Jökull - 01.01.2012, Side 84

F. Pálsson et al.
Figure 1. Location of Langjökull (L), Hofsjökull (H), Eyjafjallajökull (E), Vatnajökull (V), Snæfellsjökull (Sn),
Tindfjallajökull (T) and Torfajökull (To) ice caps, Iceland. Stykkishólmur (St) and Hveravellir (Hv) are lo-
cations of meteorological stations. – Íslandskort sem sýnir legu helstu jökla og tveggja veðurstöðva (St:
Stykkishólmur, Hv: Hveravellir).
that some of the other outlets may also surge (Palmer
et al., 2009). Due to the low lying surface eleva-
tion span, and distribution of surface area with ele-
vation (large areas close to typical ELA), (Figure 2),
Langjökull is more sensitive to climatic variation than
the higher elevated Vatnajökull ice cap (8100 km2;
50–2110 m elevation span) and Hofsjökull ice cap
(900 km2; 500–1800 m) (Aðalgeirsdóttir et al., 2006;
Gudmundsson et al., 2009a; Figure 1).
In April/May 1997 the bedrock and surface topo-
graphy of Langjökull was mapped by radio echo
sounding and DGPS surveys along profiles approxi-
mately 1 km apart (Figures 2 and 3). The surface mass
balance and velocity has been measured at 22–23 sites
each year since 1997 and glacio-meteorological ob-
servations were initiated in 2001 (Figure 4a; Björns-
son et al., 2002; Gudmundsson et al., 2009b). This
data has been used to calibrate models to predict the
past evolution and future outlook of the glacier (Flow-
ers et al., 2007, Gudmundsson et al., 2009a; 2009b).
In addition to the 1997 surface map, three surface
maps exist based on measurements in 1937, 1945
and 1986, and a surface map constructed using stereo
image pairs, acquired on 12, 14, 17 and 19 August
2004 with the optical HRG sensor onboard the SPOT5
satellite (Berthier et al., 2004, 2006). Furthermore, the
margin of the Little Ice Age maximum extent (∼1890)
was mapped and area-volume scaling method used to
estimate the corresponding ice volume.
In this paper, we explain the methods we use to
construct the 1997 and 2004 maps of the glacier and
how those maps, along with DGPS and kinematic
GPS observations on and around the glacier, are used
to improve the accuracy of older maps. We describe
the mass balance based on annual in situ measure-
ments, the long term net balance variation from vol-
ume changes deduced from the existing surface maps,
as well as the mass balance sensitivity to both summer
and annual temperature variations.
82 JÖKULL No. 62, 2012