Jökull - 01.01.2020, Side 8

Glacier extent in Iceland, 1890–2019
HISTORICAL AND GLACIOLOGICAL
BACKGROUND
Widespread glacier advances manifest the LIA cool-
ing in Iceland, and most glaciers reached their great-
est historical (that is after 874 CE in Iceland) ex-
tent during the LIA, with a maximum recorded in the
late 19th century (e.g. Þórarinsson, 1943; Eyþórsson,
1935, 1981; Guðmundsson, 1997; Sigurðsson, 2005;
Flowers et al., 2007; Kirkbride and Dugmore, 2008;
Geirsdóttir et al., 2009, 2019; Larsen et al., 2011;
Hannesdóttir et al., 2015a; Björnsson 2009, 2017),
although some glaciers reached a similar extent al-
ready during the 18th century (e.g. Þórarinsson, 1943;
Thoroddsen, 1958; Bradwell et al., 2006; Kirkbride
and Dugmore, 2008; Harning et al., 2016). The maxi-
mum LIA extent is the largest post-Preboreal extent of
many glaciers, in particular the larger outlet glaciers
of the main ice caps (Þórarinsson, 1943; Eyþórsson,
1981; Flowers et al., 2008; Geirsdóttir et al., 2009,
2019). Mapping and dating of Neoglacial moraines
have revealed glacier advances of similar extent as
during the LIA in a few locations (e.g. Guðmunds-
son, 1997; Kirkbride and Dugmore, 2006). However,
pre-LIA moraine remnants are found tens to hundreds
of metres outside the LIA limit of some glaciers –
for example the Stóralda moraines of Svínafellsjök-
ull (Þórarinsson, 1956), the outermost moraines of
Sólheimajökull (e.g. Schomacker et al., 2012) and
in front of Fjallsjökull and Kvíárjökull (Björnsson,
1998), Kaldalónsjökull (Brynjólfsson et al., 2015)
and Kötlujökull (Schomacker et al., 2003). The max-
imum Neoglacial extent of glaciers in Tröllaskagi
is typically only slightly beyond the maximum LIA
extent indicating that the glacier dimensions during
the LIA largely reflect the post-Preboreal Holocene
maximum extent (Stötter et al., 1999). Neverthe-
less, the Neoglacial advances for some glaciers were
more extensive than those of the LIA (e.g. Kirkbride
and Dugmore, 2001; Kellerer-Pirklbauer et al., 2007;
Fernández-Fernández et al., 2019).
Studies on glacier variations of the LIA have been
based on dating landforms in the proglacial area, by
tephrochronology, radiocarbon dating and lichenome-
try (e.g. Guðmundsson, 1997; Sigurðsson, 2005). In
recent decades, more continuous records on glacier
fluctuations have been obtained from sediment cores
from lakes proximal to the glaciers or affected by
glacial meltwater (e.g. Striberger et al., 2011; Larsen
et al., 2015; Harning et al., 2016; Geirsdóttir et al.,
2019).
Many glaciers started retreating from their LIA
terminal moraines in the last decades of the 19th cen-
tury. The retreat accelerated in the 1930s, as a result
of rapid warming starting in the 1920s (Figure 2). Due
to cooler summers after the 1940s, the glacier retreat
slowed down, and most glaciers advanced or halted
their retreat in the period 1960 to 1990. Almost all
glaciers in Iceland started retreating again in the mid-
1990s, and the retreat has been particularly rapid since
the year 2000. Figure 3 shows the relative proportion
of advance and retreat of non-surging glacier termini
since the start of regular observations of terminus vari-
ations in Iceland in the 1930s (Sigurðsson, 1998).
Glacier variations in Iceland show a clear relation-
ship with variations in climate. In-situ glacier mass-
balance measurements, geodetic mass-balance esti-
mates, degree–day mass balance and energy balance
models of selected glaciers, indicate that the mass bal-
ance is mainly governed by variation in summer tem-
perature and winter precipitation (Jóhannesson and
Sigurðsson, 1998; Aðalgeirsdóttir et al., 2006; Flow-
ers et al., 2007; Björnsson and Pálsson, 2008; Guð-
mundsson et al., 2009, 2011; Pálsson et al., 2012;
Björnsson et al., 2013; Schmidt et al., 2017; Belart
et al., 2019, 2020). There is a strong spatial mass-
balance gradient over Iceland. Glaciers located close
to the south and west coast experience higher decadal
mass-balance oscillations, and they have higher mass-
balance sensitivity to changes in summer temperature
and winter precipitation, than the more inland, eastern
and northern glaciers (e.g. Magnússon et al., 2016;
Belart et al., 2020). This difference can probably be
explained by differences in local climate, related to
the pattern of oceanic currents surrounding Iceland
(Hock and others, 2005; Björnsson et al., 2013; Be-
lart et al., 2020).
DATA AND METHODS
The outlines of Icelandic glaciers at different times
have been drawn by several research groups in re-
JÖKULL No. 70, 2020 5