GRACE Science Team Meeting

Session B.3-b: Cryosphere

(Greenland mass loss: comparing GRACE and GPS observations of crustal uplift)
Shafaqat Abbas Khan

(Mass Balance of Polar Ice sheets from GRACE)
Jianli Chen

(Recent Changes of the Earth's land ice from GRACE: methods, signals and errors)
Scott Luthcke

(Consistent patterns of Antarctic ice sheet interannual variations from ENVISAT radar altimetry and GRACE )
Martin Horwath

(Above-linear increase in Greenland and Antarctica ice mass loss from GRACE and other data)
Isabella Velicogna


Session: B.3.a - Theme: Changes of Greenland Cryosphere
Title: Greenland mass loss: comparing GRACE and GPS observations of crustal uplift
First Author: Shafaqat Abbas Khan
Presenter: John Wahr
Co-Authors: John Wahr, Micheal Bevis, Eric Kendrick, Isabella Velicogna

Abstract: Greenland's main outlet glaciers have more than doubled their contribution to global sea level rise over the last decade. Recent work has shown accelerations in ice flow and in ice mass loss in southeast Greenland starting in spring 2004. Here, we provide evidence that the ice loss is now spreading up along the northwest coast, with this acceleration likely starting in 2006. We support this with two lines of evidence. One is based on measurements from GRACE, and the other comes from continuous GPS measurements from three long-term sites on bedrock adjacent to the ice sheet. The GRACE results provide a direct measure of mass loss, averaged over scales of a few hundred km. The GPS data are used to monitor crustal uplift caused by ice mass loss close to the sites. The GRACE results can also be used to predict crustal uplift, which can be compared with the GPS results. In addition to showing that the northwest ice sheet margin is now losing ice mass, the uplift results from both the GPS measurements and the GRACE predictions show rapid acceleration in southeast Greenland in 2004, followed by a moderate de-acceleration in 2006. Because that latter de-acceleration is weak, southeast Greenland still appears to be losing ice mass at a much higher rate than it was prior to spring 2004.

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Session B.3-b: Cryosphere
Title: Mass Balance of Polar Ice sheets from GRACE
First Author: Jianli Chen
Presenter: Jianli Chen
Co-Authors: C.R. Wilson, D. Blankenship, B.D. Tapley

Abstract: Using extended records of GRACE data, we estimate long-term ice mass change over Antarctica and Greenland, at both regional and continental scales. The new GRACE estimates indicate, during the period April 2002 and February 2009, Antarctica is losing ice at a rate of about 190 77 Gt/yr, with the majority, 132 26 Gt/yr, from the West Antarctic ice sheet and the rest, 57 52 Gt/yr from East Antarctica. Amundsen Sea Embayment loss (110 3.8 Gt/yr) dominates the West Antarctic figure, with the Antarctic Peninsula losing about 38 3.4 Gt/yr. During the same period, the Greenland ice is losing 221 38 Gt/yr, mostly from Greenland's periphery, with low rates of accumulation in the interior. Accelerated ice loss since around 2005/2006 is evident for both Antarctica and Greenland. The new GRACE results suggest that the Greenland rate is increased to -267 46 km3/yr for 2005 to 2009, while the Antarctic rate can be as large as 220 89 Gt/yr for 2006-2009. In contrast with previous GRACE estimates, the GRACE data show that East Antarctica appears to start losing ice (since 2006), mostly in coastal regions. These results independently confirm recent estimates from remote sensing ice flux indicating accelerated ice losses in Antarctica and Greenland.

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Session B.3-b: Cryosphere
Title: Recent Changes of the Earth's land ice from GRACE: methods, signals and errors
First Author: Scott Luthcke
Presenter: Scott Luthcke
Co-Authors: D.D. Rowlands, J.J. McCarthy, T. Sabaka, F.G. Lemoine, J.P. Boy

Abstract: The NASA/DLR Gravity Recovery and Climate Experiment (GRACE) mission has been acquiring ultra-precise inter-satellite K-band range and range-rate (KBRR) measurements providing new observations of the complex evolution of the Earth's land ice. The fidelity of the ice mass flux solutions depends on many factors including solution method, parameterization and processing of the GRACE level 1B data including forward modeling of various non-ice mass signals (e.g. hydrology, oceans, atmosphere). In this talk we present the latest GRACE derived ice mass solutions for the Gulf of Alaska glaciers, Greenland and Antarctica. We compare solutions derived from various solution techniques and explore the impact of forward modeling and parameterization on the final solutions. Solution results as well as errors and limitations will be discussed.

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Session B.3-b: Cryosphere
Title: Consistent patterns of Antarctic ice sheet interannual variations from ENVISAT radar altimetry and GRACE
First Author: Martin Horwath
Presenter: Martin Horwath
Co-Authors: Benoit Legresy, Guillaume Ramillien, Fabien Blarel, Frederique Remy, Jean-Michel Lemoine

Abstract: By observing temporal volume and mass changes, respectively, satellite radar altimetry (RA) and satellite gravimetry are complementary tools for ice sheet mass balance studies. Here we compare and jointly interpret results from ENVISAT RA and GRACE. The underlying RA products were generated with the Along-Track Satellite Radar Altimetry approach which exploits all observations along the repeat track and accounts for time-variable volume echo effects through analyzing the temporal variations of the radar echo shape. The used GRACE products are the CNES/GRGS 10-daily global gravity solutions obtained with a regularisation during processing and requiring no additional filtering. In order to render the spatial resolution of both datasets comparable we rigorously describe the spatial filtering of geophysical signals that is inherent to the GRACE processing. We then apply the same filtering to the maps of altimetric height changes.

After correction for GIA, the spatial patterns of linear trends shown by RA and GRACE over the common period (10/2002 to 04/2008) agree well, not only for the extreme ice losses in the Amundsen Sea Sector of West Antarctica but also for an alternating sequence of gains and losses along the East Antarctic coast. Differences between ENVISAT ice sheet thickness changes and GRACE equivalent ice thickness changes are primarily due to the lack of RA coverage and secondarily due to changes in the firn density structure associated with surface mass balance fluctuations as well as due to errors in either observational data sets.

Moreover, the general patterns of year-to-year nonlinear variations on top of the trends agree between the two data sets. This agreement gives confidence in the interannual variations of both data sets. As a consequence, the high-resolution patterns provided by RA can be used to relate the interannual mass variations observed by GRACE to either flow variations or surface mass balance (SMB) variations and to validate atmospheric modeling results on SMB fluctuations. For example, we find that the dynamic ice mass loss in the Amundsen Sea Basin was temporarily mitigated by a snow accumulation excess in the second half of 2005 and that in most of East Antarctica, linear trends over 5 years are predominantly due to fluctuations in snow accumulation.

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Session B.3-b: Cryosphere
Title: Above-linear increase in Greenland and Antarctica ice mass loss from GRACE and other data
First Author: Isabella Velicogna
Presenter: Isabella Velicogna
Co-Authors: Eric Rignot

Abstract: We use monthly measurements of time-variable gravity from the GRACE (Gravity Recovery and Climate Experiment) satellite gravity mission to determine the ice mass-loss for the Greenland and Antarctic Ice Sheets during the period between April 2002 and February 2009. We find that during this time period the mass loss of the ice sheets is not a constant, but accelerating with time, i.e. that the GRACE observations are better represented by a quadratic trend than by a linear one. Implying that the ice sheets contribution to sea level becomes larger with time. In Greenland, the mass loss increased from 137 Gt/yr in 2002-2003 to 286 Gt/yr in 2007-2009, i.e. an acceleration of -30 +/-11 Gt/yr^2 in 2002-2009. In Antarctica the mass loss increased from 104 Gt/yr in 2002-2006 to 246 Gt/yr in 2006-2009, i.e. an acceleration of -26+/-14 Gt/yr^2 in 2002-2009. The observed acceleration in ice sheet mass loss helps reconcile GRACE ice mass estimates obtained for different time periods. Acceleration rates in mass loss from GRACE are compared with the one estimated using InSAR based mass budget approach.

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