News date: 25h October, 2016 compiled by Dr. Alvarinho J. Luis Accelerated glacier melting in West Antarctica documented Study findings will help improve predictions about global sea level rise Summary: Two new studies have found the fastest ongoing rates of glacier retreat ever observed in West Antarctica and offer an unprecedented look at ice melting on the floating undersides of glaciers. The results highlight how the interaction between ocean conditions and the bedrock beneath a glacier can influence the frozen mass, helping scientists better predict future Antarctica ice loss and global sea level rise. Two new studies by researchers at the Univ. of California and NASA have found the fastest ongoing rates of glacier retreat ever observed in West Antarctica and offer an unprecedented look at ice melting on the floating undersides of glaciers. The results highlight how the interaction between ocean conditions and the bedrock beneath a glacier can influence the frozen mass, helping scientists better predict future Antarctica ice loss and global sea level rise. The studies examined three neighboring glaciers that are melting and retreating at different rates. The Smith, Pope and Kohler glaciers flow into the Dotson and Crosson ice shelves in the Amundsen Sea embayment in West Antarctica, the part of the continent with the largest decline in ice (see Figure). The primary question is how the Amundsen Sea sector of West Antarctica will contribute to sea level rise in the future, particularly following observations of massive changes in the area over the last two decades. Using satellite data, Scientists continue to measure the evolution of the grounding line of these glaciers, which helps determine their stability and how much mass the glacier is gaining or losing. Scheuchl's team compared radar interferometry data from the European Space Agency's Sentinel-1a C band satellite in Terrain Observation with Progressive Scans mode and compared with data from the earlier ERS-1 and ERS-2 satellites to identify changes in each glacier's grounding line -- the boundary where it loses contact with bedrock and begins to float on the ocean. The grounding line is important because nearly all glacier melting takes place on the underside of this floating portion, called the ice shelf. If a glacier loses mass from enhanced melting, it may start floating farther inland from its former grounding line, just as a boat stuck on a sandbar may be able to float again if a heavy cargo is removed. This is called grounding line retreat. They found that the Smith Glacier's grounding line had retreated 2 km/yr since 1996, the Pope Glacier's grounding line receded more slowly, at 0.5 km/yr since 1996. and the Kohler Glacier's grounding line, which had gradually retreated, actually readvanced 2 km/yr since 2011. For a separate study, the NASA Jet Propulsion Laboratory's Ala Khazendar measured ice loss at the bottom of the three glaciers, which he suspected might be influencing the changes in their grounding lines. His work, published today in the journal Nature Communications, involved gauging the thickness and height of the ice via radar and laser altimetry instruments utilized in NASA's Operation IceBridge and earlier NASA airborne campaigns. Radar waves penetrate glaciers all the way to their base, allowing direct assessment of how the bottom profiles of the three glaciers at their grounding lines differed between 2002 and 2014. Laser measurements of surface elevation were used to infer changes in the thickness of the floating ice shelves. Previous studies using other techniques estimated the average melting rates at the bottom of the Dotson and Crosson ice shelves to be about 12 m/yr. Khazendar and his team, analyzing their direct radar measurements, found stunning rates of ice loss from the glaciers' undersides on the ocean sides of their grounding lines. The fastest-melting glacier, Smith, lost between 300 and 490 m in thickness between 2002 and 2009 near its grounding line, or up to 70 m/yr. Those years encompass a period when rapid mass loss was seen around the Amundsen Sea. The regional scale of the decline made scientists strongly suspect that an increase in the influx of ocean heat beneath the ice shelves must have taken place. Khazendar said Smith's fast retreat and thinning are likely related to the shape of the underlying bedrock over which it was retreating between 1996 & 2014, which sloped downward toward the continental interior, and oceanic conditions in the cavity beneath the glacier. As the grounding line receded, warm and dense ocean water could reach the newly uncovered deeper parts of this cavity, causing more melting. As a result, more sections of the glacier become thinner and float, meaning that the grounding line continues retreating, and so on. Smith's retreat might slow down now that its grounding line has reached bedrock that rises farther inland of the 2014 grounding line. Pope and Kohler, in contrast, are on bedrock that slopes upward toward the interior. The question remains whether other glaciers in West Antarctica will behave more like Smith or more like Pope and Kohler. Many glaciers in this sector of Antarctica are on beds that deepen farther inland, like Smith's. However, Khazendar and Scheuchl said, researchers need more information on the shape of the bedrock and seafloor beneath the ice, as well as more data on ocean circulation and temperatures, to be able to better project how much ice these glaciers will contribute to the ocean in a changing climate. Source: University of California, Irvine Reference: B. Scheuchl, et al. Grounding line retreat of Pope, Smith, and Kohler Glaciers, West Antarctica, measured with Sentinel-1a radar interferometry data. Geophysical Research Letters, 2016; 43 (16): 8572 DOI: 10.1002/2016GL069287 pdf reprint follows….. Geophysical Research Letters RESEARCH LETTER Grounding line retreat of Pope, Smith, and Kohler Glaciers, 10.1002/2016GL069287 West Antarctica, measured with Sentinel-1a radar Key Points: interferometry data • Sentinel-1 is a breakthrough mission for systematic, widespread, 1 1 1,2 1 2 continued, grounding line mapping B. Scheuchl , J. Mouginot , E. Rignot , M. Morlighem , and A. Khazendar in Antarctica • Pope, Smith, and Kohler Glaciers 1Department of Earth System Science, University of California, Irvine, CA, USA, 2Jet Propulsion Laboratory, California are undergoing major changes. Institute of Technology, Pasadena, California, USA The Grounding line of Smith Glacier continues to retreat at 2 km/yr • Crosson and Dotson Ice Shelves lost many pinning points, which Abstract We employ Sentinel-1a C band satellite radar interferometry data in Terrain Observation with significantly threatens their stability Progressive Scans mode to map the grounding line and ice velocity of Pope, Smith, and Kohler glaciers, in in years to come West Antarctica, for the years 2014–2016 and compare the results with those obtained using Earth Remote Sensing Satellites (ERS-1/2) in 1992, 1996, and 2011. We observe an ongoing, rapid grounding line retreat Supporting Information: of Smith at 2 km/yr (40 km since 1996), an 11 km retreat of Pope (0.5 km/yr), and a 2 km readvance of • Supporting Information S1 Kohler since 2011. The variability in glacier retreat is consistent with the distribution of basal slopes, i.e., fast along retrograde beds and slow along prograde beds. We find that several pinning points holding Correspondence to: Dotson and Crosson ice shelves disappeared since 1996 due to ice shelf thinning, which signal the ongoing B. Scheuchl, weakening of these ice shelves. Overall, the results indicate that ice shelf and glacier retreat in this sector [email protected] remain unabated. Citation: Scheuchl, B., J. Mouginot, E. Rignot, M. Morlighem, and A. Khazendar 1. Introduction (2016), Grounding line retreat of Pope, Smith, and Kohler Glaciers, The Amundsen Sea Embayment (ASE) sector of West Antarctica has been undergoing significant changes over West Antarctica, measured with the past few decades and is the largest contributor to sea level rise from Antarctica at present [McMillan et al., Sentinel-1a radar interferometry data, Geophys. Res. Lett., 43, 8572–8579, 2014; Mouginot et al., 2014; Rignot et al., 2014; Sutterley et al., 2014]. Spaceborne remote sensing is crucial to doi:10.1002/2016GL069287. assess its state of mass balance, detect temporal and spatial changes in mass balance, and provide ice sheet numerical models with suitable observations and boundary conditions to model the evolution of ice in this Received 21 APR 2016 sector. The position of the grounding line, where ice detaches from the bed and becomes afloat in the ocean, is Accepted 10 JUN 2016 especially critical to know with precision in order to define where basal friction of glacier ice drops to zero and Published online 29 AUG 2016 where ice melts in contact with the ocean or breaks up into icebergs. Prior attempts at mapping grounding lines have been limited because few spaceborne SAR missions offer the short-term repeat pass capability required to map the differential vertical displacement of floating ice at tidal frequencies with sufficient detail to resolve grounding line boundaries in areas of fast ice deformation [Rignot et al., 2011a; Li et al., 2015]. On the eastern part of ASE, the satellite record has indicated that Pine Island and Thwaites glaciers have been thinning, their grounding lines have been retreating at rates of 1–2 km/yr, and their speeds have been increas- ing with time [Rignot, 2008; Park et al., 2013; Rignot et al., 2014; Mouginot et al., 2014]. On the western part of ASE, Pope, Smith, and Kohler glaciers also contribute to ice drainage from the ASE through Crosson and Dotson ice shelves. In the 1980s, when the glaciers were estimated to be close to a state of mass balance, they drained about 15% of the mass flux from the ASE.