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A
Abdalati, W, Zwally HJ, Bindschadler R, Csatho B, Farrell SL, Fricker HA, Harding D, Kwok R, Lefsky M, Markus T, Marshak A, Neumann T, Palm S, Schutz B, Smith B, Spinhirne J, Webb C.  2010.  The ICESat-2 Laser Altimetry Mission. Proceedings of the IEEE . 98:735-751.   10.1109/jproc.2009.2034765   AbstractWebsite

Satellite and aircraft observations have revealed that remarkable changes in the Earth's polar ice cover have occurred in the last decade. The impacts of these changes, which include dramatic ice loss from ice sheets and rapid declines in Arctic sea ice, could be quite large in terms of sea level rise and global climate. NASA's Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), currently planned for launch in 2015, is specifically intended to quantify the amount of change in ice sheets and sea ice and provide key insights into their behavior. It will achieve these objectives through the use of precise laser measurements of surface elevation, building on the groundbreaking capabilities of its predecessor, the Ice Cloud and Land Elevation Satellite (ICESat). In particular, ICESat-2 will measure the temporal and spatial character of ice sheet elevation change to enable assessment of ice sheet mass balance and examination of the underlying mechanisms that control it. The precision of ICESat-2's elevation measurement will also allow for accurate measurements of sea ice freeboard height, from which sea ice thickness and its temporal changes can be estimated. ICESat-2 will provide important information on other components of the Earth System as well, most notably large-scale vegetation biomass estimates through the measurement of vegetation canopy height. When combined with the original ICESat observations, ICESat-2 will provide ice change measurements across more than a 15-year time span. Its significantly improved laser system will also provide observations with much greater spatial resolution, temporal resolution, and accuracy than has ever been possible before.

B
Brunt, KM, Fricker HA, Padman L, Scambos TA, O'Neel S.  2010.  Mapping the grounding zone of the Ross Ice Shelf, Antarctica, using ICESat laser altimetry. Annals of Glaciology. 51:71-79. AbstractWebsite

We use laser altimetry from the Ice, Cloud, and land Elevation Satellite (ICESat) to map the grounding zone (CZ) of the Ross Ice Shelf, Antarctica, at 491 locations where ICESat tracks cross the grounding line (GL). Ice flexure in the GZ occurs as the ice shelf responds to short-term sea-level changes due primarily to tides. ICESat repeat-track analysis can be used to detect this region of flexure since each repeated pass is acquired at a different tidal phase; the technique provides estimates for both the landward limit of flexure and the point where the ice becomes hydrostatically balanced. We find that the ICESat-derived landward limits of tidal flexure are, in many places, offset by several km (and up to similar to 60 km) from the GL mapped previously using other satellite methods. We discuss the reasons why different mapping methods lead to different GL estimates, including: instrument limitations; variability in the surface topographic structure of the GZ; and the presence of ice plains. We conclude that reliable and accurate mapping of the GL is most likely to be achieved when based on synthesis of several satellite datasets.

C
Carter, SP, Fricker HA, Siegfried MR.  2017.  Antarctic subglacial lakes drain through sediment-floored canals: theory and model testing on real and idealized domains. Cryosphere. 11:381-405.   10.5194/tc-11-381-2017   AbstractWebsite

Over the past decade, satellite observations of ice surface height have revealed that active subglacial lake systems are widespread under the Antarctic Ice Sheet, including the ice streams. For some of these systems, additional observations of ice-stream motion have shown that lake activity can affect ice-stream dynamics. Despite all this new information, we still have insufficient understanding of the lake-drainage process to incorporate it into ice-sheet models. Process models for drainage of ice-dammed lakes based on conventional "R-channels" incised into the base of the ice through melting are unable to reproduce the timing and magnitude of drainage from Antarctic subglacial lakes estimated from satellite altimetry given the low hydraulic gradients along which such lakes drain. We have developed an alternative process model, in which channels are mechanically eroded into the underlying deformable subglacial sediment. When applied to the known active lakes of the Whillans-Mercer ice-stream system, the model successfully reproduced both the inferred magnitudes and recurrence intervals of lake-volume changes, derived from Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data for the period 2003-2009. Water pressures in our model changed as the flood evolved: during drainage, water pressures initially increased as water flowed out of the lake primarily via a distributed system, then decreased as the channelized system grew, establishing a pressure gradient that drew water away from the distributed system. This evolution of the drainage system can result in the observed internal variability of ice flow over time. If we are correct that active subglacial lakes drain through canals in the sediment, this mechanism also implies that active lakes are typically located in regions underlain by thick subglacial sediment, which may explain why they are not readily observed using radio-echo-sounding techniques.

Carter, SP, Fricker HA, Blankenship DD, Johnson JV, Lipscomb WH, Price SF, Young DA.  2011.  Modeling 5 years of subglacial lake activity in the MacAyeal Ice Stream (Antarctica) catchment through assimilation of ICESat laser altimetry. Journal of Glaciology. 57:1098-1112. AbstractWebsite

Subglacial lakes beneath Antarctica's fast-moving ice streams are known to undergo similar to 1 km(3) volume changes on annual timescales. Focusing on the MacAyeal Ice Stream (MacIS) lake system, we create a simple model for the response of subglacial water distribution to lake discharge events through assimilation of lake volume changes estimated from Ice, Cloud and land Elevation Satellite (ICESat) laser altimetry. We construct a steady-state water transport model in which known subglacial lakes are treated as either sinks or sources depending on the ICESat-derived filling or draining rates. The modeled volume change rates of five large subglacial lakes in the downstream portion of MacIS are shown to be consistent with observed filling rates if the dynamics of all upstream lakes are considered. However, the variable filling rate of the northernmost lake suggests the presence of an undetected lake of similar size upstream. Overall, we show that, for this fast-flowing ice stream, most subglacial lakes receive >90% of their water from distant distributed sources throughout the catchment, and we confirm that water is transported from regions of net basal melt to regions of net basal freezing. Our study provides a geophysically based means of validating subglacial water models in Antarctica and is a potential way to parameterize subglacial lake discharge events in large-scale ice-sheet models where adequate data are available.

Christoffersen, P, Bougamont M, Carter SP, Fricker HA, Tulaczyk S.  2014.  Significant groundwater contribution to Antarctic ice streams hydrologic budget. Geophysical Research Letters. 41:2003-2010.   10.1002/2014gl059250   AbstractWebsite

Satellite observations have revealed active hydrologic systems beneath Antarctic ice streams, but sources and sinks of water within these systems are uncertain. Here we use numerical simulations of ice streams to estimate the generation, flux, and budget of water beneath five ice streams on the Siple Coast. We estimate that 47% of the total hydrologic input (0.98km(3)yr(-1)) to Whillans (WIS), Mercer (MIS), and Kamb (KIS) ice streams comes from the ice sheet interior and that only 8% forms by local basal melting. The remaining 45% comes from a groundwater reservoir, an overlooked source in which depletion significantly exceeds recharge. Of the total input to Bindschadler (BIS) and MacAyeal (MacIS) ice streams (0.56km(3)yr(-1)), 72% comes from the interior, 19% from groundwater, and 9% from local melting. This contrasting hydrologic setting modulates the ice streams flow and has important implications for the search for life in subglacial lakes.

D
Damsgaard, A, Suckale J, Piotrowski JA, Houssais M, Siegfried MR, Fricker HA.  2017.  Sediment behavior controls equilibrium width of subglacial channels. Journal of Glaciology. 63:1034-1048.   10.1017/jog.2017.71   AbstractWebsite

Flow-frictional resistance at the base of glaciers and ice sheets is strongly linked to subglacial water pressure. Understanding the physical mechanisms that govern meltwater fluxes in subglacial channels is hence critical for constraining variations in ice flow. Previous mathematical descriptions of soft-bed subglacial channels assume a viscous till rheology, which is inconsistent with laboratory data and the majority of field studies. Here, we use a grain-scale numerical formulation coupled to pore-water dynamics to analyze the structural stability of channels carved into soft beds. Contrary to the soft-bed channel models assuming viscous till rheology, we show that the flanks of till channels can support substantial ice loads without creep closure of the channel, because the sediment has finite frictional strength. Increased normal stress on the channel flanks causes plastic failure of the sediment, and the channel rapidly shrinks to increase the ice-bed contact area. We derive a new parameterization for subglacial channelized flow on soft beds and show that channel dynamics are dominated by fluvial erosion and deposition processes with thresholds linked to the plastic rheology of subglacial tills. We infer that the described limits to channel size may cause subglacial drainage to arrange in networks of multiple closely spaced channels.

F
Fricker, HA, Siegfried MR, Carter SP, Scambos TA.  2016.  A decade of progress in observing and modelling Antarctic subglacial water systems. Philosophical Transactions of the Royal Society a-Mathematical Physical and Engineering Sciences. 374   10.1098/rsta.2014.0294   AbstractWebsite

In the decade since the discovery of active Antarctic subglacial water systems by detection of subtle surface displacements, much progress has been made in our understanding of these dynamic systems. Here, we present some of the key results of observations derived from ICESat laser altimetry, CryoSat-2 radar altimetry, Operation IceBridge airborne laser altimetry, satellite image differencing and ground-based continuous Global Positioning System (GPS) experiments deployed in hydrologically active regions. These observations provide us with an increased understanding of various lake systems in Antarctica: Whillans/Mercer Ice Streams, Crane Glacier, Recovery Ice Stream, Byrd Glacier and eastern Wilkes Land. In several cases, subglacial water systems are shown to control ice flux through the glacier system. For some lake systems, we have been able to construct more than a decade of continuous lake activity, revealing internal variability on time scales ranging from days to years. This variability indicates that continuous, accurate time series of altimetry data are critical to understanding these systems. On Whillans Ice Stream, our results from a 5-year continuous GPS record demonstrate that subglacial lake flood events significantly change the regional ice dynamics. We also show how models for subglacial water flow have evolved since the availability of observations of lake volume change, from regional-scale models of water routeing to process models of channels carved into the subglacial sediment instead of the overlying ice. We show that progress in understanding the processes governing lake drainage now allows us to create simulated lake volume time series that reproduce time series from satellite observations. This transformational decade in Antarctic subglacial water research has moved us significantly closer to understanding the processes of water transfer sufficiently for inclusion in continental-scale ice-sheet models.

Fricker, HA, Carter SP, Bell RE, Scambos T.  2014.  Active lakes of Recovery Ice Stream, East Antarctica: a bedrock-controlled subglacial hydrological system. Journal of Glaciology. 60:1015-1030.   10.3189/2014JoG14J063   AbstractWebsite

A connected system of active subglacial lakes was revealed beneath Recovery Ice Stream, East Antarctica, by ICESat laser altimetry. Here we combine repeat-track analysis of ICESat (2003-09), Operation IceBridge laser altimetry and radio-echo sounding (2011 and 2012), and MODIS image differencing (2009-2011) to learn more about the lake activity history, the surface and bedrock topographic setting of the lakes and the constraints on water flow through the system. We extend the lake activity time series to 2012 for the three lower lakes and capture two major lake drainages. One lake underwent a large deflation between 2009 and 2011 while another lake, which had been continuously filling between 2003 and 2010, started to drain after 2011. Most of the active lakes are located in a similar to 1000 km long bedrock trough under the main trunk of Recovery Ice Stream, whose base is similar to 1500-2000 m below present-day sea level. The hydrologic system beneath Recovery Ice Stream is controlled by this unusually pronounced bedrock topography, in contrast to most Antarctic systems studied to date, which are controlled by the ice surface topography. Hydrologic connections among the lakes appear to be direct and responsive, and we reproduce the lake activity using a simple subglacial water model. We discuss potential causes of non-steady hydrologic behavior in major Antarctic catchments.

J
Jacka, TH, Abdalati W, Allison I, Carsey F, Casassa G, Fily M, Frezzotti M, Fricker HA, Genthon C, Goodwin I, Guo Z, Hamilton GS, Hindmarsh RCA, Hulbe CL, Jacka TH, Jezek KC, Scambos TA, Shuman C, Skvarca P, Takahashi S, van de Wal RSW, Vaughan DG, Wang WL, Warner RC, Wingham DJ, Young NW, Zwally HJ, Comm I.  2004.  Recommendations for the collection and synthesis of Antarctic Ice Sheet mass balance data. Global and Planetary Change. 42:1-15.   10.1016/j.gloplacha.2003.11.008   AbstractWebsite

Recent unexpected changes in the Antarctic Ice Sheet, including ice sheet thinning, ice shelf collapse and changes in ice velocities, along with the recent realization that as much as one third of ice shelf mass loss is due to bottom melt, place a new urgency on understanding the processes involved in these changes. Technological advances, including very new or forthcoming satellite-based (e.g. ICESat, CryoSat) remote sensing missions, will improve our ability to make meaningful determinations of changes in Antarctic Ice Sheet mass balance. This paper is the result of a workshop held to develop a strategy for international collaboration aimed at the collection and synthesis of Antarctic Ice Sheet mass balance data, and at understanding the processes involved so that we might predict future change. Nine sets of recommendations are made, concerning the most important and sensitive measurements, temporal ranges and study areas. A final tenth recommendation calls for increased synthesis of ice sheet data and communication between the field measurement, satellite observation and modelling communities. (C) 2004 Published by Elsevier B.V.

M
Markus, T, Neumann T, Martino A, Abdalati W, Brunt K, Csatho B, Farrell S, Fricker H, Gardner A, Harding D, Jasinski M, Kwok R, Magruder L, Lubin D, Luthcke S, Morison J, Nelson R, Neuenschwander A, Palm S, Popescu S, Shum CK, Schutz BE, Smith B, Yang YK, Zwally J.  2017.  The Ice, Cloud, and land Elevation Satellite-2 (ICESat-2): Science requirements, concept, and implementation. Remote Sensing of Environment. 190:260-273.   10.1016/j.rse.2016.12.029   AbstractWebsite

The Ice, Cloud, and land Elevation Satellite (ICESat) mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets, as well as sea ice thickness distribution. These measurements have provided important information on the response of the cryopshere (Earth's frozen surfaces) to changes in atmosphere and ocean condition. ICESat operated from 2003 to 2009 and provided repeat altimetry measurements not only to the cryosphere scientific community but also to the ocean, terrestrial and atmospheric scientific communities. The conclusive assessment of significant ongoing rapid changes in the Earth's ice cover, in part supported by ICESat observations, has strengthened the need for sustained, high accuracy, repeat observations similar to what was provided by the ICESat mission. Following recommendations from the National Research Council for an ICESat follow-on mission, the ICESat-2 mission is now under development for planned launch in 2018. The primary scientific aims of the ICESat-2 mission are to continue measurements of sea ice freeboard and ice sheet elevation to determine their changes at scales from outlet glaciers to the entire ice sheet, and from 105 of meters to the entire polar oceans for sea ice freeboard. ICESat carried a single beam profiling laser altimeter that produced similar to 70 m diameter footprints on the surface of the Earth at similar to 150 m along-track intervals. In contrast, ICESat-2 will operate with three pairs of beams, each pair separated by about 3 km cross-track with a pair spacing of 90 m. Each of the beams will have a nominal 17 m diameter footprint with an along -track sampling interval of 0.7 m. The differences in the ICESat-2 measurement concept are a result of overcoming some limitations associated with the approach used in the ICESat mission. The beam pair configuration of ICESat-2 allows for the determination of local cross -track slope, a significant factor in measuring elevation change for the outlet glaciers surrounding the Greenland and Antarctica coasts. The multiple beam pairs also provide improved spatial coverage. The dense spatial sampling eliminates along -track measurement gaps, and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals. The ICESat-2 instrumentation concept uses a low energy 532 nm (green) laser in conjunction with single-photon sensitive detectors to measure range. Combining ICESat-2 data with altimetry data collected since the start of the ICESat mission in 2003, such as Operation IceBridge and ESA's CryoSat-2, will yield a 15+ year record of changes in ice sheet elevation and sea ice thickness. ICESat-2 will also provide information of mountain glacier and ice cap elevations changes, land and vegetation heights, inland water elevations, sea surface heights, and cloud layering and optical thickness. Published by Elsevier Inc. This is an open access article under the CC BY license

Marsh, OJ, Fricker HA, Siegfried MR, Christianson K, Nicholls KW, Corr HFJ, Catania G.  2016.  High basal melting forming a channel at the grounding line of Ross Ice Shelf, Antarctica. Geophysical Research Letters. 43:250-255.   10.1002/2015gl066612   AbstractWebsite

Antarctica's ice shelves are thinning at an increasing rate, affecting their buttressing ability. Channels in the ice shelf base unevenly distribute melting, and their evolution provides insight into changing subglacial and oceanic conditions. Here we used phase-sensitive radar measurements to estimate basal melt rates in a channel beneath the currently stable Ross Ice Shelf. Melt rates of 22.20.2ma(-1) (>2500% the overall background rate) were observed 1.7km seaward of Mercer/Whillans Ice Stream grounding line, close to where subglacial water discharge is expected. Laser altimetry shows a corresponding, steadily deepening surface channel. Two relict channels to the north suggest recent subglacial drainage reorganization beneath Whillans Ice Stream approximately coincident with the shutdown of Kamb Ice Stream. This rapid channel formation implies that shifts in subglacial hydrology may impact ice shelf stability.

P
Padman, L, Siegfried MR, Fricker HA.  2018.  Ocean tide influences on the Antarctic and Greenland ice sheets. Reviews of Geophysics. 56:142-184.   10.1002/2016rg000546   AbstractWebsite

Ocean tides are the main source of high-frequency variability in the vertical and horizontal motion of ice sheets near their marine margins. Floating ice shelves, which occupy about three quarters of the perimeter of Antarctica and the termini of four outlet glaciers in northern Greenland, rise and fall in synchrony with the ocean tide. Lateral motion of floating and grounded portions of ice sheets near their marine margins can also include a tidal component. These tide-induced signals provide insight into the processes by which the oceans can affect ice sheet mass balance and dynamics. In this review, we summarize in situ and satellite-based measurements of the tidal response of ice shelves and grounded ice, and spatial variability of ocean tide heights and currents around the ice sheets. We review sensitivity of tide heights and currents as ocean geometry responds to variations in sea level, ice shelf thickness, and ice sheet mass and extent. We then describe coupled ice-ocean models and analytical glacier models that quantify the effect of ocean tides on lower-frequency ice sheet mass loss and motion. We suggest new observations and model developments to improve the representation of tides in coupled models that are used to predict future ice sheet mass loss and the associated contribution to sea level change. The most critical need is for new data to improve maps of bathymetry, ice shelf draft, spatial variability of the drag coefficient at the ice-ocean interface, and higher-resolution models with improved representation of tidal energy sinks.

Paolo, FS, Padman L, Fricker HA, Adusumilli S, Howard S, Siegfried MR.  2018.  Response of Pacific-sector Antarctic ice shelves to the El Nino/Southern Oscillation. Nature Geoscience. 11:121-+.   10.1038/s41561-017-0033-0   AbstractWebsite

Satellite observations over the past two decades have revealed increasing loss of grounded ice in West Antarctica, associated with floating ice shelves that have been thinning. Thinning reduces an ice shelf's ability to restrain grounded-ice discharge, yet our understanding of the climate processes that drive mass changes is limited. Here, we use ice-shelf height data from four satellite altimeter missions (1994-2017) to show a direct link between ice-shelf height variability in the Antarctic Pacific sector and changes in regional atmospheric circulation driven by the El Nino/Southern Oscillation. This link is strongest from the Dotson to Ross ice shelves and weaker elsewhere. During intense El Nino years, height increase by accumulation exceeds the height decrease by basal melting, but net ice-shelf mass declines as basal ice loss exceeds ice gain by lower-density snow. Our results demonstrate a substantial response of Amundsen Sea ice shelves to global and regional climate variability, with rates of change in height and mass on interannual timescales that can be comparable to the longer-term trend, and with mass changes from surface accumulation offsetting a significant fraction of the changes in basal melting. This implies that ice-shelf height and mass variability will increase as interannual atmospheric variability increases in a warming climate.

S
Siegfried, MR, Fricker HA.  2018.  Thirteen years of subglacial lake activity in Antarctica from multi-mission satellite altimetry. Annals of Glaciology. 59:42-55.   10.1017/aog.2017.36   AbstractWebsite

The ability to detect the surface expression of moving water beneath the Antarctic ice sheet by satellite has revealed a dynamic basal environment, with implications for regional ice dynamics, grounding-line stability, and fluxes of freshwater and nutrients to the Southern Ocean. Knowledge of subglacial activity on timescales important for near-term prediction of ice-sheet fluctuations (decadal to century) is limited by the short observational record of NASA's Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry mission used to generate the last continent-wide survey (2003-08). Here, we use synthetic aperture radar-interferometric-mode data from ESA's CryoSat-2 radar altimetry mission (2010-present), which samples 45 of the ICESat-derived subglacial lakes, to extend their time series to the end of 2016. The extended time series show that there have been surface-height changes at 20 of the 45 lakes since 2008, indicating that some of these features are persistent and potentially cyclic, while other features show negligible changes, suggesting these may be transient or nonhydrological features. Continued monitoring of active lakes for both height and velocity changes, as well as developing methods for identifying additional lakes, is critical to quantifying the full distribution of active subglacial lakes in Antarctica.

Smith, BE, Fricker HA, Joughin IR, Tulaczyk S.  2009.  An inventory of active subglacial lakes in Antarctica detected by ICESat (2003-2008). Journal of Glaciology. 55:573-595. AbstractWebsite

Through the detection of surface deformation in response to water movement, recent satellite studies have demonstrated the existence of subglacial lakes in Antarctica that fill and drain on timescales of months to years. These studies, however, were confined to specific regions of the ice sheet. Here we present the first comprehensive study of these 'active' lakes for the Antarctic ice sheet north of 86 degrees S, based on 4.5 years (2003-08) of NASA's Ice, Cloud and land Elevation Satellite (ICESat) laser altimeter data. Our analysis has detected 124 lakes that were active during this period, and we estimate volume changes for each lake. The ICESat-detected lakes are prevalent in coastal Antarctica, and are present under most of the largest ice-stream catchments. Lakes sometimes appear to transfer water from one to another, but also often exchange water with distributed sources undetectable by ICESat, suggesting that the lakes may provide water to or withdraw water from the hydrologic systems that lubricate glacier flow. Thus, these reservoirs may contribute pulses of water to produce rapid temporal changes in glacier speeds, but also may withdraw water at other times to slow flow.