Recent results from GRACE in Greenland and Antarctica Isabella Velicogna* and John Wahr** * ESS, University of California Irvine, Irvine CA ** Dept Of.

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Presentation transcript:

Recent results from GRACE in Greenland and Antarctica Isabella Velicogna* and John Wahr** * ESS, University of California Irvine, Irvine CA ** Dept Of Physics and CIRES University of Colorado, Boulder CO

Methods to estimate Ice Mass Variation: Remote Sensing Measurements: - Snow input with respect to ice discharge ( InSAR ), fluxes. - Height of surface (radar, and laser altimetry), volume. Problem: difficulty of monitoring the entire ice sheet, sparse data on accumulation (InSAR), ambiguity in density conversion (Altimetry). - Mass variation (GRACE): comprehensive survey of the entire ice sheet, First direct measure of mass. Models: Ice sheet modeling has many limitations that lead to underestimating ice mass rate (Alley et al., Science 2005).

Greenland

To estimate ice sheet change we need to account for: -GRACE errors -Measurements, processing and aliasing errors -Contamination from other geophysical signals: 1- caused by signals outside Greenland - continental hydrology outside Greenland (using GLDAS monthly global water storage fields, and CPC). - ocean mass variability using a JPL version of the ECCO general circulation model. 2- from Greenland signals unrelated to snow and ice: - error in atmospheric mass correction (<3% of GRACE variance) - PGR signal. We use two independent ice history models: ICE-5G and GREEN1 Best Estimate : 5 +/- 19 km 3 /yr

Trend, Apr 02-Jun07: -238 km 3 /yr __ GRACE - hydrology leakage - PGR Trend Apr 2002-Dec 2006: -238+/-36 km 3 /yr Greenland Mass Variation From GRACE

NORTH GREENLAND ----Apr ‘02-Jun ‘07: -65/-18 km 3 /yr SOUTH GREENLAND ----Apr ‘02-Jun ‘07: -164+/-24 km 3 /yr

Linear trend fit to same 33 months, between 4/2003 and 6/2006. km 3 /yr of ice. CSR RL1: -254 CSR RL4: -256 JPL RL GFZ RL4: N/A GFZ RL3: -195 Goddard: -188 Total Greenland. PGR not removed. Linear trend fit to same 30 months, between 2/2003 and 12/2005. km 3 /yr CSR RL1: -214 CSR RL4: -230 JPL RL4 -99 GFZ RL4: -222 GFZ RL3: -185 Goddard: N/A

Helheim glacier. Kangerdludssuaq glacier.

Elastic uplift of ∼ 35 mm in Kulusuk. Khan et al GRL (2007) Ku

ENVISAT (Davis, 2007 ) Mass loss increased from 90 to 220 km^3/yr in (Rignot & Kanagaratnam, 2006) Melt area in Greenland increasing (K. Steffen) Ekström et al., Science 2006 Krabill et al., 2006 Watkins et al, 2006

Greenland mass balance

Antarctica

To estimate ice sheet change we need to account for: -GRACE errors -Measurements, processing and aliasing errors -Contamination from other geophysical signals: 1- caused by signals outside Antarctica - continental hydrology outside Antarctica (using GLDAS monthly global water storage fields). - ocean mass variability (using a JPL version of the ECCO general circulation model. (Negligible) 2- from Antarctic signals unrelated to snow and ice: - error in atmospheric mass correction (~10 km 3 /yr) - PGR: we use two independent ice history models: ICE-5G and IJ05 Best Estimate : the midpoint of this range /- 79 km 3 /yr,

__ GRACE __ GRACE - hydrology leakage - PGR ---- Apr ‘02-Jun ‘07: /-80 km 3 /yr Antarctica Ice mass Change

WAIS and EAIS Mass Variation From GRACE WAIS (Apr ‘02-Jun ‘07) :-112+/-21 km 3 /yr EAIS (Apr ‘02-Jun ‘07) :-24+/-56 km 3 /yr

Linear trend fit to same 33 months, between 4/2003 and 6/2006. km 3 /yr CSR RL1: -42 CSR RL4: -77 JPL RL3: -75 JPL RL4 -83 GFZ RL3: -48 GFZ RL4: N/A Goddard: -28 West Antarctica. ICE-5 PGR model removed. PGR uncertainty: ±20 km 3 /yr. Linear trend fit to same 30 months, between 2/2003 and 12/2005. km 3 /yr CSR RL1: -86 CSR RL4: -118 JPL RL3: -110 JPL RL GFZ RL3: -84 GFZ RL4: -121 Goddard: N/A

Linear trend fit to same 33 months, between 4/2003 and 6/2006. km 3 /yr CSR RL1: 54 CSR RL4: 12 JPL RL3: -70 JPL RL4 -61 GFZ RL3: 120 GFZ RL4: N/A Goddard: -3 East Antarctica. ICE-5 PGR model removed. PGR uncertainty: ±55 km 3 /yr. Linear trend fit to same 30 months, between 2/2003 and 12/2005. km 3 /yr CSR RL1: 46 CSR RL4: -14 JPL RL3: -61 JPL RL4 -76 GFZ RL3: 117 GFZ RL4: 3 Goddard: N/A

East/West dividing line

ICESat Rignot et al Harpold, Urban, & Schutz Davis, 2006 Watkins et al. 2006

Summary - GRACE delivers mass estimates for the polar ice sheets. Strengths (relative to other techniques): ---GRACE samples the entire ice sheet, both the interior and the edges. It does not require interpolation between ground points. ---GRACE provides a direct observation of mass. ---GRACE has a dense enough (monthly) temporal sampling to allow separation between trends and seasonal terms. Weaknesses: ---GRACE is more sensitive to PGR model errors than other techniques. ---GRACE cannot provide resolution below a few hundred km. -- Greenland is losing mass significantly. -- Mass loss increased significantly in the last few years (Grace observed timing is consistent with Kangerdlugssuaq and Helheim glaciers observed acceleration). -- Antarctica appears to have lost significant mass between 2002 and GRACE can help identify problems in other techniques. -- COMBINING GRACE, InSAR and Altimetry we can improve interpretation of ice mass changes

Antarctica from range acceleration data Preliminary plot of the trend in Greenland ice mass over derived from analysis of range acceleration data Watkins et al. 2006

Greenland Mass Variation From GRACE Velicogna and Wahr, Nature 2006 SOUTH GREENLAND ----Apr ‘02-Apr ‘06: -161+/-24 km 3 /yr ---- Apr ‘02-Apr ‘04: -20+/-26 km 3 /yr ---- May‘04-Apr ‘06: -246+/-36 km 3 /yr NORTH GREENLAND ----Apr ‘02-Apr ‘06: -83+/-18 km 3 /yr Apr ‘02-Apr ‘04: -80+/-28 km 3 /yr May‘04-Apr ‘06: -90+/-28 km 3 /yr

Greenland Mass Variation From GRACE Velicogna and Wahr,Nature 2006 __ GRACE - hydrology leakage - PGR Trend Apr 2002-Apr 2006: -248+/-36 km 3 /yr ~0.5+/-0.1 mm/yr sea level rise Trend Apr 2002-Apr 2004: -104+/-54 km 3 /yr Trend May 2004-Apr 2006: -342+/-66 km 3 /yr

Melt area in Greenland increasing ( K. Steffen) Red shows seasonal surface melt extent (slush and ponds of meltwater) for two extreme years. Surface melt has increased 16% between 1979 and Ekström et al., Science 2006 Krabill et al., 2006 Glacier changes (Rignot and Kanagaratnam, 2006) -- Mass loss increased from 90 to 220 km^3/yr in /3 loss caused by glacier mechanics. -- 1/3 caused by enhanced ice melt minus enhanced snowfall (Hanna et al., 2004). Other evidence of changes of the Greenland Ice sheet More snowfall …. (Johannessen et al., 2005) Accumulation 575 km 3 /yr

Secular trend from CSR Release 4 (de-striped). 4/02 –6/ km smoothing radius. The same, but after removing a post-glacial- rebound model (ICE-5G).

Rate of ice volume change: All Antarctica: -145 km 3 /yr West Antarctica: -112 km 3 /yr East Antarctica: -24 km 3 /yr East/West dividing line PGR uncertainty : All Antarctica: ± 80 km 3 /yr West Antarctica: ± 20km 3 /yr East Antarctica: ± 55 km 3 /yr