Description: We present regional-scale mass balances for 25 drainage basins of the Antarctic Ice Sheet (AIS) from satellite observations of the Gravity and Climate Experiment (GRACE) for the years 2002–2011. Satellite gravimetry estimates of the AIS mass balance are strongly influenced by mass movement in the Earth interior caused by ice advance and retreat during the last glacial cycle. Here, we develop an improved glacial-isostatic adjustment (GIA) estimate for Antarctica using newly available GPS uplift rates, allowing us to more accurately separate GIA-induced trends in the GRACE gravity fields from those caused by current imbalances of the AIS. Our revised GIA estimate is considerably lower than previous predictions, yielding an (upper) estimate of apparent mass change of 48 ± 18 Gt yr−1. Therefore, our AIS mass balance of −103 ± 23 Gt yr−1 is considerably less negative than previous GRACE estimates. The Northern Antarctic Peninsula and the Amundsen Sea Sector exhibit the largest mass loss (−25 ± 6 Gt yr−1 and −126 ± 11 Gt yr−1, respectively). In contrast, East Antarctica exhibits a slightly positive mass balance (19 ± 16 Gt yr−1), which is, however, mostly the consequence of compensating mass anomalies in Dronning Maud and Enderby Land (positive) and Wilkes and George V Land (negative) due to interannual accumulation variations. In total, 7% of the area constitute more than half of the AIS imbalance (53%), contributing −151 ± 9 Gt yr−1 to global mean sea-level change. Most of this imbalance is caused by long-term ice-dynamic speed up expected to prevail in the future.

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: Two regions in West Antarctica have received increased attention over the last years due to their potential sensitivity to climate change: the Antarctic Peninsula, which currently experiences warming at rates greater than the global average (Vaughan, 2006), ongoing ice shelf disintegration and subsequent glacier acceleration (Scambos et al., 2004); and the Amundsen Sea Sector (AS), where ice velocities, ice discharge and glacial imbalances are extreme compared to the rest of the continent (Rignot et al., 2008). The satellite gravimetry mission GRACE reveals interannual variations in ice mass related to varying snowfall, which is strongly influenced by the global climate phenomenon El Nino (Sasgen et al., 2010).

Description: The Greenland and Antarctic ice sheets have been reported to be losing mass at accelerating rates1, 2. If sustained, this accelerating mass loss will result in a global mean sea-level rise by the year 2100 that is approximately 43 cm greater than if a linear trend is assumed2. However, at present there is no scientific consensus on whether these reported accelerations result from variability inherent to the ice-sheet–climate system, or reflect long-term changes and thus permit extrapolation to the future3. Here we compare mass loss trends and accelerations in satellite data collected between January 2003 and September 2012 from the Gravity Recovery and Climate Experiment to long-term mass balance time series from a regional surface mass balance model forced by re-analysis data. We find that the record length of spaceborne gravity observations is too short at present to meaningfully separate long-term accelerations from short-term ice sheet variability. We also find that the detection threshold of mass loss acceleration depends on record length: to detect an acceleration at an accuracy within ±10 Gt yr−2, a period of 10 years or more of observations is required for Antarctica and about 20 years for Greenland. Therefore, climate variability adds uncertainty to extrapolations of future mass loss and sea-level rise, underscoring the need for continuous long-term satellite monitoring.

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.