Jökull - 01.01.2012, Page 92

F. Pálsson et al.
1.0
1.5
2.0
2.5
b w
(
m
w
e )
-2.0 -1.0 0.0
bn ( mwe )
-5
-4
-3
-2
-1
b s
(
m
w
e )
1.8 m
-1.8 m
Figure 10. Langjökull: measured bn plotted against
bs and bw both suggest zero balance turnover of 1.8
mwe. – Samband ársafkomu við sumar- og vetrar-
afkomu. Það bendir til að jökullinn yrði í massajafn-
vægi við vetrarákomu og sumarleysingu sem næmi 1.8
m að vatnsgildi.
1000
1200
1400
E
LA
(m
a
.s
.l.
)
-2.0 -1.0 0.0
bn ( mwe )
0
20
40
60
A
A
R
(
%
)
Figure 11. Langjökull: measured bn plotted against
ELA (red on the south mass balance profile, blue -
western profile on the north dome) and AAR. – Vensl
ársafkomu og hæðar hjarnmarka hvers árs (efri) og
hlutfalls safnsvæðis af heildarflatarmáli (neðri).
firn) must also be considered. Over long periods of
mass loss, most of the mass lost is ice and hence the
specific density can be assumed 900 kg m−3.
The volume change of Langjökull from autumn
1997 to autumn 2004 was calculated by integrating
over the elevation difference of the surface DEMs of
spring 1997 and autumn 2004 (Figure 13), correcting
for the summer balance of 1997 (-2.98 km3we), yields
the total volume difference 8.5 km3we. The mass loss
by summing the annual mass balance DEMs (derived
by in situ field measurements) yields 8.8 km3we. The
difference between the two estimates is only in total
0.3 km3we or ∼0.05 mwe yr−1 (in terms of specific
balance), two orders of magnitude less than the zero
mass balance turnover (∼1.8 mwe yr−1). Considering
our estimate of 15% accuracy in the mass measure-
ments the agreement is remarkable, and indicates that
the methodology used to create the mass balance maps
from the in situ measurements is random for each year
rather than creating a bias.
The glacier area and ice volume estimates, derived
from available surface maps, are given in Table 2 and
Figure 14. The glacier area extent has decreased from
∼1029 km2 in 1937 to 906 km2 in 2004 (∼12%) and
the corresponding volume from 229 km3 to 188 km3
(∼18%). According to our observation, the LIA maxi-
mum area extent was 1093 km2. Area-volume scaling
relationship of V = cAγ has been suggested by Bahr
(1997). The observed volume change of Langjökull
during the study period (1937–2004) is however too
small compared to the present ice volume, to quan-
tify the equation coefficients. Furthermore, the coef-
ficients are extremely sensitive to only small (or neg-
ligible) changes in our data. Over a narrow band of
90 JÖKULL No. 62, 2012