Geodetic signatures of glacial changes in Antarctica

I. Sasgen et al.

MAGMA Seminar, May 25, 2005, Prague

Geodetic signaturesof glacial changes in Antarctica

Ingo Sasgen

Supervision: Detlef Wolf, Zdeněk MartinecGeoForschungsZentrum Potsdam

Email: [email protected]

Geodetic signatures of Antarctica I. Sasgen

Page 2MAGMA Seminar, May 25, 2005, Prague

Larsen B ice-shelf collapse

National Snow and Ice DataCenter, http://nsidc.gov (2005)

Jan. 31 – Mar. 05, 2002



Causes and consequences of collapse

mean temperature trend of +1.2°C in 100 a for all Antarctic stations

regional warming of +2.5°C in 50 a along the Antarctic Peninsula

warming likely cause for collapse of the Larsen B ice shelf

glacier acceleration observed after desintegration of the Larsen B ice shelf, i.e. ice-velocity increase by factor of 5 to 8



Scientific importance

Antarctic ice sheet (AIS) largest ice mass on earth 10 times larger than the Greenland ice sheet glacial variations closely linked to global climate

and sea-level changes knowledge on present-day state and near-future

developement improves climate models, which predict global temperature and sea-level changes

for the centuries to come



Antarctic ice sheet

13.6 x 106 km2 grounded portion,i.e. 95 % of the continent

volume of 61 m equivalent sea level (ESL)

~ 4 km maximum ice thickness drained by ice streams which feed ice

shelves mountain glaciers along

Antarcic Peninsula



Ice sheet volume changes

accumulation: ~ + 5 mm ESL/a accumulation – discharge: ~ - 0.1 mm ESL/a discharge since last glacial maximum (LGM),

21 ka BP: ~ -12 m ESL

present sea-level rise: ~ 1.8 mm/a ESL sea-level rise since LGM: ~ 110 m ESL



Antarctic continent

area of rock outcrops < 0.4 % East Antarctica is a

Precambrian shield West Antarctica comprises

younger, tectonically more active terranes



Transantarctic Mountains

Transantarctic Mountains

Transantarctic Mountains mark tectonic suture zone lateral variations of the lithosphere thickness and the

viscosity expected



Modelling components

Elastic

Last glaciation~ 1000 a

Seasonal~ 1 a

Secluar~ 10 – 100 a

Viscoelastic

Geoid-height change

Radial displacement

Gravity Recovery andClimate Experiment (GRACE)

Global PositioningSystem (GPS)

Load model

Earth model

Earth response



Seasonal ice-mass changes (VAUG)

Vaughan et al. (1999) Cazenave et al. (2000)

accumulation of ~ 5 mm ESL/a based on ice cores

temporal variation from global mean sea-level changes inferred from satellite altimetry



Seasonal earth response

Geoid-height change Radial displacementElastic,

cut-off degree 256



East Antarctica roughly in balance most prominent changes of up to 1

m/a for glaciers draining into the Amundsen Sea (PIG, THW, SMI, KOH)

East Antarctica in balance Byrd (BYR) likely in balance (former

0.05 mm ESL/a) ice-thickness changes along Antarctic

Peninsula and West Antarctic coast several m/a

Update 2004:

Secular ice-mass balance (RT02)

Region Mass change Volume change

Gt/a mm ESL/a

West Antarctica (75 %) - 45 - 0.12

East Antarctica (55 %) 19 0.05

Antarctica (58 %) - 26 - 0.07

Rignot & Thomas (2002)



Earth response to secular changes

Geoid-height change Radial displacementElastic,

cut-off degree 256



Viscoelastic earth model

present-day post-glacial rebound (PGR) due tolast glaciation is calculated with a

lateral homogenous viscoelastic earth model based on the

spectral-finite element code developed by Martinec (2000)



Earth model parameters

Model Lith.

thickness Viscosity Elasticity

hL (km) UM (Pa s) LM (Pa s)

LVM 100 5.2 1020 5.9 1021 PREM

MP 100 1.0 1021 4.0 1021 PREM

HVM 100 2.0 1021 1.0 1022 PREM

WestAntarctica

East Antarctica



15 ka BP7 ka BP4 ka BP

Last glaciation and its retreat (HUY)

thermomechanical model allows regional retreat history, e. g. late

retreat from the Ronne ice shelf ice volume of – 12 m ESL at the LGM

compared to present day

Huybrechts (2002)



Earth response to last glaciation

Geoid-height change Radial displacementViscoelastic,

cut-off degree 256



International GPS Service stations

7 stations along the Antarcic coast

continuous time series > 6 a

nominal accuracy ~ 1 mm/a



Land-uplift rates at GPS stations

mm/a mm/a



Possible GPS transects

GPS measurements along A-A‘, B-B1 and B1-B2 can constrain the glacial history

Measurements along B2-B questionable: tectonic displacements large and influenced by rheological transition



Summary of GPS comparison

Interpretation of IGS data difficult, because stations located

at rheological transition (e.g. Mawson, Davis) lateral heterogenous earth model

at ice margin where rebound is complex accurate last glaciation models

in tectonically active regions (e.g. Mc Murdo) ignore particular station

not in the former load center

Large solution differences between regional networks, e.g. Amery ice shelf region



GRACE satellite mission



GRACE satellite mission

Primary mission objective of the GRACE:monthly high-accuracy determination of the earth‘s gravity field,i.e. temporal variations of the geoid height

Possible application:mass balance of ice sheetsocean-current changespost-glacial rebound

Mission status:operational since October 200218 monthly solutions existcurrent spatial resolution ~ 1000 km with anestimated accuracy ~ mm/a



Schematic principle of GRACE

Satellite A Satellite B

∆m

Satellite-satellite distance ∆l tracked by microwave

link

Satellite B

∆l = f (∆m)

GPS GPS GPS GPS

Precise orbitdetermination by GPS



May. 2003 – Apr. 2003May 2003 – Apr. 2002Nov. 2002 – Aug. 2002

Comparison of spectral geoid change

Nov. 2003 – Nov. 2002



May 2003 – Apr. 2003

Spatial geoid change comparison

Aug. 2003 – Aug. 2002 Nov. 2002 – Aug. 2002

Prediction,

Observation,

cut-off degree 13

cut-off degree 13



Discussion of geoid-height interpretation

Predicted and GRACE measured geoid changes do not correspond yet:

Strong anomalies over the ocean dominate the signal artificial ocean phenomenon: tides not successfully removed? real ocean phenonmenon: circumpolar current?

Seasonal changes not visible (not even the sign) temporal variation not realistic?

Secular changes not detectable at the current resolution expected 8 a of measurments sufficient for a linear-tend estimate?



Outlook: seasonal ice-mass changes

Include metereological parameters accumulation from moisture flux onto

the Antarctic continent discharge, i.e. mainly calving, from

surface-air temperature Patagonia as proxy for the Antarctic

Peninsula?

accumulation discharge



Outlook: GRACE data

Quantify errors introduced by ocean model

Remove ocean signal (e.g. by filtering)

Focus on (regional) total ice-mass changes, not spatial distribution

Allow error dependent weighing of degree power to include maximum information

May 2003 – May 2002 minusAug. 2003 – Aug. 2002



Glacial changes of the AIS induce geoid changes and land uplift : with measurable magnitudes and specific signatures

Observations by GRACE and GPS

do not correspond to the predictions yet

need to be refined and extended according to the expected signature

Summary



Questions

???X?!!?!



Glacial changes of the AIS induce geoid and surface displacement changes: with measurable magnitudes and specific signatures

Observations of the geoid (GRACE) and the surface displacement changes (GPS)

do not correspond to the predictions yet

need to be refined and extended according to the expected signature



Present ice-mass balance (updated)

Rignot & Thomas (2002), Thomas et al. (2004), Rignot et al. (2004)

Sasgen et al. (2005)



Secular

Spectral geoid change

Last glaciation

Secular ice-mass balance induces geoid change well above GRACE accuracy

High power even at high degrees However, seasonal changes ~ one

order of magnitude larger Interannual variation can introduce

„pseudo“-secular trend

Last glaciation induces high power at degree low degrees well above the GRACE accuracy

Up to degree 9 the employed earth model is not importance

Documents

Geodetic signatures of glacial changes in Antarctica