Jökull - 01.01.2020, Side 12

Glacier extent in Iceland, 1890–2019
Tafla 2, frh. – Flatarmál jökla á mismunandi tímum frá lok-
um litlu ísaldar, upplýsingar um gögn sem notuð eru til þess
að draga útlínur jöklanna og heimildir eftir því sem við á.
Dálkhausar gefa til kynna gögn sem notuð eru í hverju til-
viki. GE: jökulummerki frá litlu ísöld. Ef engra heimilda
er getið, þá hefur útlína þess jökuls verið dregin eða end-
urtúlkuð í þessari grein. Flatarmál minnstu jöklanna (átta
neðstu línurnar) er innan sviga í þeim tilfellum sem flatar-
málið er áætlað út frá þekktum flatarmálsbreytingum fimm
jökla af svipaðri gerð/stærð. Athugið að í sumum tilfell-
um er flatarmál jökla frábrugðið fyrri útgefnum tölum, eins
og til dæmis þeim sem birtust í grein Helga Björnssonar
og Finns Pálssonar (2008) sem og á Jöklakortinu (Oddur
Sigurðsson o.fl., 2017) vegna þess að urðarkápur jökla eru
samkvæmt GLIMS skilgreiningu hluti af jöklinum en einn-
ig hafa útlínur verið endurtúlkaðar í einhverjum tilfellum.
Síðasti dálkurinn sýnir hlutfallslega stærð jökuls árið 2019
miðað við hámarksstærð hans á litlu ísöld.
Digital elevation model (DEM) differencing can
help identifying the active part of the glacier termi-
nus, as distinct from stagnant ice, isolated from the
surroundings and not moving, which is thus practi-
cally not a part of the glacier (e.g. Vincent et al., 2016;
Mölg et al., 2018; Tanarro et al., 2019). For glaciers,
that terminate at higher elevation, snow often makes
the glacier margin hard to distinguish in many areas,
for example on the Fimmvörðuháls mountain pass
and on the south side of Drangajökull. Small, peren-
nial or late-summer seasonal snow patches are some-
times difficult to distinguish from glacier ice on satel-
lite and aerial images (e.g. Sigurðsson et al., 2014;
DeVisser and Fountain, 2015; Selkowitz and Forster
2016; Leigh et al., 2019, and references therein).
Ice patches are ice bodies without movement by flow
or internal motion (e.g. Serrano et al., 2011). Dis-
tinguishing seasonal snow patches from glaciers or
perennial snow based on one-time photography is im-
possible. Only by tracking the features over a num-
ber of years can the seasonal or perennial nature of
each feature be determined. The large ice caps domi-
nate the area covered by glaciers in Iceland so uncer-
tainty about small snow or ice patches, or perennial
and seasonal snow, does not have a large effect on the
estimate of the total area of the glaciers in a relative
sense.
The area of some of the small glaciers (< 3 km2)
(Figure 2 and Table 1) is known from the mapping
of the glacier outline ∼1890, around 2000, in 2014
and 2017. Their areal extent in 1945, 1973, ∼2010
and 2019 is estimated with statistical regression based
on the known area of other small glaciers at those
times (Snæfellsjökull, Hrútfellsjökull, Kaldaklofsjök-
ull, Snæfell, Ok glacier). For all maps presented be-
low, a combination of elevation hillshades from three
main sources, ArcticDEM mosaic tiles (Porter et al.,
2018), lidar data sets from the Icelandic IPY glacier
mapping campaign (Jóhannesson et al., 2013), and an
elevation data set from the National Land Survey of
Iceland published in 2016, was used as a background.
The reconstructed Little Ice Age maximum extent
In many areas, well preserved glacial geomorpholog-
ical features, including terminal and lateral moraines,
trimlines and glacier erratics, delineate the maxi-
mum LIA extent of the glaciers. Glacially eroded
and sculptured landscapes and differences in vege-
tation cover also give an indication of the possible
extent of the glaciers during the LIA. Reconstruc-
tion of the maximum LIA glacier extent has been
based on glacial geomorphological features identified
on oblique and vertical aerial photographs and satel-
lite images as well as by detailed field investigations
(e.g. Þórarinsson, 1943; Sigurðsson, 2005; Pálsson et
al., 2012; Hannesdóttir et al., 2015a; Evans, 2016a,
2016b; Guðmundsson et al., 2017). In some cases,
the LIA terminal moraines are shown on the oldest
reliable maps from 1905 (surveyed in 1902–1904),
which were based on the geodetic surveys of the Dan-
ish General Staff (Nørlund, 1944; Böðvarsson, 1996).
These maps do not cover the whole country, but they
include the southern stretch of Vatnajökull, a few out-
lets of Mýrdalsjökull, Eyjafjallajökull, Snæfellsjök-
ull and Drangajökull. Additionally, historical docu-
ments, maps and photographs from the 19th century
to the early 20th century have been used in previous
studies to constrain the maximum LIA extent (e.g.
Thoroddsen, 1911, 1958; Þorkelsson, 1918; Bárðar-
son, 1934; Magnússon, 1955; F. Björnsson, 1993,
1998; Guðmundsson et al., 2012). The maximum LIA
extent of some glaciers has been studied in more de-
tail by glacial geomorphological mapping in the field.
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