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.

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?

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

Observations: melt change

Steffen and Huff, 2003.

Greenland melt extent increased by 16% between 1979-2002

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.

Modelling: conventional evolution

Alley et al., 2005, Science

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

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)

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.

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

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

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.

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.

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.

• 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

•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

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.

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!

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).

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).