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Seasonal Evolution of Supraglacial Lakes at the Margins of the Greenland Ice Sheet

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Title: Seasonal Evolution of Supraglacial Lakes at the Margins of the Greenland Ice Sheet


1
Seasonal Evolution of Supra-glacial Lakes at the
Margins of the Greenland Ice Sheet
  • Malcolm McMillan1, Peter Nienow1,
  • Andrew Shepherd1 Toby Benham2
  • School of GeoSciences, University of Edinburgh.
  • Scott Polar Research Institute, University of
    Cambridge.

2
Aim To investigate seasonal changes in the
volume of water stored by and released from
supra-glacial lakes on the margin of the
Greenland Ice Sheet.
Combine satellite remote sensing and in-situ
meteorology.
WHY?
Images from http//www.whoi.edu/oceanus
3
Observations volume change
Airborne laser altimetry 1993-99
Satellite (ERS) radar altimetry 1992-2003
cm/yr
Krabill et al, 2000, Science
Johannessen et al, 2005, Science
new GRACE and IceSat measurements
4
Observations melt change
Steffen and Huff, 2003.
Greenland melt extent increased by 16 between
1979-2002
5
Observations dynamic thinning
  • Ice sheet outlet glaciers are accelerating
  • 150 acceleration of Jakobshavn Isbrae between
    1985 and 2003

Joughin et al., 2004, Nature
  • 2/3 of recent ice mass loss the result of
    dynamic thinning
  • Rignot and Kanagaratnam, 2006, Science.

6
Modelling conventional evolution
Alley et al., 2005, Science
7
Modelling uncertainties
  • Predicting the evolution of the GIS is
    complicated because of a
  • critical lack of understanding of certain key
    processes
  • Dynamic response of ice sheet to increased
    meltwater production.

8
Dynamic response
Zwally et al., 2002, Science
  • Ice sheet interior accelerated during periods of
    summer melt.
  • Hypothesis increased melt induces speed-up
    through enhanced basal sliding.
  • Behaviour typical of many polythermal and
    temperate glaciers.

Bingham et al., 2003, Ann. Glac.
Glacier velocity ( increase/decrease from annual
mean)
9
Supra-glacial lakes
  • During summer, lakes up to several kilometers
    square form on the surface of the ice near the
    ice sheet margins.

22 June 1990.
Landsat image in Zwally et al., 2002, Science.
How do these lakes behave during the course of a
melt-season?
Image from http//www.whoi.edu/oceanus
10
Study Area
  • 2 sites at the margin of the western GIS
  • Site 1) Swiss Camp scenes (70N, 49W),
  • 18,800 km2, 400 - 2100 m elevation.
  • Site 2) Russell Glacier scenes (67N, 48W)
  • 3000 km2, 500 - 1200 m elevation.

Landsat 7 ETM 30 m resolution
Swiss Camp Scenes
Greenland
ASTER 15 m resolution
Russell Glacier Scenes
11
Remote Sensing Data
Compared images of both sites from early July
2001 and early August 2001. Survey of lake area
conducted for lakes gt 0.01 km2 on both dates.
Swiss Camp Scenes
Russell Glacier Scenes
Landsat scene, 7th July 2001
12
Results - Remote Sensing Survey
July 2001
August 2001
Substantial drainage of lakes at both sites.
1 km
Draining of lakes at 950m, Russell Glacier
13
Variations in lake behaviour with altitude.
Russell Glacier
  • Russell Glacier
  • Net decrease in lake area between 3rd July and
    1st August at all altitudes.
  • Swiss Camp
  • Below 1200m decrease in net lake area between
    7th July and 1st August.
  • Above 1200m increase in net lake area.

Swiss Camp
14
Hydrological Cycle
  • These results suggest lakes situated at higher
    elevations / latitudes (i.e. lower temperatures)
    will be at an earlier stage in their seasonal
    evolution during the survey period.
  • High elevation Swiss Camp lakes
  • early stage of seasonal evolution lakes filling
  • Low elevation Swiss Camp all Russell
    Glacier lakes
  • later stage of seasonal evolution lakes draining

Observations suggest a melt, fill and drainage
cycle.
15
What about volume changes?
  • 1996 - 2005 Positive Degree Days (PDDs)
    calculated from
  • GC-Net met. stations temp. data Steffen and Box,
    2001.

Russell Glacier Scene
JAR1
JAR2
Swiss Camp Scene
JAR3
SC
Swiss Camp
JAR1
JAR2
JAR3
GC-Net meteorological stations locations
16
Degree day model
  • PDD model used to estimate melt upglacier of
    the Swiss Camp lakes.
  • Used degree day factors for ice (8 mm w.e.) and
    snow (3mm w.e.) from Braithwaite, J. Glac., 1995.

2nd image taken
1st image taken
2001 Positive Degree Days, Swiss Camp Region
17
Volume Results
  • Total annual runoff produced in 2001 upglacier of
    the Swiss Camp lakes was estimated to be 3.15
    km3.
  • 0.12 km3 before 7th July (first scene)
  • A further 0.97 km3 before 1st August (second
    scene).

From surveyed lake area and estimated melt
volumes Mean July depth of Swiss Camp lakes 2
meters (assuming no drainage pre-July). Mean
August depth of lakes continuing to fill 8
meters.
18
Volume Results cont.
Volume water drained (c/w stored) by Aug.
estimated 0.65 km3 This equates to a mean
sub-glacial water depth of 11cm below the area
spanned by the lakes, should the water penetrate
simultaneously to the ice sheet bed. It wont
but what is the rate of drainage? better
temporal resolution required ground truthing!
19
Conclusions
  • 1) Supra-glacial lakes show a clear seasonal
    evolution.
  • 2) Volumes of meltwater draining from the lakes
    are substantial and could perturb ice-dynamics.
  • Two critical unknowns
  • Does meltwater reach the bed?
  • If yes, does it
  • i) rapidly develop a hydraulically efficient
    channelised drainage system ( ? Pw) or
  • ii) does ice overburden ensure the survival of a
    hydraulically inefficient distributed drainage (
    ? Pw).

20
Acknowledgements
This work was supported by a UK Natural
Environment Research Council studentship
(NER/S/M/2005/13876). Satellite data were
provided by the Global Land Cover Facility
(http//www.lancover.org) and the Land Processes
Distributed Archive Centre (http//LPDAAC.usgs.gov
). Meteorological data were provided by the
Steffen Research Group (http//cires.colorado.edu/
science/groups/steffen).
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