Ice Sheets and Ice Sheet Mass Balance 2

Satellite observations have transformed our knowledge of the contemporary mass changes of the polar ice sheets, and have provided important observational constraints for ice sheet models. Currently, three satellite based methods are used for estimating ice sheet mass balance: radar and laser altimetry measurement of ice sheet volume change, measurement of the ice sheets’ changing gravitational attraction, and differencing rates of ice sheet mass input and output. The longest continuous mass balance record is provided by radar altimetry. Since 1991, ESA's European Remote-Sensing (ERS-1 and ERS-2), ENVISAT, and CryoSat-2 satellites have acquired a continuous time-series of pulse-limited altrimeter observations, allowing monitoring of ice sheet volume change at monthly intervals.In this paper, we combine these measurements to determine changes in the volume and mass of the East Antarctic (EAIS) and West Antarctic (WAIS) ice sheets between 1991 and 2016 - by far the longest record available to date. We examine the 25-year volume and mass change trends of the ice sheets’ principle drainage basins and provide an estimate of the imbalances within the ice sheet areas unsurveyed by the radar altimeter. The long data record also allows us to compare trends within different epochs to quantify the reported mass loss acceleration from the Amundsen Sea and Bellinghsausen Sea sectors. We estimate that the mass imbalance of areas unsurveyed by the radar altimeter is small when compared to regional signals and to their uncertainties. Ice dynamical imbalance now affects the majority of the Amundsen Sea sector, and we observe accelerating rates of mass loss from this region in each 5-year epoch of our survey. Altogether, ice losses from the Pine Island and Thwaites glacier drainage basins are now 5 to 10 times greater, respectively, than at the start of our survey.

The Larsen Ice Shelf on the Weddell Coast of the Antarctic Peninsula has been subject to extensive retreat during the last decades. Sequences of successive retreat culminated in the collapse of the two northernmost sections, the Prince Gustav Channel (PGC) and Larsen-A ice shelves, in January 1995, followed by disintegration of the main section of Larsen-B in March 2002. Larsen-C Ice Shelf has been comparatively stable, except for major calving events in 1986 and 2004/5. However, also for Larsen-C progressive decrease in ice shelf stability might be imminent, concluding from flow acceleration, thinning, and the propagation of major rifts. In order to study the flow dynamics of ice shelves and tributary glaciers during different phases of ice shelf retreat and collapse, we generated velocity maps for the period 1995 to 2015 from synthetic aperture radar (SAR) data of the satellite missions ERS-1, ERS-2, Envisat, ALOS, TerraSAR-X, TanDEM-X and Sentinel-1. We derived also mass fluxes of the outlet glaciers across the grounding line or calving front, using satellite-derived flow velocities and ice thickness data. The collapse of the PGC and Larsen-A ice shelves and of the main section of Larsen-B triggered a near immediate response of tributary glaciers with increased velocities maintained to date. After an initial speed up after collapse, several of the main glaciers slowed down in later years, resulting in significant decrease of ice export across the calving gates compared to the first years after collapse. Other glaciers continue to show high velocities. Major flow acceleration, which started soon after the 2002 collapse, is also observed for the remnant section of Larsen-B Ice Shelf in SCAR Inlet. Whereas the acceleration of Larsen-B during the pre-collapse years did not affect the velocity of the tributary glaciers, the acceleration of SCAR Inlet Ice Shelf after 2002 triggered major acceleration of its large tributaries. On the northern section of Larsen-C the analysis of ERS-1/-2 InSAR data shows flow acceleration up to 10% between November 1995 and February 2000. Moderate acceleration rates are observed also for the years after 2000. The frontal section downstream of a major rift accelerated significantly during 2014/15. The tributary glaciers to Larsen-C do not show any significant acceleration by now. This, however, is not necessarily a sign of continuing ice shelf stability, as the stable ice flow on tributary glaciers to Larsen-A and-B during the years before ice shelf collapse shows.

Substantial environmental changes are occurring over the Antarctic Peninsula (AP), including rapid climate warming, ice shelf collapse, and accelerated glacier thinning and flow. These changes have major implications for the regional ice sheet mass balance and for global sea level rise. Geodetic estimates of the AP Ice Sheet (APIS) mass balance indicate that it lost mass at an average rate of 20 ± 14 Gt/yr over the period 1992-2011 (Shepherd et al., 2012); this equates to approximately 25% of all Antarctic ice sheet mass losses, despite occupying only 4% of the continental area. A key shortcoming of past estimates of AP mass change is that they have either been at low spatial resolution (gravimetry) or have had sparse spatial sampling (the mass budget method and altimetry). As such, a comprehensive assessment which combines completeness of coverage with the spatial detail required to resolve and understand regional patterns of change has been lacking.

Here we present the final results from the ESA Support To Science Element (STSE) Antarctic Peninsula Mass Balance (APMB) project. Using the three geodetic techniques of Gravimetry, Altimetry and the Mass Budget Method, we have sought to maximise the sampling and accuracy of the mass balance measurements by optimising the methodologies for the specific conditions of the region. The high level of consistency found between estimates from the different techniques in time periods and regions of overlap gives confidence in our findings. We take advantage of the relative sampling strengths of the different measurements by combining them to produce time series of mass balance measurements at basin to ice sheet scales.  Our results show that there has been a doubling in the rate of mass loss from the APIS in the past decade and that the mass loss has primarily originated from three key regions of dynamic imbalance; the Larsen B region, the Fleming Glacier region, and most recently, from the Western Palmer Land region.

We combine measurements of ice velocity from Landsat feature tracking and satellite radar interferometry from ERS-1/2, RADARSAT-1/2, ALOS PALSAR, TanDEM-X and Sentinel-1a, with ice thickness from existing compilations to document 43 years of mass flux from the Amundsen Sea Embayment (ASE) of West Antarctica. We also measure the grounding line retreat of glaciers draining into ASE using ERS-1/2 satellite radar differential interferometry from 1992 to 2011. The total ice discharge has increased by 77% since 1973. Half of the increase occurred between 2003 and 2009. Ice speeds of Pine Island Glacier stabilized between 2009 and 2015, following a decade of rapid acceleration. 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. Associated with the flux and speed increases, the grounding lines have retreated 31 km on Pine Island, 14 km on Thwaites, 10 km on Hayne and 35 km on Smith/Kohler glaciers. We conclude on the possible development of a marine ice sheet instability in this part of Antarctica and how future observations from Sentinel-1a/b are crucial to follow the rapid evolution of ASE in the coming decades.

Ice sheets are acknowledged by the World Meteorological Organization (WMO) and the United Nations Framework Convention on Climate Change (UNFCCC) as an Essential Climate Variable (ECV) needed to make significant progress in the generation of global climate products and derived information. Spaceborne Synthetic Aperture Radar (SAR) data prove an extremely useful source to provide relevant information on ice sheets. Specifically, ice velocity, grounding line, and ice front location can be extracted. Here, we provide an overview of the activities of our group and report on available Earth System Data Records (ESDR) in Antarctica. Currently available ESDR’s include: the first complete mapping of surface ice velocity over the Antarctic continent, a SAR based grounding line of Antarctica, ice velocity time series for selected regions (Ross and Ronne-Filchner, Amundsen Sea Embayment), as well as an Ice shelf boundary map based on ice front and grounding line. Products currently under development include a time series the Antarctic Peninsula and a revisit of the 2000 RADARSAT Antarctic Mapping Mission data set.

We provide an update on our time series of the Larsen-B- and C- ice shelves and their tributaries. Our results show a continued increase in surface velocity on Remnant Larsen-B ice shelf, in contrast to the relative stability of the Larsen-C Ice shelf. We use data from many satellite missions that acquired science data in the region to assemble the time series. Data continuity is a crucial aspect to this work. Given the lifetime of satellite missions and their respective primary mandates, not all data is available at all times. The availability of SAR data for polar science is currently coordinated by the Polar Space Task Group (PSTG) was established under the auspices of the World Meteorological Organization’s (WMO) Executive Council Panel of Experts on Polar Observations Research and Services (EC-PORS) with a mandate to provide coordination across Space Agencies to facilitate acquisition and distribution of fundamental satellite datasets, and to contribute to or support development of specific derived products in support of cryospheric and polar scientific research and applications. Our results directly benefit from PSTG efforts and products derived are being made available free of charge to the science community.

Data analysis and ESDR production is conducted at the Department of Earth System Science, University of California Irvine under a contract with the National Aeronautics and Space Administration's MEaSUREs program. Spaceborne SAR data were made available for this project courtesy of the Polar Space Task Group.