B.2 - Greenland/Antarctica Secular Change
(Moderator: Steve Nerem)

   Isabella Velicogna (12)
   Michael Watkins (12)
   Scott Luthcke (12)
   Jianli Chen (12)
   Ingo Sasgen (10)
   Discussion (22)

Isabella Velicogna (12)
Presenter: Isabella Velicogna
Co-Authors: John Wahr
Title: Greenland and Antarctica ice mass variations

Abstract: We use monthly GRACE gravity fields to estimate the linear trends in Greenland and Antarctic ice mass.

Both ice sheets display a large mass imbalance during the analyzed period. The mass of the Antarctica ice sheet decreases significantly from 2002 to 2006. Most of this mass loss is generated by the West Antarctic Ice Sheet. The Greenland ice sheet display significant mass loss during the same period with significant acceleration after April 2004. The acceleration is almost entirely due to accelerated rates of ice mass loss in southern Greenland. We will analyze the difference between mass balance estimates obtained using data processed by different centers.

Michael Watkins (12)
Presenter: Michael Watkins
Co-Authors: Dah-Ning Yuan; Willy Bertiger; Da Kuang; Gerhard Kruizinga; Sung Byun; Wenwen Lu, Sien Wu
Title: Recent JPL Results: Implications for Polar Ice Mass Studies

Abstract: In an earlier abstract at the GRACE Science Team meeting, we outline a complete set of solution approaches that we are pursuing at JPL, which cover a broad range of harmonics and mascons. In this talk, we focus specifically on the results from these various solutions for polar ice mass, and Greenland in particular. In some cases, we find significant differences in mass loss magnitude and spatial distribution, depending on the parameterization. We will outline these trends, as well as our best solution and estimates of true uncertainties.

Scott Luthcke (12)
Presenter: Scott Luthcke
Co-Authors: D.D. Rowlands; R.D. Ray; F. Lemoine; R.S. Nerem; H.J. Zwally; W. Abdalati; D. Chinn
Title: Recent ice sheet mass change observations from GRACE mascon solutions

Abstract: On multi-decadal time scales or longer, the most important processes affecting sea level are those associated with the mass balance over the EarthÕs ice sheets. The vulnerability of the cryosphere to climate change along with the difficulty in acquiring uniform in situ observations in these inhospitable regions, makes the problem of understanding ice sheet mass trends a high Earth science research priority at NASA. The Gravity Recovery and Climate Experiment (GRACE) mission has acquired ultra-precise inter-satellite ranging data since 2002. These data provide new opportunities to observe and understand ice mass changes at unprecedented temporal and spatial resolution. In order to improve upon the ice mass trend observations obtained from GRACE, we have employed unique data analysis approaches to obtain high-resolution local mass change (mascon solutions) from GRACE inter-satellite observations alone. We have applied our mascon solution technique to the Greenland and Antarctic ice sheets estimating surface mass change for irregularly shaped regions defined by the ice sheet drainage basins and further sub-divided by elevation. We estimate the surface mass change of each ice sheet drainage basin sub-divided by elevation at 10-day intervals. We have computed a 3-year time series of surface mass change for each ice sheet drainage basin. These mascon solutions provide unprecedented observations of the seasonal and inter-annual evolution of ice-sheet mass flux. In this presentation, we discuss the details of our ice sheet mascon solutions, as well as compare our solution results with mass change observations derived from alternative methods and data.

Jianli Chen (12)
Presenter: Clark Wilson
Co-Authors: CR Wilson, BD Tapley
Title: Ice Mass Balance Estimates from GRACE

Abstract: We have estimated ice mass changes over Antarctic and Greenland ice sheets, and the coast of the Gulf of Alaska (GOA), using time-variable gravity measurements from GRACE. Estimates are constructed by fitting linear functions to time series at 1 x 1 degree grid points, then using a forward-model approach to find mass rates consistent with these. Rates must be corrected for effects of spatial filtering and limited resolution of GRACE data.

Estimated total ice melting rate over Greenland is Ð 234 ± 24 km3/year during the period April 2002 to November 2005, mostly from East Greenland. This estimate agrees well with a recent assessment of Ð 224 ± 41 km3/year, based on satellite radar interferometry data. A separate estimate by Velicogna and Wahr, from the same data using different estimation methods gives a similar rate. GRACE changes in southeast Greenland suggest accelerated melting since the summer of 2004, consistent with the latest remote sensing measurements.

GRACE data over Antarctic reveal a clear mass decrease in the Amundsen Sea Embayment in West Antarctica, and an increase in the Enderby Land region in East Antarctica. The mass deficit in the Amundsen Sea is ~ Ð 69 ± 17 km3/yr, which is in surprising agreement with the assessment (~ Ð72 km3/yr) from other remote sensing techniques. The mass accumulation in the Enderby Land region in East Antarctica is equivalent to ~ 96 ± 14 km3/yr.

In the GOA, GRACE estimates of mass loss correlate with glacial balance data at Gulkana and Wolverine, two benchmark glaciers of the U.S. Geological Survey, although GRACE estimates are smaller in magnitude. After correcting for leakage errors and removing terrestrial water storage contributions using model estimates, GRACE estimates of glacial melting in the GOA region is ~ Ð 101 ± 22 km3/year, which agrees with ~ Ð 96 ± 35 km3/year based on airborne laser altimetry data.

Ingo Sasgen (10)
Presenter: Ingo Sasgen
Co-Authors: Z. Martinec; K. Fleming
Title: Regional ice-mass changes and glacial-isostatic adjustment in Antarctica from GRACE

Abstract: We infer regional mass changes in Antarctica using ca. 3.5 years of CNES RL01C and GFZ RL03L GRACE level 2 data. We apply a two-step noise reduction to the estimated temporal trends of the GRACE potential coefficients consisting of a statistical reliability test of the inferred trends and an optimized smoothing of the gravity field. The most prominent areas of geoid changes over Antarctica are then fitted by a three-region model describing the mass changes due to glacier melting in the Amundsen Sea Sector and the Antarctic Peninsula, and glacial-isostatic adjustment around the Ronne Ice Shelf.

We obtain rates of mass change in the Amundsen Sea sector corresponding to a sea-level change of 0.23 (CNES RL01C) and 0.26 (GFZ RL03L) mm/a, the error lying between 0.01 and 0.02 mm/a, which is an improvement of about a factor of two with respect two previous GRACE estimates. The values agree with those inferred from satellite altimetry and are robust with respect to the remaining observational noise in the GRACE data. The values inferred for the Antarctic Peninsula, 0.07 to 0.22±(0.01 to 0.06) mm/a, and for the GIA signal (equivalent ice-mass loss since the last-glacial maximum of 7.2 to 13.2±(0.8 to 2.3) m are plausible, but less robust, owing to their lower magnitude and the likely contaminating signals from ocean processes associated with the Antarctic Circumpolar Current and ocean tides beneath the Ronne Ice Shelf.

Discussion (22)