Description: Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in key regions. Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth to ice and ocean load changes occurring since the Last Glacial Maximum (LGM; ~21 thousand years ago) and may be used to constrain the GrIS deglaciation history. We use data from the Greenland Global Positioning System network to directly measure GIA and estimate basin-wide mass changes since the LGM. Unpredicted, large GIA uplift rates of +12 mm/year are found in southeast Greenland. These rates are due to low upper mantle viscosity in the region, from when Greenland passed over the Iceland hot spot about 40 million years ago. This region of concentrated soft rheology has a profound influence on reconstructing the deglaciation history of Greenland. We reevaluate the evolution of the GrIS since LGM and obtain a loss of 1.5-m sea-level equivalent from the northwest and southeast. These same sectors are dominating modern mass loss. We suggest that the present destabilization of these marine-based sectors may increase sea level for centuries to come. Our new deglaciation history and GIA uplift estimates suggest that studies that use the Gravity Recovery and Climate Experiment satellite mission to infer present-day changes in the GrIS may have erroneously corrected for GIA and underestimated the mass loss by about 20 gigatons/year.

Description: Accurate quantification of the millennial-scale mass balance of the Greenland ice sheet (GrIS) and its contribution to global sea-level rise remain challenging because of sparse in situ observations in key regions. Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth to ice and ocean load changes occurring since the Last Glacial Maximum (LGM; ~21 thousand years ago) and may be used to constrain the GrIS deglaciation history. We use data from the Greenland Global Positioning System network to directly measure GIA and estimate basin-wide mass changes since the LGM. Unpredicted, large GIA uplift rates of +12 mm/year are found in southeast Greenland. These rates are due to low upper mantle viscosity in the region, from when Greenland passed over the Iceland hot spot about 40 million years ago. This region of concentrated soft rheology has a profound influence on reconstructing the deglaciation history of Greenland. We reevaluate the evolution of the GrIS since LGM and obtain a loss of 1.5-m sea-level equivalent from the northwest and southeast. These same sectors are dominating modern mass loss. We suggest that the present destabilization of these marine-based sectors may increase sea level for centuries to come. Our new deglaciation history and GIA uplift estimates suggest that studies that use the Gravity Recovery and Climate Experiment satellite mission to infer present-day changes in the GrIS may have erroneously corrected for GIA and underestimated the mass loss by about 20 gigatons/year.

Description: Der glazial-isostatische Ausgleich in Island infolge des rezenten Abschmelzens der Vatnajökull-Eiskappe wird durch die Viskositätsverteilung im Erdinnern und durch die Details der Abschmelzgeschichte kontrolliert. Interpretationen der Ergebnisse von GPS- und Schweremeßkampagnen im Zeitintervall 1991–2000 bzw. 1992–1999 mit Hilfe lateral homogener Erdmodelle zur Bestimmung der Lithosphärenmächtigkeit, Asthenosphärenmächtigkeit und Asthenosphärenviskosität sind bislang nicht voll zufriedenstellend gewesen. Insbesondere nahe des Eisrandes war die Anpassung der berechneten Landhebung und Schwereänderung an die Beobachtungsdaten nur unzureichend, was mit der Nichtberücksichtigung des Island-Plumes in den lateral homogenen Erdmodellen zusammenhängen kann. In der vorliegenden Arbeit wird für die Modellierung der Landhebung und Schwereänderung ein Programmpaket verwendet, daß die Berechnung auflastinduzierter Störungen eines Maxwellviskoelastischen, inkompressiblen, selbstgravitierenden, sphärischen Erdmodells gestattet. Um das Vorhandensein des Plumes unter dem Vatnajökull zu simulieren, wird eine axialsymmetrische Viskositätsverteilung verwendet, wobei der Plumeradius und die Plumeviskosität freie Parameter sind. Basierend auf seismischen Ergebnissen wird über dem Plume eine 6 km mächtige Lithosphäre angenommen, die sich im peripheren Bereich des Plumes auf 35 km verdickt. Die Abschmelzgeschichte des Vatnajökulls beruht auf Interpretationen geomorphologischer und klimatologischer Untersuchungen und wird durch eine mit dem Plume koaxiale Last mit parabolischem Profil und zeitabhängigem Radius simuliert. Die Ergebnisse der Modellierung favorisieren einen Plumeradius von ~ 80 km und eine Plumeviskosität von (0.3–1.0) × 1018 Pa s.

Description: The Gravity Recovery and Climate Experiment (GRACE) has so far seen around 4 years worth of monthly gravity-field solutions being released to the scientific community. These are provided in the form of Stokes potential coefficients by the GRACE Science Data Service centers; the Center for Space Research, University of Texas (CSR), the GeoForschungsZentrum Potsdam (GFZ) and the Jet Propulsion Laboratory (JPL), as well as the Centre National d'Études Spatiales (CNES). We make use of the releases from these centers that have the longest time series, and infer temporal changes in the geoid for Australia by fitting a model incorporating secular, annual, and semi-annual terms to the time series of each of the Stokes potential coefficients that make up the solutions. Geoid change in Australia, neglecting oceanic and atmospheric contributions, arises mainly from hydrological processes, ongoing glacial-isostatic adjustment and present-day global ice-mass changes. Predictions are made of these contributions using models describing changes in continental water storage, ice-volume changes in the areas of major present-day ice cover, and the continuing viscoelastic response of the Earth to the last glacial-interglacial transition. The reliability of the inferred geoid-change terms is examined using several classical statistical tests, namely the Student t-test and the Fisher F-test. In addition, we apply the Wiener Optimal Evaluator to the original GRACE solutions to determine the preferred release.

Description: In this study, a new method for computing the sensitivity of the glacial isostatic adjustment (GIA) forward solution with respect to the Earth's mantle viscosity, the so-called the forward sensitivity method (FSM), and a method for computing the gradient of data misfit with respect to viscosity parameters, the so-called adjoint-state method (ASM), are presented. These advanced formal methods complement each other in the inverse modelling of GIA-related observations. When solving this inverse problem, the first step is to calculate the forward sensitivities by the FSM and use them to fix the model parameters that do not affect the forward model solution, as well as identifying and removing redundant parts of the inferred viscosity structure. Once the viscosity model is optimized in view of the forward sensitivities, the minimization of the data misfit with respect to the viscosity parameters can be carried out by a gradient technique which makes use of the ASM. The aim is this paper is to derive the FSM and ASM in the forms that are closely associated with the forward solver of GIA developed by Martinec. Since this method is based on a continuous form of the forward model equations, which are then discretized by spectral and finite elements, we first derive the continuous forms of the FSM and ASM and then discretize them by the spectral and finite elements used in the discretization of the forward model equations. The advantage of this approach is that all three methods (forward, FSM and ASM) have the same matrix of equations and use the same methodology for the implementation of the time evolution of stresses. The only difference between the forward method and the FSM and ASM is that the different numerical differencing schemes for the time evolution of the Maxwell and generalized Maxwell viscous stresses are applied in the respective methods. However, it requires only a little extra computational time for carrying out the FSM and ASM numerically. An straightforward approach to compute the gradient of the data misfit is the brute-force method, whereby the partial derivatives of the misfit with respect to model parameters are approximated by the centred difference of two forward model runs. Although the brute-force method is useful for computing the gradient of the data misfit with respect to a small number of model parameters, it becomes expensive for a viscosity model with a large number of parameters. The ASM offers an efficient alternative for computing the gradient of the misfit since the computational time of the ASM is independent of the number of viscosity parameters. The ASM is thus highly efficient for calculating the gradient of the misfit for models with large numbers of parameters. However, the forward-model solution for each time step must be stored, hence the memory demands scale linearly with the number of time steps. This is the main drawback of the ASM.

Description: A problem that arises when investigating the spatial representation of the monthly gravity-field solutions from the Gravity Recovery and Climate Experiment (GRACE) space mission is the presence of north-south oriented striped noise. This is a result of the high inclination of the GRACE spacecrafts (89.5degree) and the configuration of the GRACE satellites, resulting in less-dense sampling in the east-west direction. We compare the results from the application of three different post-processing filters to the GRACE gravity-field solutions, and their ability to reduce the characteristic "striping". These are: (1) a statistical filter, where temporal trends inferred from the time series of the GRACE solution Stokes coefficients are tested for their statistical reliability, (2) an order-dependent filter, as applied by Swenson and Wahr (2006), and (3), an anisotropic moving average filter applied to the solutions in the spatial domain. We discuss the similarities and limitations of each filter, and their benefits for various aspects of GRACE analysis.

Description: The GFZ German Research Centre for Geosciences as part of the GRACE Science Data System (SDS) is currently reprocessing the complete GRACE mission data. This new Level-2 data release (RL06 in the SDS nomenclature) will be based on reprocessed Level-1B instrument data (RL03), updated processing standards and background models and will take care of limitations known from previous RL05. Examples are the application of the latest RL06 Atmosphere and Ocean Dealiasing Model, update of the ocean tide model, implementation of the most recent IERS conventions or improvements in GFZ´s GPS data processing. This 15+ year time series of monthly Level-2 spherical harmonics and underlying processing standards will then serve for the continuation with GRACE-FO (Follow-on) data expected for early 2018. In parallel a team of GFZ, the Alfred-Wegener-Institute Bremerhaven and TU Dresden has developed and implemented a portal at GFZ where users can download dedicated Level-3 products for hydrological, oceanic and polar research activities. This portal is expected to be made public by the end of 2017. The presentation will show the status and examples of these new RL06 Level-2 products and prototype Level-3 products based on GFZ’s RL05a Level-2 monthly solutions.

Description: This study is concerned with the forward modelling of the present-day glacial-isostatic adjustment (GIA) of the earth to present and past changes of the Antarctic ice sheet (AIS). We predict temporal variations in the geoid height and topographic height within the context of the Gravity Recovery and Climate Experiment (GRACE) satellite mission and terrestrial Global Positioning System (GPS) stations in Antarctica.