Bernd Scheuchl

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.

Ice Flow of the Antarctic Ice Sheet

A high-resolution map of ice motion in Antarctica shows the details of ice movement in a warming climate. We present a reference, comprehensive, high-resolution, digital mosaic of ice motion in Antarctica assembled from multiple satellite interferometric synthetic-aperture radar data acquired during the International Polar Year 2007 to 2009. The data reveal widespread, patterned, enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent. This view of ice sheet motion emphasizes the importance of basal-slip–dominated tributary flow over deformation-dominated ice sheet flow, redefines our understanding of ice sheet dynamics, and has far-reaching implications for the reconstruction and prediction of ice sheet evolution.

Ice-Shelf Melting Around Antarctica

Major Meltdown The ice shelves and floating ice tongues that surround Antarctica cover more than 1.5 million square kilometers—approximately the size of the entire Greenland Ice Sheet. Conventional wisdom has held that ice shelves around Antarctica lose mass mostly by iceberg calving, but recently it has become increasingly clear that melting by a warming ocean may also be important. Rignot et al. (p. 266, published 13 June) present detailed glaciological estimates of ice-shelf melting around the entire continent of Antarctica, which show that basal melting accounts for as much mass loss as does calving. Basal melting of Antarctic ice shelves accounts for as much mass loss as does iceberg calving. We compare the volume flux divergence of Antarctic ice shelves in 2007 and 2008 with 1979 to 2010 surface accumulation and 2003 to 2008 thinning to determine their rates of melting and mass balance. Basal melt of 1325 ± 235 gigatons per year (Gt/year) exceeds a calving flux of 1089 ± 139 Gt/year, making ice-shelf melting the largest ablation process in Antarctica. The giant cold-cavity Ross, Filchner, and Ronne ice shelves covering two-thirds of the total ice-shelf area account for only 15% of net melting. Half of the meltwater comes from 10 small, warm-cavity Southeast Pacific ice shelves occupying 8% of the area. A similar high melt/area ratio is found for six East Antarctic ice shelves, implying undocumented strong ocean thermal forcing on their deep grounding lines.

Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011

We measure the grounding line retreat of glaciers draining the Amundsen Sea sector of West Antarctica using Earth Remote Sensing (ERS-1/2) satellite radar interferometry from 1992 to 2011. Pine Island Glacier retreated 31 km at its center, with most retreat in 2005-2009 when the glacier ungrounded from its ice plain. Thwaites Glacier retreated 14 km along its fast flow core and 1 to 9 km along the sides. Haynes Glacier retreated 10 km along its flanks. Smith/Kohler glaciers retreated the most, 35 km along its ice plain, and its ice shelf pinning points are vanishing. These rapid retreats proceed along regions of retrograde bed elevation mapped at a high spatial resolution using a mass conservation technique that removes residual ambiguities from prior mappings. Upstream of the 2011 grounding line positions, we find no major bed obstacle that would prevent the glaciers from further retreat and draw down the entire basin.

Sustained increase in ice discharge from the Amundsen Sea Embayment, West Antarctica, from 1973 to 2013

We combine measurements of ice velocity from Landsat feature tracking and satellite radar interferometry, and ice thickness from existing compilations to document 41 years of mass flux from the Amundsen Sea Embayment (ASE) of West Antarctica. The total ice discharge has increased by 77% since 1973. Half of the increase occurred between 2003 and 2009. Grounding-line ice speeds of Pine Island Glacier stabilized between 2009 and 2013, following a decade of rapid acceleration, but that acceleration reached far inland and occurred at a rate faster than predicted by advective processes. Flow speeds across Thwaites Glacier increased rapidly after 2006, following a decade of near-stability, leading to a 33% increase in flux between 2006 and 2013. Haynes, Smith, Pope, and Kohler Glaciers all accelerated during the entire study period. The sustained increase in ice discharge is a possible indicator of the development of a marine ice sheet instability in this part of Antarctica. Key Points Sustained ASE mass flux increase: 77% since 1973 Thwaites accelerated by 33% during the last 6 years Speed changes are pervasive and rapid: major implications for ice flow modeling ©2014. American Geophysical Union. All Rights Reserved.

Monitoring very slow slope movements by means of SAR interferometry : A case study from a mass waste above a reservoir in the Ötztal Alps, Austria

The application of radar interferometry to detect slope movements on the order of millimeters to centimeters per year is demonstrated. The deformation field of a slope above a hydropower reservoir in the Austrian Alps was derived from SAR interferometric pairs of the satellites ERS-1 and ERS-2, acquired between July 1992 and August 1998. Above the treeline it was possible to map the motion from interferograms covering time spans up to three years. Whereas ground-based geodetic measurements focussed on the movements of the lower slope section, the interferometric analysis shows that the mass wasting processes affect the entire slope of 1000 m vertical extent. Significant interannual differences of the displacement rates became apparent which were found to be related to the pattern of summer rainfall.

Mapping of Ice Motion in Antarctica Using Synthetic-Aperture Radar Data

Ice velocity is a fundamental parameter in studying the dynamics of ice sheets. Until recently, no complete mapping of Antarctic ice motion had been available due to calibration uncertainties and lack of basic data. Here, we present a method for calibrating and mosaicking an ensemble of InSAR satellite measurements of ice motion from six sensors: the Japanese ALOS PALSAR, the European Envisat ASAR, ERS-1 and ERS-2, and the Canadian RADARSAT-1 and RADARSAT-2. Ice motion calibration is made difficult by the sparsity of in-situ reference points and the shear size of the study area. A sensor-dependent data stacking scheme is applied to reduce measurement uncertainties. The resulting ice velocity mosaic has errors in magnitude ranging from 1 m/yr in the interior regions to 17 m/yr in coastal sectors and errors in flow direction ranging from less than 0.5 in areas of fast flow to unconstrained direction in sectors of slow motion. It is important to understand how these mosaics are calibrated to understand the inner characteristics of the velocity products as well as to plan future InSAR acquisitions in the Antarctic. As an example, we show that in broad sectors devoid of ice-motion control, it is critical to operate ice motion mapping on a large scale to avoid pitfalls of calibration uncertainties that would make it difficult to obtain quality products and especially construct reliable time series of ice motion needed to detect temporal changes.

Fast retreat of Zachariæ Isstrøm, northeast Greenland

Shrinking shelf and faster flow Zachariæ Isstrøm, a large glacier in northeast Greenland, began a rapid retreat after detaching from a stabilizing sill in the late 1990s. Mouginot et al. report that between 2002 and 2014, the area covered by the glaciers ice shelf shrank by 95%; since 1999, the glaciers flow rate has nearly doubled; and its acceleration increased threefold in the fall of 2012. These dramatic changes appear to be the result of a combination of warmer air and ocean temperatures and the topography of the ocean floor at the head of the glacier. Rising sea levels should continue to destabilize the marine portion of Zachariæ Isstrøm for decades. Science, this issue p. 1357 A large glacier in northeast Greenland is retreating rapidly as air and ocean warm. After 8 years of decay of its ice shelf, Zachariæ Isstrøm, a major glacier of northeast Greenland that holds a 0.5-meter sea-level rise equivalent, entered a phase of accelerated retreat in fall 2012. The acceleration rate of its ice velocity tripled, melting of its residual ice shelf and thinning of its grounded portion doubled, and calving is now occurring at its grounding line. Warmer air and ocean temperatures have caused the glacier to detach from a stabilizing sill and retreat rapidly along a downward-sloping, marine-based bed. Its equal-ice-volume neighbor, Nioghalvfjerdsfjorden, is also melting rapidly but retreating slowly along an upward-sloping bed. The destabilization of this marine-based sector will increase sea-level rise from the Greenland Ice Sheet for decades to come.

Grounding line retreat of Totten Glacier, East Antarctica, 1996 to 2013

© 2015. American Geophysical Union. All Rights Reserved. Totten Glacier, East Antarctica, a glacier that holds a 3.9 m sea level change equivalent, has thinned and lost mass for decades. We map its grounding line positions in 1996 and 2013 using differential radar interferometry (InSAR) data and develop precise, high-resolution topographies of its ice surface and ice draft using NASA Operation IceBridge data, InSAR data, and a mass conservation method. We detect a 1 to 3 km retreat of the grounding line in 17 years. The retreat is asymmetrical along a two-lobe pattern, where ice is only grounded a few 10 m above sea level, or ice plain, which may unground further with only modest amounts of ice thinning. The pattern of retreat indicates ice thinning of 12 m in 17 years or 0.7±0.1 m/yr at the grounding line on average. Sustained thinning will cause further grounding line retreat but may not be conducive to a marine instability. Key Points Grounding line of Totten Glacier is retreating, not as fast as West Antarctica Retreat pattern explained by the newly inferred bed geometry If ice thinning maintains, bed geometry conducive to further retreat

Modeling of ocean‐induced ice melt rates of five west Greenland glaciers over the past two decades

Geophysical Research Letters RESEARCH LETTER 10.1002/2016GL068784 Key Points: • How does the ocean control the melting of Greenland glaciers? • Glaciers dominated by calving processes are less sensitive to ice-ocean interactions • The study provided a basis for quantifying ice-ocean interaction elsewhere around Greenland Supporting Information: • Supporting Information S1 Correspondence to: E. Rignot, erignot@uci.edu Citation: Rignot, E., et al. (2016), Modeling of ocean-induced ice melt rates of five west Greenland glaciers over the past two decades, Geophys. Res. Lett., 43, doi:10.1002/2016GL068784. Received 25 MAR 2016 Accepted 25 MAY 2016 Accepted article online 30 MAY 2016 Modeling of ocean-induced ice melt rates of five west Greenland glaciers over the past two decades E. Rignot 1,2 , Y. Xu 1 , D. Menemenlis 2 , J. Mouginot 1 , B. Scheuchl 1 , X. Li 1 , M. Morlighem 1 , H. Seroussi 1 , M. van den Broeke 3 , I. Fenty 2 , C. Cai 1 , L. An 1 , and B. de Fleurian 1 1 Department of Earth System Science, University of California, Irvine, California, USA, 2 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, 3 Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, Netherlands Abstract High-resolution, three-dimensional simulations from the Massachusetts Institute of Technology general circulation model ocean model are used to calculate the subaqueous melt rate of the calving faces of Umiamako, Rinks, Kangerdlugssup, Store, and Kangilerngata glaciers, west Greenland, from 1992 to 2015. Model forcing is from monthly reconstructions of ocean state and ice sheet runoff. Results are analyzed in combination with observations of bathymetry, bed elevation, ice front retreat, and glacier speed. We calculate that subaqueous melt rates are 2–3 times larger in summer compared to winter and doubled in magnitude since the 1990s due to enhanced subglacial runoff and 1.6±0.3 ∘ C warmer ocean temperature. Umiamako and Kangilerngata retreated rapidly in the 2000s when subaqueous melt rates exceeded the calving rates and ice front retreated to deeper bed elevation. In contrast, Store, Kangerdlugssup, and Rinks have remained stable because their subaqueous melt rates are 3–4 times lower than their calving rates, i.e., the glaciers are dominated by calving processes. 1. Introduction The Greenland Ice Sheet has been experiencing an accelerating rate of mass loss over the past few decades from a combination of enhanced surface melt and increased glacier ice discharge [e.g., Rignot and Kanagaratnam, 2006; van den Broeke et al., 2009; Enderlin et al., 2014]. Widespread acceleration of its marine-terminating glaciers and fluctuations in the position of their calving margins has been attributed to the enhanced advection of warm subsurface ocean waters of subtropical origin at the ice-ocean boundary [e.g., Holland et al., 2008; Rignot et al., 2010; Straneo et al., 2010; Rignot et al., 2012; Bevan et al., 2012; Christoffersen et al., 2011]. Yet there has been no quantitative demonstration of the role of subaqueous melt in driving tide- water glaciers into a retreat, in part due to our poor knowledge of the magnitude of the subaqueous melt rates, their spatial pattern, horizontally and vertically, and their seasonal to interannual variability. One hypothesis is that subaqueous melting at the ice front has increased sufficiently in magnitude to counteract the advection of ice from upstream and force the glacier front to retreat. As the glacier retreats, the removal of grounded ice at the ice front will reduce basal resistance, which will enable glacier ice from upstream to speedup. Speedup will cause ice to thin from longitudinal stretching, which will be conducive to further retreat. The retreat rate will depend on the glacier geometry, especially the bed elevation below sea level and whether the bed ele- vation increases or decreases farther inland [e.g., Post et al., 2011]. When examining the details of the glacier response to ocean thermal forcing, large variations have been reported from one glacier to the next or within the same fjord system [e.g., Box and Decker, 2011; Moon et al., 2012], which suggests a complex interplay between subaqueous melting, iceberg calving, glacier geometry, and other factors [Benn et al., 2007]. ©2016. American Geophysical Union. All Rights Reserved. RIGNOT ET AL. Here we consider the recent history of five glaciers in central west Greenland for which we have quality data to reconstruct ocean-induced melt rates. We review their time evolution and response to atmospheric and oceanic forcings. We use three-dimensional, high-resolution simulations of ice melt into the ocean to model their horizontal, vertical, seasonal, and interannual variability. Model forcing is constrained using in situ oceanographic data, reconstructions of ice sheet runoff, novel bed topography, and bathymetry data. We compare changes in subaqueous melt rate through time with ice velocity at the ice front. We conclude on the impact of ice-ocean interaction on the glacier dynamics and in turn on glacier stability. MODELING OF GLACIER MELT IN GREENLAND