Description: Using recently released reprocessed gravity solutions from the Gravity Recovery and Climate Experiment (GRACE), we estimate the ice loss rate for the Patagonia Icefield (PIF) of South America, for the period April 2002 through December 2006. After postglacial rebound and hydrological effects are corrected, the estimated rate is -27.9 ± 11 km3/year, equivalent to an average loss of ~-1.6 m/year ice thickness change if evenly distributed over the entire PIF area. The estimated contribution to global sea level rise is 0.078 ± 0.031 mm/year. This is an independent confirmation of relatively large melting rate estimates from earlier studies employing topographic and cartographic data.

Description: Earth’s spin axis has been wandering along the Greenwich meridian since about 2000, representing a 75° eastward shift from its long-term drift direction. The past 115 years have seen unequivocal evidence for a quasi-decadal periodicity, and these motions persist throughout the recent record of pole position, in spite of the new drift direction. We analyze space geodetic and satellite gravimetric data for the period 2003–2015 to show that all of the main features of polar motion are explained by global-scale continent-ocean mass transport. The changes in terrestrial water storage (TWS) and global cryosphere together explain nearly the entire amplitude (83 ± 23%) and mean directional shift (within 5.9 ° ± 7.6 ° ) of the observed motion. We also find that the TWS variability fully explains the decadal-like changes in polar motion observed during the study period, thus offering a clue to resolving the long-standing quest for determining the origins of decadal oscillations. This newly discovered link between polar motion and global-scale TWS variability has broad implications for the study of past and future climate.

Description: There is a general consensus among Earth scientists that melting of land ice greatly contributes to sea-level rise (SLR) and that future warming will exacerbate the risks posed to human civilization. As land ice is lost to the oceans, both the Earth’s gravitational and rotational potentials are perturbed, resulting in strong spatial patterns in SLR, termed sea-level fingerprints. We lack robust forecasting models for future ice changes, which diminishes our ability to use these fingerprints to accurately predict local sea-level (LSL) changes. We exploit an advanced mathematical property of adjoint systems and determine the exact gradient of sea-level fingerprints with respect to local variations in the ice thickness of all of the world’s ice drainage systems. By exhaustively mapping these fingerprint gradients, we form a new diagnosis tool, henceforth referred to as gradient fingerprint mapping (GFM), that readily allows for improved assessments of future coastal inundation or emergence. We demonstrate that for Antarctica and Greenland, changes in the predictions of inundation at major port cities depend on the location of the drainage system. For example, in London, GFM shows LSL that is significantly affected by changes on the western part of the Greenland Ice Sheet (GrIS), whereas in New York, LSL change predictions are greatly sensitive to changes in the northeastern portions of the GrIS. We apply GFM to 293 major port cities to allow coastal planners to readily calculate LSL change as more reliable predictions of cryospheric mass changes become available.

Description: The annual cycle and secular trend of Greenland mass loading are well recorded in measurements of solid Earth deformation. Horizontal crustal displacements can potentially track the spatiotemporal detail of mass changes with great fidelity. Our analysis of Greenland crustal motion data reveals that a significant excitation of horizontal amplitudes occurs during the intense melt years. We discover that solitary seasonal waves of substantial mass transport (1.67 ± 0.54 Gt/month) traveled at a maximum speed of 7.1 km/month through Rink Glacier in 2012. We deduce that intense surface melting enhanced either basal lubrication or softening of shear margins, or both, causing the glacier to thin dynamically in summer. The newly routed upstream subglacial water was likely to be both retarded and inefficient, thus providing a causal mechanism for the prolonged ice transport to continue well into the winter months. As the climate continues to produce increasingly warmer spring and summer, amplified seasonal waves of mass transport may become ever more present with important ramifications for the future sea level rise.

Description: We provide a new analysis of glacial isostatic adjustment (GIA) with the goal of assembling the model uncertainty statistics required for rigorously extracting trends in surface mass from the Gravity Recovery and Climate Experiment (GRACE) mission. Such statistics are essential for deciphering sea level, ocean mass, and hydrological changes because the latter signals can be relatively small (≤2 mm/yr water height equivalent) over very large regions, such as major ocean basins and watersheds. With abundant new 〉7 year continuous measurements of vertical land motion (VLM) reported by Global Positioning System stations on bedrock and new relative sea level records, our new statistical evaluation of GIA uncertainties incorporates Bayesian methodologies. A unique aspect of the method is that both the ice history and 1-D Earth structure vary through a total of 128,000 forward models. We find that best fit models poorly capture the statistical inferences needed to correctly invert for lower mantle viscosity and that GIA uncertainty exceeds the uncertainty ascribed to trends from 14 years of GRACE data in polar regions.

Description: Lateral heterogeneities in the Earth’s crust and mantle are demanded from seismic tomographic models, surface data and constraints derived from mantle dynamics. Nevertheless, such structural features are often neglected in GIA and only a 1D structure is assumed for the prediction of the earth’s response to glacial loading as for the inversion of mantle viscosity. 1D model assumption is valid when focussing on vertical motions which are less sensitive to lateral variations in mantle structure but it is questionable for the prediction of horizontal motions. In this presentation, we discuss the consequences which arise for the deformational behaviour of the Earth’s interior if we consider lateral viscosity variations. In particular, our study focusses on viscosity variations in the upper mantle including the mantle lithosphere and implications for plate motions as for the global gravity field are discussed.

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: The measurement of glacial isostatic adjustment (GIA) is one of the key ways in which geophysicists probe the long-term mantle rheology and Pleistocene ice history. GIA models are also tied to global and regional relative sea-level (RSL) histories, to 20th century tide-gauge (TG) data and to space and terrestrial geodetic measurements. Two new types of observation are related to the high-resolution space–gravity data recovered from the Gravity and Climate Experiment (GRACE) satellite pair and the soon-to-be launched Gravity and Ocean Circulation Experiment (GOCE) with on-board three-component gradiometer. Gravity mapping has the unique capability of isolating those regions that lack isostatic equilibrium. When coupled with other space and terrestrial geodetic measurements, such as those of the Global Positioning System (GPS) networks and with multi-decade terrestrial gravity data, new constraints on GIA are in the offing and should soon illuminate new interpretations of ice-sheet history and mantle response. GIA studies also incorporate space-based altimetry data, which now provide multi-decadal coverage over continents, oceans and lakes. As we are approaching 72 monthly solutions of GRACE gravity coefficients for determining the Earth's secular component of gravity change over the continents, a new issue has surfaced: the problem of relying on interannual hydrological modeling to determine the hydrological contribution to the linear trend in the gravity field. Correctly extracting this contribution is germane to using the GIA-driven component for modeling solid-Earth and paleo-climatic parameters. Seismic and heat-flux-based models of the Earth's interior are emerging with ever higher levels of sophistication regarding material strength (or viscosity). A basic question raised is: how good are traditional Newtonian and non-Newtonian viscosity models that only allow radial variations of Earth parameters? In other words: under what circumstances must this assumption be abandoned for joint interpretations of new and traditional data sets. In this short review we summarize the issues raised in the papers forming this special issue (SI) dedicated to GIA.