Jökull - 01.01.2001, Blaðsíða 1

Jökulsárlón at Breiðamerkursandur, Vatnajökull, Iceland:
20th century changes and future outlook
Helgi Björnsson, Finnur Pálsson and Sverrir Guðmundsson
Science Institute, University of Iceland, Hagi, Hofsvallagata 53, IS-107 Reykjavík, Iceland
Abstract — We describe ice fluxes and calving rates of the terminus of Breiðamerkurjökull, SE–Vatnajökull,
since the formation of the proglacial lake Jökulsárlón in the 1930s. Breiðamerkurjökull retreated 4 km during
the 20th century and the lake is now 15 km

in area. Field observations in 1997 to 1999 revealed ice flow
toward the lake with an average velocity of 258 m yr


. The calving rate was estimated at 582 m yr


and the
calving ice flux at
 


m  yr


. The thermal energy required to melt the ice calving into Jökulsárlón is at
present about 2500 MW, of which approximately half to two–thirds, is supplied by warm seawater flowing into
the lake. An empirical relationship is derived for the calving rate as a function of the water depth at the calving
front. Past ice fluxes at the terminus of Breiðamerkurjökull are simulated using theoretical models and possible
future changes predicted for given mass balance scenarios. Both the ice fluxes toward the lake and the calving
rates into the lake have increased rapidly during the last two decades. Model calculations suggest similar
retreat rate of the calving front position for the next  70 years which would imply an average lake growth rate
of 0.5 km

yr


. At that rate Breiðamerkurjökull would retreat from the lake depression after  200 years and
almost vanish in  400 years.
INTRODUCTION
The lake Jökulsárlón was created during the warm
climate of the 20th century when the glacier outlet,
Breiðamerkurjökull of Vatnajökull (Figure 1, Plate 1),
retreated rapidly from an overdeepening which the
glacier excavated during its Little Ice Age advance
(Björnsson, 1979, 1996). The terminus calves into the
expanding lake that obtains thermal energy from so-
lar radiation and ocean tidal water flowing upstream
the glacial river Jökulsá (Plate 2). The formation of
the lake has had significant geomorphologic impact.
Glacial sediment is now deposited in the lake instead
of compensating for coastal abrasion at the mouth of
the river. During the period 1904 to 1989 the coast-
line receded inland by 700 m along an 8 km wide strip
at the river mouth, at an average rate of 8.5 m yr


(Víkingsson, 1991; Jóhannesson, 1994, 1995; Björns-
son, 1998). At this retreat rate the highway across
Breiðamerkursandur will be destroyed in a few years.
This is a setting for challenging practical and scientific
problems and as background for further study of these
problems we have assimilated available data pertain-
ing the development of Jökulsárlón and Breiðamerk-
urjökull. This study provides a quantitative descrip-
tion of the ice fluxes and calving rates at the front of
the outlet glacier during its 20th century retreat.
Field measurements to estimate the various mass
balance components were made in 1997 to 1999 and
the data used to tune theoretical ice flow and mass bal-
ance models. We use the models to reconstruct the
annual mass balance components at the lake from its
birth, and we compute possible near-future changes
in the lake extent. In this paper we also provide es-
timates of the energy components contributing to ice
ablation and discuss the impact of artificially reducing
the inflow of tidal seawater into the lake by damming
the river outlet.
JÖKULL No. 50 1