René Forsberg

A Reconciled Estimate of Ice-Sheet Mass Balance

Warming and Melting Mass loss from the ice sheets of Greenland and Antarctica account for a large fraction of global sea-level rise. Part of this loss is because of the effects of warmer air temperatures, and another because of the rising ocean temperatures to which they are being exposed. Joughin et al. (p. 1172) review how ocean-ice interactions are impacting ice sheets and discuss the possible ways that exposure of floating ice shelves and grounded ice margins are subject to the influences of warming ocean currents. Estimates of the mass balance of the ice sheets of Greenland and Antarctica have differed greatly—in some cases, not even agreeing about whether there is a net loss or a net gain—making it more difficult to project accurately future sea-level change. Shepherd et al. (p. 1183) combined data sets produced by satellite altimetry, interferometry, and gravimetry to construct a more robust ice-sheet mass balance for the period between 1992 and 2011. All major regions of the two ice sheets appear to be losing mass, except for East Antarctica. All told, mass loss from the polar ice sheets is contributing about 0.6 millimeters per year (roughly 20% of the total) to the current rate of global sea-level rise. The mass balance of the polar ice sheets is estimated by combining the results of existing independent techniques. We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth’s polar ice sheets. We find that there is good agreement between different satellite methods—especially in Greenland and West Antarctica—and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by –142 ± 49, +14 ± 43, –65 ± 26, and –20 ± 14 gigatonnes year−1, respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year−1 to the rate of global sea-level rise.

Exploring Arctic Transpolar Drift During Dramatic Sea Ice Retreat

The Arctic is undergoing significant environmental changes due to climate warming. The most evident signal of this warming is the shrinking and thinning of the ice cover of the Arctic Ocean. If the warming continues, as global climate models predict, the Arctic Ocean will change from a perennially ice-covered to a seasonally ice-free ocean. Estimates as to when this will occur vary from the 2030s to the end of this century. One reason for this huge uncertainty is the lack of systematic observations describing the state, variability, and changes in the Arctic Ocean.

Bedrock displacements in Greenland manifest ice mass variations, climate cycles and climate change

The Greenland GPS Network (GNET) uses the Global Positioning System (GPS) to measure the displacement of bedrock exposed near the margins of the Greenland ice sheet. The entire network is uplifting in response to past and present-day changes in ice mass. Crustal displacement is largely accounted for by an annual oscillation superimposed on a sustained trend. The oscillation is driven by earth’s elastic response to seasonal variations in ice mass and air mass (i.e., atmospheric pressure). Observed vertical velocities are higher and often much higher than predicted rates of postglacial rebound (PGR), implying that uplift is usually dominated by the solid earth’s instantaneous elastic response to contemporary losses in ice mass rather than PGR. Superimposed on longer-term trends, an anomalous ‘pulse’ of uplift accumulated at many GNET stations during an approximate six-month period in 2010. This anomalous uplift is spatially correlated with the 2010 melting day anomaly.

New Gravity Field for the Arctic

The study of the Arctic Ocean has been hampered by incomplete basic knowledge of the basin and its structure. An improved gravity anomaly map (Figure 1) and grid have been developed that complement the new International Chart of the Arctic Ocean [Jakobsson et al., 2008]. This article announces the availability of the Arctic Gravity Project (ArcGP) grid version 2.0 and discusses the genesis of the project.

Topography and penetration of the Greenland Ice Sheet measured with Airborne SAR Interferometry

A digital elevation model (DEM) of the Geikie ice cap in East Greenland has been generated from interferometric C-band synthetic aperture radar (SAR) data acquired with the airborne EMISAR system. GPS surveyed radar reflectors and an airborne laser altimeter supplemented the experiment. The accuracy of the SAR DEM is about 1.5m. The mean difference between the laser heights and the SAR heights changes from 0 m in the soaked zone to a maximum of 13 m in the percolation zone. This is explained by the fact that the snow in the soaked zone contains liquid water which attenuates the radar signals, while the transparency of the firn in the percolation zone makes volume scattering dominate at the higher elevations. For the first time, the effective penetration has been measured directly as the difference between the interferometric heights and reference heights obtained with GPS and laser altimetry.

A low-cost glacier-mapping system

An old portable 60 MHz radar has been upgraded with a new digital data-processing and -acquisition system and a new antenna construction enabling a fast and low-cost installation on aTwin Otter aircraft. Augmented by a laser altimeter and kinematic global positioning system (GPS), the system has the capability of acquiring accurate data on location and ice-surface elevation, and adequate-quality data on ice thickness. The system has been applied successfully in mapping the Nioghalvfjerdsfjorden glacier, northeast Greenland, in spite of the difficult conditions with melting water on the glacier surface. The measurements from the floating part of the glacier have been evaluated by comparison of radar data with laser-altimeter and in situ measurements.

Modelling the fine-structure of the geoid: Methods, data requirements and some results

The requirements for precise geoid models on local and regional scales have increased in recent years, primarily due to the ongoing developments in height determination by GPS on land, but also due to oceanographic requirements in using satellite altimetry for recovering dynamic sea-surface topography. Suitable methods for geoid computations from gravity data include Stokes integration, FFT methods, and least-squares collocation. Especially the FFT methods are efficient in handling large amounts of gravity data, and new variants of the methods taking earth curvature rigorously into account provide attractive methods for obtaining continental-scale, high-resolution geoid models. The accuracy of such models may be from 2–5 cm locally, to 50–100 cm on regional scales, depending on gravity data coverage, long wave-length gravity field errors, and datum problems. When approaching the cm-level geoid basic geoid definition questions (geoid or quasigeoid?) become very significant, especially in rugged areas.In the paper the geoid modelling methods and problems are reviewed, and some investigations on local data requirements for cm-level geoid prediction are presented. Some actual results are presented from Scandinavia, where a recent regional high-resolution geoid model yields apparent accuracies of 2–10 cm over GPS baselines of 50 to 2000 km.

New map compiled of Europe's gravity field

A gravity anomaly map of Europe was recently compiled that incorporates significant new data, especially from the eastern European countries. The map (Figure 1a), which was developed by a group of geophysicists and geodesists, goes a long way toward establishing a common and reliable database. Geological information contained in the regional gravity field are highlighted in the map, and all known large-scale geological structures are well coordinated with the anomaly patterns. The map offers basic gravity information for geophysical studies related to subsurface geology and large-scale tectonic features in Europe and reinforces or supports previous interpretations of largescale tectonic features.

Airborne Gravity Field Determination

Airborne measurement of gravity has long been a goal for geodesy and geophysics, both to serve geodetic needs (such as geoid determination) and in order to provide efficient and economic mapping of gravity anomalies for geophysical exploration. Although airborne gravimetry has been attempted since the 1960s (LaCoste 1967), it is only in the 1990s, with the development of carrier-phase kinematic GPS methods, that the accuracy has reached a useful level. In later years new gravity acceleration sensors and improved GPS processing methods have resulted in airborne survey accuracies of 1 mGal (10–5 m/s2) or less at a resolution of a few kilometers for several commercial operators (Williams and MacQueen 2001), typically operating in relatively small regions for geophysical exploration and flying during optimal conditions (e.g., at night when turbulence is minimal).

Satellite-based estimates of sea-ice volume flux through Fram Strait

Abstract Sea-ice volume fluxes through Fram Strait, Arctic Ocean, are estimated for the two Icesat measurement periods in February/March and October/November 2003 by combining Sea-ice area fluxes, determined from Space-borne microwave observations, with estimates of the Sea-ice thickness distribution, inferred from measurements of Icesat’s Geoscience Laser Altimeter System (GLAs) instrument. The thickness is derived from Icesat data by converting its Surface elevation measurements into an ice freeboard estimate. Combined with prior information about ice density and Snow depth and density, the freeboard is converted into ice thickness. Uncertainties in freeboard estimates due to geoid model errors are reduced through the use of the recent geoid from the Arctic Gravity Project. Missing information about the ocean circulation and ocean tides is approximated locally by interpolating the Sea Surface height linearly between open leads. Meridional ice volume fluxes estimated for 79˚N using ice drift observed by AMSR-E (QuiksCAT) amount to 168 km3 (236km3) and 62 km3 (77 km3) for 30 day periods in February/March and October/November 2003, respectively. These values lie in the range of previous results from Similar Studies, but are considerably Smaller than the average ice flux during the 1990s, most likely because of a Smaller ice-drift Speed during 2003.