2011 - Cooperative Institute for Research in Environmental Sciences ...

Konrad Steffen
with William Colgan
Melt-Flow Acceleration
of the Greenland Ice Sheet
FUNDING: NASA GODDARD SPACE FLIGHT CENTER
This research is in
support of the continuous
GPS network on the
Greenland Ice Sheet in
the vicinity of the Swiss
Camp (SC, Figure 1),
at the western slope of
the ice sheet at approximately
70° N. The GPS
network consists of a
total of eight GPS Trimble
4000 SSE, R5 and R7 receivers
with the objective
to monitor the meltflow
acceleration and
surface-height change.
The GPS receivers are
aligned along the main
flow direction of the ice
sheet (UP50 to JAR2),
and three receivers were placed at the mean equilibrium
line altitude (ELA) at Swiss Camp, Up50 and S16. Up50 is
located 50 km from SC, N70 is 70 km north of SC and S16
is approximately 16 km south of SC.
At present, Greenland’s mass loss appears to be equally
split between surface mass balance (i.e., melt and runoff)
and ice dynamics (i.e., ice discharge). Predicting the relative
contributions of these two terms to future sea-level
rise is complicated by potential nonlinear feedbacks.
Generally, however, the future ice-dynamic contribution
is regarded as more difficult to forecast than its surfacemass-balance
counterpart. This is due to the inability to
establish the mechanism responsible for the recent widespread
acceleration of outlet glaciers.
The research conducted over the past year has focused
on better understanding mechanisms by which (i) increased
surface meltwater production results in enhanced
basal sliding and (ii) the recent acceleration may be the onset
of a long-term response to increased effective driving
stress stemming from a loss of terminus back-stress. The
ablation zone northeast of Ilulissat has an annual velocity
cycle comprised of a summer speedup event followed
by a fall slowdown event
(Figure 2). At the onset
of this project, it was not
evident whether seasonally
enhanced basal sliding
or seasonally reduced
terminus back-stress was
responsible for this annual
velocity cycle. Our findings
now suggest that enhanced
basal sliding is more important
than reduced terminus
back-stress in determining
inland ice velocities.
Figure 1: GPS network on the western slope of the Greenland Ice Sheet
monitors the melt-flow acceleration northwest of Ilulissat. The box
around the graph gives the latitude and longitude coordinates; the blue
line depicts the ice-sheet margin.
Figure 2: Observed 10-day mean ice-surface velocities (grey) and
cumulative positive-degree days (PDD; red) in 2005 and 2006 at Swiss
Camp (SC), JAR1 and JAR2 (where available) versus Julian Date. Black
lines denote the bi-Gaussian characterization of the annual ice-surface
velocity cycle at each station.
Above: Nunataks on the eastern slope of the Greenland Ice Sheet close to
Kulusuk—blue ice areas form on the downwind side of the mountains.
Left: GPS unit with solar panels and climate instruments at S16 (see Figure 1).
CIRES Annual Report 2011 51