support from the
Friends of Vatnajökull Association (www.friendsof-
vatnajokull.is) and the Landsvirkjun Energy Research
Fund. We thank Dan Shugar and an anonymous re-
viewer for constructive comments on an early version
of the manuscript.
Jaðarlón við sunnanverðan Vatnajökul
Á 20. öld tóku lón að myndast framan við jökulsporða
við sunnanverðan Vatnajökul, sem þá voru farnir að
hopa eftir að hafa náð sögulegri hámarksútbreiðslu í
lok litlu ísaldar, um 1890. Mörg sporðlón mynduðust
á fjórða áratug 20. aldar og eftir því sem leið á öldina
fjölgaði þeim. Flest þeirra stækkuðu hægt uns kom
fram á miðjan tíunda áratuginn. Þá tóku þau að stækka
ört ár frá ári og vaxa nú hratt vegna hlýnandi loftslags.
Á sama tíma hafa jökulstífluð lón í inndölum rýrnað
vegna þess að jöklarnir hafa þynnst og jökulhlaup úr
þeim orðið minni en áður var eða tekið fyrir þau með
öllu.
Í þessari grein eru raktar breytingar á jökullónum
á Suðausturlandi, frá Skeiðarárjökli vestan Öræfa, og
austur að Hoffellsjökli í Hornafirði. Megináherslan er
á sporðlón en einnig sagt lítillega frá jökulstífluðum
JÖKULL No. 69, 2019 29