Jökull - 01.01.2012, Side 91

Mass and volume changes of Langjökull ice cap, Iceland, ∼1890 to 2009
Table 1. Specific winter (bw), summer (bs) and net
balance (bn) for Langjökull. A conservative error
estimate is on the order of 15% for both bw and bs.
– Meðal vetrar-, sumar- og ársafkoma Langjökuls.
Year bw bs bn
(m yr−1 ) (m yr−1 ) (m yr−1 )
( w.e.) (w.e.) (w.e.)
1996–1997 1.9 -3.2 -1.3
1997–1998 1.12 -2.82 -1.7
1998–1999 1.39 -2.11 -0.71
1999–2000 2.13 -2.88 -0.75
2000–2001 1.28 -2.55 -1.27
2001–2002 1.57 -3.22 -1.66
2002–2003 2.11 -4.05 -1.95
2003–2004 1.79 -3.28 -1.49
2004–2005 1.62 -2.51 -0.89
2005–2006 2 -3.08 -1.08
2006–2007 1.65 -3.06 -1.41
2007–2008 2 -3.84 -1.84
2008–2009 2.02 -2.39 -0.36
bs
bn
bw
1998 2000 2002 2004 2006 2008
-4
-3
-2
-1
0
1
2
S
pe
ci
fic
m
as
s
ba
la
nc
e
(m
w
e)
Figure 8. Langjökull mass balance record: winter- (bw),
summer- (bs) and net- (bn) specific balance. – Mæld
meðal vetrar-, sumar og ársafkoma Langjökuls.
-3.0 mwe yr−1, or 1.7 times b0−bal; the negative bal-
ance of those 13 years is due to extreme summer abla-
tion. Scatter plots of bn against equilibrium line alti-
tude (ELA) and accumulation area ratio (AAR) (Fig-
ure 11) suggest zero mass balance ELA at ∼1000 m
on the southern dome and ∼1200 m on the northern
dome, and a zero mass balance AAR of 56% (inter-
section between the straight line and zero bn in Fig-
ure 11); AAR has however varied between 20 to 45%
from the years 1996–1997 to 2008–2009.
In this paper we do not include the mass bal-
ance survey results for 2009–2010 and 2010–2011,
both years the summer melting was greatly enhanced
(∼threefold in 2010) by tephra spread over the glacier
surface from eruptions, in Eyjafjallajökull in April
2010 and Grímsvötn (center of Vatnajökull) in May
2011. Hence, both those years are outliers and be-
yond the scope of the present study. The winter pre-
cipitation at Hveravellir and the winter balance of
Langjökull are correlated (in spite of high scatter), and
the summer balance is strongly correlated to the aver-
age summer temperatures (Figure 12).
1998 2000 2002 2004 2006 2008
-16
-14
-12
-10
-8
-6
-4
-2
C
um
ul
at
iv
e
sp
ec
ifi
c
m
as
s
ba
la
nc
e
(m
w
e)
Figure 9. Langjökull cumulative mass balance
1996–1997 to 2008–2009. – Uppsöfnuð afkoma
Langjökuls jökulárin 1996–1997 til 2008–2009.
Mass balance and volume changes from differen-
tial DEMs
When volume change is used to estimate mass balance
(especially over short time spans) care must be taken
to calculate the estimates over a number of glacier
years (i.e. from autumn to autumn). The different spe-
cific density of the volume gained or lost (ice, snow
JÖKULL No. 62, 2012 89