We present the first comprehensive high-resolution mosaic of ice velocity of the Northern (NPI) and Southern Patagonian Icefields (SPI), from multiple synthetic aperture radar and optical data collected between 1984 and 2014. The results reveal that many of the outlet glaciers extend far into the central ice plateaus, which implies that changes in ice dynamics propagate far inside the accumulation area. We report pronounced seasonal to interannual variability of ice motion on Pío XI and Jorge Montt, a doubling in speed of Jorge Montt, a major slow down of O'Higgins, significant fluctuations of Upsala and a deceleration of San Rafael, which illustrate the need for sustained, continuous time series of ice motion to understand the long-term evolution of the rapidly thinning icefields. The velocity product also resolves major ambiguities in glacier drainage in areas of relatively flat topography illustrating the need to combine topography and flow direction to map drainage basins.

A digital representation of ice surface velocity is essential for a variety of glaciological, geologic and geophysical analyses and modeling. Here, we present a new, reference, comprehensive, high-resolution, digital mosaic of ice motion in Greenland assembled from satellite radar interferometry data acquired during the International Polar Year 2008 to 2009 by the Envisat Advanced Synthetic-Aperture Radar (ASAR), the Advanced Land Observation System (ALOS)'s Phase-Array L-band SAR (PALSAR) and the RADARSAT-1 SAR that covers 99% of the ice sheet in area. The best mapping performance is obtained using ALOS PALSAR data due to higher levels of temporal coherence at the L-band frequency; but C-band frequency SAR data are less affected by the ionosphere. The ice motion map reveals various flow regimes, ranging from patterned enhanced flow into a few large glaciers in the cold, low precipitation areas of north Greenland; to diffuse, enhanced flow into numerous, narrow, fast-moving glaciers in the warmer, high precipitation sectors of northwest and southeast Greenland. We find that the 100 fastest glaciers (v > 800m/yr) drain 66% of the ice sheet in area, marine-terminating glaciers drain 88% of Greenland, and basal-sliding motion dominates internal deformation over more than 50% of the ice sheet. This view of ice sheet motion provides significant new constraints on ice flow modeling. Copyright 2012 by the American Geophysical Union.

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.

The delineation of an ice sheet grounding line, i.e., the transition boundary where ice detaches from the bed and becomes afloat in the ocean, is critical to ice sheet mass budget calculations, numerical modeling of ice sheet dynamics, iceocean interactions, oceanic tides, and subglacial environments. Here, we present 15 years of comprehensive, high-resolution mapping of grounding lines in Antarctica using differential satellite synthetic-aperture radar interferometry (DInSAR) data from the Earth Remote Sensing Satellites 1-2 (ERS-1/2), RADARSAT-1 and 2, and the Advanced Land Observing System (ALOS) PALSAR for years 1994 to 2009. DInSAR directly measures the vertical motion of floating ice shelves in response to tidal oceanic forcing with millimeter precision, at a sample spacing better than 50 m, simultaneously over areas several 100 km wide; in contrast with earlier methods that detect abrupt changes in surface slope in satellite visible imagery or altimetry data. On stagnant and slow-moving areas, we find that breaks in surface slope are reliable indicators of grounding lines; but on most fast-moving glaciers and ice streams, our DInSAR results reveal that prior mappings have positioning errors ranging from a few km to over 100 km. A better agreement is found with ICESat's data, also based on measurements of vertical motion, but with a detection noise one order of magnitude larger than with DInSAR. Overall, the DInSAR mapping of Antarctic grounding lines completely redefines the coastline of Antarctica. Copyright 2011 by the American Geophysical Union.

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.

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.

We report changes in ice velocity of a 6.5 million km 2 region around South Pole encompassing the Filchner-Ronne and Ross Ice Shelves and a significant portion of the ice streams and glaciers that constitute their catchment areas. Using the first full interferometric synthetic aperture radar (InSAR) coverage of the region completed in 2009 and partial coverage acquired in 1997, we processed the data to assemble a comprehensive map of ice speed changes between those two years. On the Ross Ice Shelf, our results confirm a continued deceleration of Mercer and Whillans Ice Streams with a 12-yr velocity difference of-'50 m yr-' 1 (-'16.7%) and-'100 m yr-' 1 (-'25.3%) at their grounding lines. The deceleration spreads 450 km upstream of the grounding line and more than 500 km onto the shelf, beyond what was previously known. Ross and Filchner Ice Shelves exhibit signs of pre-calving events, representing the largest observed changes, with an increase in speed in excess of +100 m yr-' 1 in 12 yr. Other changes in the Ross Ice Shelf region are less significant. The observed changes in glacier speed extend on the Ross Ice Shelf along the ice streams' flow lines. Most tributaries of the Filchner-Ronne Ice Shelf show a modest deceleration or no change between 1997 and 2009. Slessor Glacier shows a small deceleration over a large sector. No change is detected on the Bailey, Rutford, and Institute Ice Streams. On the Filchner Ice Shelf itself, ice decelerated rather uniformly with a 12-yr difference in speed of-'50 m yr-' 1, or-'5% of its ice front speed, which we attribute to a 12 km advance in its ice front position. Our results show that dynamic changes are present in the region. They highlight the need for continued observation of the area with a primary focus on the Siple Coast. The dynamic changes in Central Antarctica between 1997 and 2009 are generally second-order effects in comparison to losses on glaciers in the Bellingshausen and Amundsen Seas region and on the Antarctic Peninsula. We therefore conclude that the dynamic changes shown here do not have a strong impact on the mass budget of the Antarctic continent. © 2012 Author(s).

The bed topography beneath the Greenland ice sheet controls the flow of ice and its discharge into the ocean. Outlet glaciers move through a set of narrow valleys whose detailed geometry is poorly known, especially along the southern coasts1-3. As a result, the contribution of the Greenland ice sheet and its glaciers to sea-level change in the coming century is uncertain. Here, we combine sparse ice-thickness data derived from airborne radar soundings with satellite-derived high-resolution ice motion data through a mass conservation optimization scheme5. We infer ice thickness and bed topography along the entire periphery of the Greenland ice sheet at an unprecedented level of spatial detail and precision. We detect widespread ice-covered valleys that extend significantly deeper below sea level and farther inland than previously thought. Our findings imply that the outlet glaciers of Greenland, and the ice sheet as a whole, are probably more vulnerable to ocean thermal forcing and peripheral thinning than inferred previously from existing numerical ice-sheet models. © 2014 Macmillan Publishers Limited.

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