seasonal evolution of supra-glacial lakes at the margins of the greenland ice sheet malcolm mcmillan...
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![Page 1: Seasonal Evolution of Supra-glacial Lakes at the Margins of the Greenland Ice Sheet Malcolm McMillan 1, Peter Nienow 1, Andrew Shepherd 1 & Toby Benham](https://reader030.vdocuments.us/reader030/viewer/2022032708/56649e7a5503460f94b79dcc/html5/thumbnails/1.jpg)
Seasonal Evolution of Supra-glacial Lakes at the Margins of the
Greenland Ice Sheet
Malcolm McMillan1, Peter Nienow1,
Andrew Shepherd1 & Toby Benham2
1. School of GeoSciences, University of Edinburgh.2. Scott Polar Research Institute, University of Cambridge.
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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.
Images from http://www.whoi.edu/oceanus
WHY?
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Krabill et al, 2000, Science
Airborne laser altimetry1993-99
cm/yr
Johannessen et al, 2005, Science
Satellite (ERS) radar altimetry1992-2003
Observations: volume change
+ new GRACE and IceSat measurements
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Observations: melt change
Steffen and Huff, 2003.
Greenland melt extent increased by 16% between 1979-2002
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Observations: dynamic thinning
Joughin et al., 2004, Nature
• Ice sheet outlet glaciers are accelerating
• 150 % acceleration of Jakobshavn Isbrae between 1985 and 2003
• ~2/3 of recent ice mass loss the result of dynamic thinningRignot and Kanagaratnam, 2006, Science.
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Modelling: conventional evolution
Alley et al., 2005, Science
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Predicting the evolution of the GIS is complicated because of acritical lack of understanding of certain key processes:
Dynamic response of ice sheet to increased meltwater production.
Modelling: uncertainties
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SummerWinter
0 – 5km
Dynamic response Zwally et al., 2002, Science
Bingham et al., 2003, Ann. Glac.
• 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.
Glacier velocity (% increase/decrease from annual mean)
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During summer, lakes up to several kilometers square form on the surface of the ice near the ice sheet margins.
How do these lakes behave during the course of a melt-season?
Image from http://www.whoi.edu/oceanus
Supra-glacial lakes
Landsat image in Zwally et al., 2002, Science.
22 June 1990.
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Study Area2 sites at the margin of the western GIS:
Site 1) ‘Swiss Camp’ scenes (~70°N, ~49°W),
18,800 km2, ~400 - 2100 m elevation.
Site 2) ‘Russell Glacier’ scenes (~67°N, ~48°W)
3000 km2, ~ 500 - 1200 m elevation.
Swiss Camp Scenes
Russell Glacier Scenes
Greenland
Landsat 7 ETM30 m resolution
ASTER15 m resolution
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Remote Sensing Data
Swiss Camp Scenes
Russell Glacier Scenes
Compared images of both sites from early July 2001 and early August 2001.
Survey of lake area conducted for lakes > 0.01 km2 on both dates.
Landsat scene, 7th July 2001
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Results - Remote Sensing Survey
1 km
July 2001 August 2001
Draining of lakes at ~ 950m, Russell Glacier
Russell Glacier Swiss Camp
Area surveyed 3 007 km2 18 808 km2
July lake area 18.0 km2 56.5 km2
August lake area 7.4 km2 (-10.5 km2) 58.4 km2 (+1.9 km2)
% of lakes with decreased area 86% 61%
Aerial decrease of original lakes - 11.1 km2 - 25.4 km2
Substantial drainage of lakes at both sites.
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Russell Glacier:
• Net decrease in lake area between 3rd July and 1st August at all altitudes.
Variations in lake behaviour with altitude.
0
2
4
6
8
10
12
14
16
18
500 700 900 1100 1300 1500
Altitude (m)
Lak
e A
rea
( k
m2 )
July
August
0
2
4
6
8
10
12
14
16
18
500 700 900 1100 1300 1500
Altitude (m)
Lak
e A
rea
( k
m2 )
July
August
Russell Glacier
Swiss CampSwiss Camp:
• Below 1200m: decrease in net lake area between 7th July and 1st August.
• Above 1200m: increase in net lake area.
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Hydrological CycleThese 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.
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• 1996 - 2005 Positive Degree Days (PDD’s) calculated fromGC-Net met. stations temp. data [Steffen and Box, 2001].
What about volume changes?
Swiss CampJAR1
JAR2JAR3
Swiss Camp Scene
Russell Glacier Scene
GC-Net meteorological stations locations
y = -0.38x + 524.29
R2 = 0.99
050
100150200250300350400450
0 500 1000 1500
Elevation (m)
Ave
rag
e an
nu
al P
DD
JAR1
JAR2
JAR3SC
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•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.
Degree day model
1st image taken
2nd image taken
2001 Positive Degree Days, Swiss Camp Region
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Volume ResultsTotal 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.
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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 won’t but what is the rate of drainage? – better temporal resolution required + ground truthing!
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Conclusions1) 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:
1) Does meltwater reach the bed?
2) 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).
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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).