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Satellite radar interferometry data reveals that the grounding line of Petermann Glacier, Greenland migrates by several kilometers during the tidal cycle, bringing pressurized, subsurface, warm ocean waters in regular contact with a large sector of grounded ice. We use the Massachusetts Institute of Technology general circulation model in two dimensions to calculate the ice melt rates as a function of grounding zone (GZ) length and ocean Thermal Forcing (TF). Ice melt rates are found to be higher in the GZ cavity than anywhere else in the ice shelf cavity. The melt rates increase sub‐linearly with the length of the GZ and ocean TF. The model results agree well with remote sensing estimates of ice melt. High basal ice melt rates in tidally flushed grounding zones imply that marine‐terminating glaciers are more sensitive to ocean TF than anticipated, which will increase their projected contribution to sea level rise. Plain Language Summary: The traditional view of ice melting in contact with ocean waters is that melt rates drop to zero at the grounding line, which is a semi‐fixed boundary at the junction between grounded ice and the ocean. In reality, the grounding line migrates by kilometers during the tidal cycle, more than 10 times beyond the range expected from hydrostatic equilibrium, which brings warm, saline water in rapid contact with broad sectors of grounded ice. We use an ocean model to calculate the melt rates caused by seawater intrusions. We find that the melt rates in the GZ are higher than anywhere else in the ice shelf cavity and increase as the GZ becomes wider and the ocean gets warmer. Ice melt in kilometer‐size grounding zones will reduce the basal resistance to flow and will increase the sensitivity of the glacier flow to ocean warming, hence projections of sea level rise from the glacier will go up. Key Points: First modeling of ice melt rates from seawater intrusions in the kilometer‐size grounding zone (GZ) of Petermann Glacier using an ocean modelModeled melt rates are highest in the GZ and increase linearly with GZ width and ocean thermal forcingHigh melt rates in kilometer‐size grounding zones imply a higher sensitivity of glaciers to ocean warming than anticipated [ABSTRACT FROM AUTHOR]

Denman Glacier, East Antarctica, holds an ice volume equivalent to a 1.5 m rise in global sea level. Using satellite radar interferometry from the COSMO‐SkyMed constellation, we detect a 5.4 ± 0.3 km grounding line retreat between 1996 and 2017–2018. A novel reconstruction of the glacier bed topography indicates that the retreat proceeds on the western flank along a previously unknown 5 km wide, 1,800 m deep trough, deepening to 3,400 m below sea level. On the eastern flank, the grounding line is stabilized by a 10 km wide ridge. At tidal frequencies, the grounding line extends over a several kilometer‐wide grounding zone, enabling warm ocean water to melt ice at critical locations for glacier stability. If warm, modified Circumpolar Deep Water reaches the sub‐ice‐shelf cavity and continues to melt ice at a rate exceeding balance conditions, the potential exists for Denman Glacier to retreat irreversibly into the deepest, marine‐based basin in Antarctica. Plain Language Summary: Using satellite radar data from the Italian COSMO‐SkyMed constellation, we document the grounding line retreat of Denman Glacier, a major glacier in East Antarctica that holds an ice volume equivalent to a 1.5 m global sea level rise. The grounding line is retreating asymmetrically. On the eastern flank, the glacier is protected by a subglacial ridge. On the western flank, we find a deep and steep trough with a bed slope that makes the glacier conducive to rapid retreat. If warm water continues to induce high rates of ice melt near the glacier grounding zone, the potential exists for Denman Glacier to undergo a rapid and irreversible retreat, with major consequences for sea level rise. Key Points: SAR observations of Denman Glacier grounding line, East Antarctica, reveal fast retreat of a glacier with a 1.5 m sea level rise equivalentDenman is retreating along a deep trough with a retrograde bed deepening up to 3.5 km below sea level, one of the deepest basin in AntarcticaWarm water intrusion in the sub‐ice shelf cavity and the retrograde bed topography makes Denman prone to marine instability in near future [ABSTRACT FROM AUTHOR]

Surface ice velocity is a fundamental characteristic of glaciers and ice sheets that quantifies the transport of ice. Changes in ice dynamics have a major impact on ice sheet mass balance and its contribution to sea level rise. Prior comprehensive mappings employed speckle and feature tracking techniques, optimized for fast‐flow areas, with precision of 2‐5 m/year, hence limiting our ability to describe ice flow in the slow interior. We present a vector map of ice velocity using the interferometric phase from multiple satellite synthetic aperture radars resulting in 10 times higher precision in speed (20 cm/year) and direction (5°) over 80% of Antarctica. Precision mapping over areas of slow motion (<1 m/year) improves from 20 to 93%, which helps better constrain drainage boundaries, improve mass balance assessment, evaluate regional atmospheric climate models, reconstruct ice thickness, and inform ice sheet numerical models. Plain Language Summary: We present a new map of Antarctic ice velocity that is 10 times more precise than prior maps and reveals ice motion at a high precision over 80% of the continent versus 20% in the past. The ice motion vector map provides novel constrains on interior ice motion and its connection with the glaciers and ice stream that control the stability and mass balance of the Antarctic Ice Sheet. Key Points: Interferometric phases from multiple satellite radar platforms are combined to derive the first comprehensive phase map of Antarctic ice velocityThe precision in ice speed (20 cm/year) and flow direction (5°) over 80% of Antarctica is a factor 10 better than prior mappings based on feature and speckle trackingThe product will improve mass balance studies, reconstruction of ice thickness, regional atmospheric climate modeling, and numerical ice sheet modeling [ABSTRACT FROM AUTHOR]

The Northern and Southern Patagonian Icefields are the largest ice masses in the Southern Hemisphere outside Antarctica, but their ice volume and bed topography are poorly known. Here, we combine airborne gravity data collected in 2012 and 2016, with radar data from the Warm Ice Experiment Sounder and Centro de Estudios Científicos's to map bed elevation and ice thickness in great detail. We perform a 3‐D inversion of the gravity data constrained by radar‐derived thickness and fjord bathymetry to infer bed elevation at 500‐m spacing, with a precision of about 60 m. We detect deep glacial valleys with ice thickness exceeding 1,400 m and sectors below sea level on the western branch of Glaciar Pio XI, Occidental, between San Rafael and Colonia, and near Fitz Roy. We calculate an ice volume of 4,756 ± 923 km3 for Northern Patagonia Icefield and Southern Patagonia Icefield, or 40 times the volume of glaciers in the European Alps. Plain Language Summary: Traditional techniques of radar depth sounding fail to resolve thick, temperate ice masses due to poor penetration of the radar signals into snow and ice and extensive scattering due to water content. We combine sparse airborne radar sounding and bathymetry data with novel high‐resolution, high‐precision airborne gravity data to infer the bed topography of the Patagonia Icefields of South America. The results reveal the full range of deep ice thickness on the plateau, portions of the icefields grounded below sea level, and the total volume content of the Patagonia Icefields. The results are critical to constrain the details of the past, present, and future evolution of this glaciated region in a warmer climate. Key Points: Comprehensive ice thickness and bed elevation measurements are conducted on the Northern and Southern Patagonian Icefields of South AmericaIce thickness in excess of 1,400 m is observed over selected regions of the plateauA multisensor approach solves the problem of mapping the thickness of temperate ice masses [ABSTRACT FROM AUTHOR]

Knowledge of the past behavior of the outlet glaciers of Southeast (SE) Greenland is necessary to understand and model spatial differences in the response of the Greenland Ice Sheet (GIS) to climatic changes. Here we use bathymetric data to map the distribution of more than 50 major moraines in SE Greenland fjords. Inner‐fjord moraines are widespread along the SE Greenland margin, occurring in 65% of the surveyed fjords. We identify, for the first time, nine midfjord moraines that span the approximately 150‐km‐long eastern margin of the Julianehåb Ice Cap. In contrast, midfjord moraines are generally absent from the deeper and wider fjords of the SE GIS. The variable distribution of midfjord moraines along the SE Greenland margin reveals contrasting behavior of the SE GIS and the eastern Julianehåb Ice Cap during the last deglaciation, which probably reflects differences in fjord geometry and exposure to ocean heat. Key Points: We use bathymetric data to identify major moraine ridges in Southeast Greenland fjordsInner‐fjord moraines, which were probably formed during the Neoglacial, are widespread along the Southeast Greenland marginMidfjord moraines are present beyond the Julianehåb Ice Cap yet are generally absent from the Southeast Greenland Ice Sheet margin [ABSTRACT FROM AUTHOR]

Abstract: In recent decades, tidewater glaciers in Northwest Greenland contributed significantly to sea level rise but exhibited a complex spatial pattern of retreat. Here we use novel observations of bathymetry and water temperature from NASA's Ocean Melting Greenland mission to quantify the role of warm, salty Atlantic Water in controlling the evolution of 37 glaciers. Modeled ocean‐induced undercutting of calving margins compared with ice advection and ice front retreat observed by satellites from 1985 to 2015 indicate that 35 glaciers retreated when cumulative anomalies in ocean‐induced undercutting rose above the range of seasonal variability of calving‐front positions, while two glaciers standing on shallow sills and colder water did not retreat. Deviations in the observed timing of retreat are explained by residual uncertainties in bathymetry, inefficient mixing of waters in shallow fjords, and the presence of small floating sections. Overall, warmer ocean temperature triggered the retreat, but calving processes dominate ablation (71%). [ABSTRACT FROM AUTHOR]

Abstract: The evolution of Greenland glaciers in a warming climate depends on their depth below sea level, flow speed, surface melt, and ocean‐induced undercutting at the calving front. We present an innovative mapping of bed topography in the frontal regions of Sermeq Avannarleq and Kujalleq, two major glaciers flowing into the ice‐choked Torssukatak Fjord, central west Greenland. The mapping combines a mass conservation algorithm inland, multibeam echo sounding data in the fjord, and high‐resolution airborne gravity data at the ice‐ocean transition where other approaches have traditionally failed. We obtain a reliable, precision (±40 m) solution for bed topography across the ice‐ocean boundary. The results reveal a 700 m deep fjord that abruptly ends on a 100–300 m deep sill along the calving fronts. The shallow sills explain the presence of stranded icebergs, the resilience of the glaciers to ocean‐induced undercutting by warm Atlantic water, and their remarkable stability over the past century. [ABSTRACT FROM AUTHOR]

Abstract: We employ National Aeronautics and Space Administration (NASA)'s Operation IceBridge high‐resolution airborne gravity from 2016, NASA's Ocean Melting Greenland bathymetry from 2015, ice thickness from Operation IceBridge from 2010 to 2015, and BedMachine v3 to analyze 20 major southeast Greenland glaciers. The results reveal glacial fjords several hundreds of meters deeper than previously thought; the full extent of the marine‐based portions of the glaciers; deep troughs enabling warm, salty Atlantic Water (AW) to reach the glacier fronts and melt them from below; and few shallow sills that limit the access of AW. The new oceanographic and topographic data help to fully resolve the complex pattern of historical ice front positions from the 1930s to 2017: glaciers exposed to AW and resting on retrograde beds have retreated rapidly, while glaciers perched on shallow sills or standing in colder waters or with major sills in the fjords have remained stable. [ABSTRACT FROM AUTHOR]

Recent studies have shown that the Northeast Greenland Ice Sheet region has been undergoing significant acceleration and dynamic thinning since 2010, and these changes are closely related to regional atmospheric and oceanic warming. Here we model the response of Nioghalvfjerdsfjorden (79North) and Zachariae Isstrøm (ZI) to ocean forcing to investigate their evolution over the coming decades. Our model suggests that 79North will retreat slowly over the next century, whereas ZI will lose its floating ice tongue completely and retreat rapidly for 70 years. After 70 years, ZI will stabilize 30 km upstream of its current position on a topographic ridge. Frontal melt rates need to reach 6 m/d in the summer to dislodge the glacier from this ridge. ZI will then continue a fast and unstoppable retreat, contributing more than 16.2 mm to global sea level rise by 2100. [ABSTRACT FROM AUTHOR]

Greenland's bed topography is a primary control on ice flow, grounding line migration, calving dynamics, and subglacial drainage. Moreover, fjord bathymetry regulates the penetration of warm Atlantic water (AW) that rapidly melts and undercuts Greenland's marine-terminating glaciers. Here we present a new compilation of Greenland bed topography that assimilates seafloor bathymetry and ice thickness data through a mass conservation approach. A new 150 m horizontal resolution bed topography/bathymetric map of Greenland is constructed with seamless transitions at the ice/ocean interface, yielding major improvements over previous data sets, particularly in the marine-terminating sectors of northwest and southeast Greenland. Our map reveals that the total sea level potential of the Greenland ice sheet is 7.42 ± 0.05 m, which is 7 cm greater than previous estimates. Furthermore, it explains recent calving front response of numerous outlet glaciers and reveals new pathways by which AW can access glaciers with marine-based basins, thereby highlighting sectors of Greenland that are most vulnerable to future oceanic forcing. [ABSTRACT FROM AUTHOR]

Recent studies suggest that the thickness of Winter Water (WW), that is, water with potential temperature below ∼−1°C located below Antarctic Surface Water and above Circumpolar Deep Water (CDW) is critical in determining the ice shelf melt rate, especially for the Pine Island Glacier (PIG). Existing model studies, however, misrepresent WW thickness and properties in the Amundsen Sea (AS). Here, we adjust a small number of model parameters in a regional Amundsen and Bellingshausen Seas configuration of the Massachusetts Institute of Technology general circulation model in order to reproduce properties and thickness of WW and CDW close to observations, with significant improvement for WW compared to previous studies. The cost, which is defined as weighted model-data difference squared, is reduced by 23%. Although a previous modeling study points out that the local surface heat loss upstream from Pine Island Polynya could be the reason for the observed 2012 PIG melt decline and WW thickening, they did not show WW freshening, which was observed at the same time. Model sensitivity experiments for surface heat loss, PIG melt rate, and precipitation fail to replicate WW freshening concurrent with PIG melt decline, implying that these processes cannot fully explain the observed PIG melt decrease. [ABSTRACT FROM AUTHOR]

We use a multisensor approach to assess the surge history over the past century of Storstrømmen and L. Bistrup Bræ, which drain the Northeast Ice Stream, Greenland. Storstrømmen surged around 1910 and Bistrup in 1913 and during the 1950s. Between 1978 and 1982, the speed of Storstrømmen peaked at 3 km/year during an active surge phase that lasted 10 years and the glacier has stayed in a quiescent phase since. Bistrup started to surge in 1988, peaked in 1993, and stopped in 1996. Both glaciers displayed a slow surge initiation and termination. Since 1993, ice builds up in the upper part of Storstrømmen at a 1 m/year, its lower part is thinning at its ablation rate of 1.4 m/year, and the grounding line has retreated by 10 km between 1992 and 2017, or 400 m/year. At these current rates, we project that Storstrømmen will meet presurge conditions in 2027–2030. Plain Language Summary: Storstrømmen and L. Bistrup Bræ in east Greenland probably are the largest surge‐type glaciers in the world. Based on the history of frontal positions, it was suggested a surge periodicity on the order of 70 years. In this study, we use a multisensor approach combining historical data sets with the modern remote sensing techniques to reassess the surge history of Storstrømmen and document the unknown behavior of L. Bistrup Bræ. We found that, between 1978 and 1982, the speed of Storstrømmen peaked at more than 3 km/year during an active surge phase that lasted 10 years and the glacier has stayed in a quiescent phase since. L. Bistrup Bræ started to surge in 1988, peaked in 1993, and stopped in 1996. Since 1993, ice builds up in upper part of Storstrømmen at a 1 m/year, its lower part is thinning at its ablation rate of 1.4 m/year, and the grounding line has retreated by 10 km between 1992 and 2017. At these current rates of mass accumulation upstream and retreat downstream, we project that Storstrømmen will meet presurge conditions in 2027–2030. Key Points: A multisensor approach combining historical with modern remote sensing data sets is used to document the history of the largest surging glaciers in the worldWe are able to analyze and interpret pattern of retreat, speedup, and thinning over the last centuryA surge cycle is accurately estimated and we forecast the next surge under current climatic conditions [ABSTRACT FROM AUTHOR]