Jökull - 01.01.2020, Síða 15

Hannesdóttir et al.
the major glaciers in Iceland. The warm spring and
summer of 2019 enhanced melting of seasonal snow,
exposing glacier margins, and enabled the delineation
of the actual ice margin for the first time in several
years at many locations. Orthorectified aerial images
from the company Loftmyndir ehf. were also used in
some areas. The smallest glaciers (< 3 km2, shown in
blue in Figure 1) and the groups listed in Table 1 were
not digitized from those images and changes in their
extent were assumed to be negligible compared with
2017.
Glacier surges
Changes in the extent of Icelandic glaciers not only
depend on variations in mass-balance driven by cli-
mate change, but also on factors such as surge activity
(Björnsson et al., 2003), subglacial volcanic eruptions
(Guðmundsson, 2005), and jökulhlaups (e.g. Guð-
mundsson and Larsen, 2013). The largest surges of
glaciers, such as the one of Brúarjökull in 1890 and
1963–1964, that resulted in an advance of the termi-
nus of the entire outlet glacier by ∼10 km with an in-
crease in the glacier area by > 160 km2 (Þórarinsson,
1969; Guðmundsson et al., 1996), have a large effect
on the glacier extent. The 1963–1964 surge of Brúar-
jökull is the only surge in Iceland of this magnitude
since the end of the LIA; other surges in Iceland in
this time period typically being in the range ∼0.3–
2 km as noted before. Most known surge-type outlet
glaciers of the main ice caps, including all the main
outlet glaciers of western Vatnajökull, surged during
the 1990s. The largest surges affecting the variations
of glacier termini are discussed for each glacier in the
corresponding subsections in the Results section be-
low.
Area calculations
In this paper, area calculations are, as is commonly
used in Iceland, made with the national ISN93 co-
ordinate system, which utilizes the Lambert con-
formal conic projection with two standard parallels
(EPSG:3057). This is slightly different from area cal-
culations in the GLIMS glacier database where area
calculations are carried out in the NSIDC EASE-
Grid that uses the WGS-84 datum and a cylindrical
equal-area projection (EPGS:3975). The differences
are, however, not noticeable, except for Vatnajökull,
where the difference amounts to a couple of km2.
Users of the dataset do, however, need to be aware
that area calculations will give different results based
on the coordinate systems they choose to use. For the
glacier area calculations, ice patches that have become
detached from the main body of each glacier since the
LIA are included in the glacier area.
RESULTS
The retreat and advance history of glaciers in Iceland
since the end of the LIA is fairly consistent across
the country according to the outline data set. Most
glaciers started to retreat from the outermost termi-
nal LIA moraines in the 1880s and 1890s. The retreat
accelerated after 1900 and almost all glacier termini
retreated rapidly in the 1930s and 1940s, with occa-
sional exceptions due to surges (Figure 3). The re-
treat slowed down, most termini stagnated and many
glaciers readvanced after 1960. The glaciers started to
retreat again due to increased temperatures after 1995
(Figure 2). The retreat rate of many glaciers since
2000 has been similar to that in the 1930s and the
1940s, although a few glaciers show up to double re-
treat rates during the latter period compared with the
earlier one (see spordakost.jorfi.is).
In the following subsections, maps of the main ice
caps and glaciers are presented with the outlines of
the maximum LIA glacier extent in ∼1890 (except for
Drangajökull at ∼1850), in 1945–1946, 1970–1980,
∼2000, 2007–2013 and 2019, see Table 2 for infor-
mation about each outline and the corresponding area.
Vatnajökull and Tungnafellsjökull
Vatnajökull ice cap lost 1069 km2 during the period
∼1890–2019, equal to 12% of its maximum LIA ex-
tent, whereof close to half of the area loss (460 km2)
occurred during the period ∼1890–1945 (Table 2).
The main area loss occurs at the large outlet glaciers
to the south, Breiðamerkurjökull and Skeiðarárjök-
ull, Tungnaárjökull to the west, and Brúarjökull and
Eyjabakkajökull to the north (Figure 4). They are all
surge-type glaciers. The rate of area change was high-
est during the first 2 decades of the 21st century, on the
order of −36 km2 a−1 to −44 km2 a−1.
12 JÖKULL No. 70, 2020