GRACE Science Team Meeting 2008

B.2 - Cryospheric Change

(Greenland and Antarctic mass balance from GRACE)
Isabella Velicogna

(GRACE Observes Small-Scale Mass Loss in Greenland)
Bert Wouters

(Present-day West Antarctic ice-mass change estimate by the constrained inversion of GRACE and InSAR data)
Ingo Sasgen

(Changes of the Greenland Ice Sheet from GRACE and ICESat)
Louise Sandberg Sorensen

Title: Greenland and Antarctic mass balance from GRACE
Session: B.2 - Cryospheric Change
First Author: Isabella Velicogna
Presenter: Isabella Velicogna
Co-Authors: John Wahr

Abstract: GRACE measurements of time variable gravity provide a unique tool to monitor ice mass variability. Using time variable gravity we can directly measure ice mass variations and provide an independent way to monitor ice sheet variability. Using measurements from the GRACE satellites we determine long term and seasonal mass variations of the Antarctic and Greenland ice sheets for over a six year period starting in Apr 2002. During this period both ice sheets displayed significant mass loss. We will analyze the current progress and uncertainties in the GRACE ice mass estimates and we will examine and interpret differences in mass balance estimates from different processing techniques, as well as the effects that various filtering technique have on the final results.

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Title: GRACE Observes Small-Scale Mass Loss in Greenland
Session: B.2 - Cryospheric Change
First Author: Bert Wouters
Presenter: Ernst Schrama
Co-Authors: D. Chambers, E. Schrama

Abstract: Using gravity data from the GRACE satellites between February 2003 and January 2008, we examine changes in Greenland's mass distribution on a regional scale. During this period, Greenland lost mass at a mean rate of 179 +/- 25 Gt/yr, equivalent to a global mean sea level change of 0.5 +/- 0.1 mm/yr. Rates increase over time and are driven by mass loss during the summers, which vary substantially over the years. The largest mass losses occurred along the southeastern and northwestern coast in the summers of 2005 and 2007, when the ice sheet lost 279 Gt and 328 Gt of ice respectively within 2 months. In 2007, a substantial mass loss is observed during summer at elevations above 2000 m, for the first time since the start of the GRACE observations.

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Title: Present-day West Antarctic ice-mass change estimate by the constrained inversion of GRACE and InSAR data
Session: B.2 - Cryospheric Change
First Author: Ingo Sasgen
Presenter: Ingo Sasgen
Co-Authors: Z. Martinec; J. Bamber

Abstract: We estimate ice mass change of drainage basins in the Amundsen Sea Sector, West Antarctica, from the Gravity Recovery and Climate Experiment (GRACE) using an inverse gravimetric approach. The inversion is constrained by the InSAR data to minimize the ambiguity of the resulting mass estimates. We determine the spatial resolution of GRACE over this region with respect to different GRACE error models. We then use this information to calculate unconstrained (GRACE only) mass balance estimates for a reduced number of drainage basins. We compare InSAR, constrained and unconstrained mass change rates, estimate their uncertainties and discuss the influence of glacial-isostatic adjustment on our results.

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Title: Changes of the Greenland Ice Sheet from GRACE and ICESat
Session: B.2 - Cryospheric Change
First Author: Louise Sandberg Sorensen
Presenter: Louise Sandberg Sorensen
Co-Authors: Rene Forsberg

Abstract: In recent years, several mass change estimates of the Greenland Ice Sheet have been published. There are large discrepancies between these published results based on different data sets and methods. There is agreement though on the fact that the Greenland ice sheet is currently experiencing a net mass loss.

In this paper we use a generalized inversion method to estimate the Greenland ice sheet mass change from the GRACE monthly global gravity solutions, provided by the different processing centers. We use a simple correction for PGR based on the ice history model ICE-5G and ground measurements in Scandinavia.

It is shown that there are significant differences in the estimates of total mass changes based on the different data sets (for example the ice mass changes derived from CSR and GFZ data are found to be -169 km3/year and -208 km3/year in water equivalent respectively), but that there is an overall agreement on the pattern of the mass change over the ice sheet. We also show preliminary results of ICESat derived height changes over the entire Greenland ice sheet, and these are compared with the GRACE results. We find that there is a spacial correlation between the GRACE mass changes and the ICESat height changes.

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