Jökull - 01.01.2020, Side 5

Hannesdóttir et al.
et al., 2012; Hannesdóttir et al., 2015a,b; Harning
et al., 2016; Guðmundsson et al., 2017; Fernández-
Fernández et al., 2017, 2019). The ongoing glacier
retreat leads to the disintegration of large glaciers into
smaller ice bodies, the formation of terminus lakes
and increased debris cover. This is increasingly chal-
lenging for glacier monitoring, for example the delin-
eation of glacier boundaries and length-change mea-
surements (e.g. Fischer et al., 2016; Paul and Bolch,
2019).
Glaciers currently cover approximately 10% of
Iceland. They are large freshwater reservoirs and con-
tain the equivalent of ∼3400 km3 of water (Björnsson,
2009, 2017), corresponding to the precipitation in the
entire country over ∼20 years (e.g. Crochet, 2007).
The glaciers influence the hydrology of the country
through the annual mass-balance cycle and changes
in ice volume due to variations in the climate, with
important implications for the hydropower industry
and other water users. The glaciers are dynamic and
highly responsive to changes in climate and have high
annual mass-turnover rates (1.5–3.0 mwe a−1, Pálsson
et al., 2012). Several of the larger ice caps and
glaciers in Iceland cover active volcanic and geo-
thermal zones, causing subglacial eruptions and fre-
quent jökulhlaups (Guðmundsson and Larsen, 2013).
They are affected by geothermal melting, which is
a substantial component in the glacier mass balance
(Björnsson et al., 2013; Jóhannesson et al., 2020), as
they are located in areas of high geothermal heat flux,
including the neovolcanic zone.
This paper describes a national glacier outline in-
ventory for Iceland for the period after the LIA maxi-
mum in the late 19th century, which has been submit-
ted to GLIMS (Global Land Ice Measurements from
Space, nsidc.org/glims). Hitherto only the glacier
outline from around the year 2000 has been avail-
able in digital form at the international snow and ice
database. The outlines were collected by several re-
search groups and institutions and are described in
more detail in other scientific papers. They have been
reviewed and updated for consistency, and the most
reliable or representative outline chosen, from several
available outlines for the same glacier. This paper pro-
vides general information about the glacier outlines,
as well as a simple interpretation of the glacier vari-
ations that they represent, but readers are referred to
the original papers for more detailed information. The
paper is not intended as a comprehensive review paper
about post-LIA glacier variations in Iceland. Rather, it
provides background information about the updated,
multi-temporal GLIMS glacier variations data set for
Iceland in order to make it more useful for other stud-
ies of glacier variations and related research.
STUDY AREA
The largest ice caps in Iceland are located in the south-
ern and central highlands (Figure 1), where prevail-
ing southerly winds deliver a large amount of pre-
cipitation, on average 4000–5000 mm a−1 in the up-
per accumulation area of Vatnajökull and Mýrdals-
jökull, reaching a maximum of ∼7000 mm a−1, and
ca. half of that on Langjökull and Hofsjökull (Björns-
son and Pálsson, 2008; Björnsson, 2009, 2017). The
balanced-budget equilibrium line altitude (ELA) of
Vatnajökull ice cap lies on average around 1000 m
a.s.l. on the southern outlet glaciers, compared with
1350 m a.s.l. on the inland outlet glaciers (Pálsson
et al., 2019). The balanced-budget ELAs of the ice
caps in central Iceland are at ca. 1100–1300 m a.s.l.
on Hofsjökull and 1000–1200 m a.s.l. on Langjök-
ull (Thorsteinsson et al., 2017; Pálsson et al., 2012,
respectively). The ELA of Mýrdalsjökull has been
estimated from satellite images (from the location
of the firn line) and is at ca. 1000 m a.s.l. on the
east- and southeast-flowing glaciers vs. 1200 m a.s.l.
on the inland and southern outlets (unpubl. data from
the IMO). Typically, only 10–20% of the bed of the
glaciers lies above the current ELA; thus, the larger
ice caps exist mainly due to their own thickness
(Björnsson and Pálsson, 2008). Several mountains
reaching over 1400 m a.s.l. maintain small glaciers in
the central highlands. A few ice-capped volcanoes ex-
ist outside the neovolcanic zone, including Snæfells-
jökull and Mt. Snæfell. Drangajökull in Vestfirðir, the
fifth largest ice cap, has the lowest ELA (600–700 m
a.s.l.) of glaciers in Iceland (Björnsson, 2009, 2017).
An overview of the ice caps and glaciers analysed in
this paper is found in Table 1, including the elevation
range and the area span in ∼1890–2019.
2 JÖKULL No. 70, 2020