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Tinto, KJ, Padman L, Siddoway CS, Springer SR, Fricker HA, Das I, Tontini FC, Porter DF, Frearson NP, Howard SL, Siegfried MR, Mosbeux C, Becker MK, Bertinato C, Boghosian A, Brady N, Burton BL, Chu W, Cordero SI, Dhakal T, Dong L, Gustafson CD, Keeshin S, Locke C, Lockett A, O'Brien G, Spergel JJ, Starke SE, Tankersley M, Wearing MG, Bell RE.  2019.  Ross Ice Shelf response to climate driven by the tectonic imprint on seafloor bathymetry. Nature Geoscience. 12:441-+.   10.1038/s41561-019-0370-2   AbstractWebsite

Ocean melting has thinned Antarctica's ice shelves at an increasing rate over the past two decades, leading to loss of grounded ice. The Ross Ice Shelf is currently close to steady state but geological records indicate that it can disintegrate rapidly, which would accelerate grounded ice loss from catchments equivalent to 11.6 m of global sea level rise. Here, we use data from the ROSETTA-Ice airborne survey and ocean simulations to identify the principal threats to Ross Ice Shelf stability. We locate the tectonic boundary between East and West Antarctica from magnetic anomalies and use gravity data to generate a new high-resolution map of sub-ice-shelf bathymetry. The tectonic imprint on the bathymetry constrains sub-ice-shelf ocean circulation, protecting the ice shelf grounding line from moderate changes in global ocean heat content. In contrast, local, seasonal production of warm upper-ocean water near the ice front drives rapid ice shelf melting east of Ross Island, where thinning would lead to faster grounded ice loss from both the East and West Antarctic ice sheets. We confirm high modelled melt rates in this region using ROSETTA-Ice radar data. Our findings highlight the significance of both the tectonic framework and local ocean-atmosphere exchange processes near the ice front in determining the future of the Antarctic Ice Sheet.

Minchew, BM, Gudmundsson GH, Gardner AS, Paolo FS, Fricker HA.  2018.  Modeling the dynamic response of outlet glaciers to observed ice-shelf thinning in the Bellingshausen Sea Sector, West Antarctica. Journal of Glaciology. 64:333-342.   10.1017/jog.2018.24   AbstractWebsite

Satellite observations of gravity anomalies, ice-surface elevation and glacier velocity show significant increases in net grounded-ice-mass loss over the past decade along the Bellingshausen Sea sector (BSS), West Antarctica, in areas where warm (> 1 degrees C) sea water floods the continental shelf. These observations provide compelling but indirect evidence that mass losses are driven primarily by reduced buttressing from the floating ice shelves caused by ocean-driven ice-shelf thinning. Here, we combine recent observations of ice velocity, thickness and thickness changes with an ice flow model to study the instantaneous dynamic response of BSS outlet glaciers to observed ice-shelf thinning, alone. Our model results show that multiple BSS outlet glaciers respond instantaneously to observed ice-shelf thinning, particularly in areas where ice shelves ground at discrete points. Increases in modeled and observed dynamic mass losses, however, account for similar to 5% of the mass loss rates estimated from gravity anomalies and changes in ice-surface elevation, suggesting that variations in surface mass balance may be key to understanding recent BSS mass loss. Our approach isolates the impact of ice-shelf thinning on glacier flow and shows that if ice-shelf thinning continues at or above current rates, total BSS mass loss will increase in the next decade.

Beem, LH, Tulaczyk SM, King MA, Bougamont M, Fricker HA, Christoffersen P.  2014.  Variable deceleration of Whillans Ice Stream, West Antarctica. Journal of Geophysical Research-Earth Surface. 119:212-224.   10.1002/2013jf002958   AbstractWebsite

The Whillans Ice Stream Ice Plain (WIP) has been slowing since at least 1963. Prior constraints on this slowdown were consistent with a constant long-term deceleration rate. New observations of ice velocity from 11 continuous and 3 seasonal GPS sites indicate the deceleration rate varies through time including on interannual time scales. Between 2009 and 2012 WIP decelerated at a rate (6.1 to 10.9 2 m/yr(2)) that was double the multidecadal average (3.0 to 5.6 2 m/yr(2)). To identify the causes of slowdown, we used new and prior velocity estimates to constrain longitudinal and transverse force budget models as well as a higher-order inverse model. All model results support the conclusion that the observed deceleration of WIP is caused by an increase in basal resistance to motion at a rate of 10 to 40 Pa/yr. Subglacial processes that may be responsible for strengthening the ice stream bed include basal freeze on, changes in subglacial hydrology, or increases in the area of resistant basal substrate through differential erosion. The observed variability in WIP deceleration rate suggests that dynamics in subglacial hydrology, plausibly driven by basal freeze on and/or activity of subglacial lakes, plays a key role in modulating basal resistance to ice motion in the region.

Fricker, HA, Padman L.  2012.  Thirty years of elevation change on Antarctic Peninsula ice shelves from multimission satellite radar altimetry. Journal of Geophysical Research-Oceans. 117   10.1029/2011jc007126   AbstractWebsite

We use data acquired between 1978 and 2008 by four satellite radar altimeter missions (Seasat, ERS-1, ERS-2 and Envisat) to determine multidecadal elevation change rates (dh(i)/dt) for six major Antarctic Peninsula (AP) ice shelves. In areas covered by the Seasat orbit (to 72.16 degrees S), regional-averaged 30-year trends were negative (surface lowering), with rates between -0.03 and -0.16 m a(-1). Surface lowering preceded the start of near-continuous radar altimeter operations that began with ERS-1 in 1992. The average rate of lowering for the first 14 years of the period was typically smaller than the 30-year average; the exception was the southern Wilkins Ice Shelf, which experienced negligible lowering between 2000 and 2008, when a series of large calving events began. Analyses of the continuous ERS/Envisat time series (to 81.5 degrees) for 1992-2008 reveal a period of strong negative dhi/dt on most ice shelves between 1992 and 1995. Based on prior studies of regional atmospheric and oceanic conditions, we hypothesize that the observed elevation changes on Larsen C Ice Shelf are driven primarily by firn compaction while the western AP ice shelves are responding to changes in both surface mass balance and basal melt rates. Our time series also show that large changes in dh(i)/dt can occur on interannual time scales, reinforcing the importance of long time series altimetry to separate long-term trends associated with climate change from interannual to interdecadal natural variability.

Chen, X, Shearer PM, Walter F, Fricker HA.  2011.  Seventeen Antarctic seismic events detected by global surface waves and a possible link to calving events from satellite images. Journal of Geophysical Research-Solid Earth. 116   10.1029/2011jb008262   AbstractWebsite

We detect 17 seismic events in Antarctica from 1997 to 2009 by applying a surface wave detector to global seismic data. We locate these events using a waveform cross-correlation method and find that most occurred near the coast of Antarctica and are clustered in three regions: four events are on the Ronne Ice Shelf, close to the location of a 1998 calving event; five events are near the Vanderford Glacier; and eight events are near the Ninnis Glacier. The observed Rayleigh and Love waves for these events have similar amplitudes and a two-lobed radiation pattern, matching the expected amplitude behavior of a single-force source model. Using such a model, we obtain best fitting horizontal force directions for the 14 events that have relatively better signal-to-noise ratios. Analysis of coastline changes from MODIS images before and after the detected events show that two events on Vanderford Glacier and one event near Ninnis Glacier are likely associated with calving events. Moreover, the inferred force directions for the seismic events appear consistent with local ice flow directions. Both satellite observations and modeling results strongly suggest a link between seismic events and calving processes in the two regions. However, the force directions on the Ronne Ice Shelf are aligned with observed rift propagation directions, suggesting that these events may arise from rifting processes.

Brunt, KM, Fricker HA, Padman L.  2011.  Analysis of ice plains of the Filchner-Ronne Ice Shelf, Antarctica, using ICESat laser altimetry. Journal of Glaciology. 57:965-975. AbstractWebsite

We use repeat-track laser altimeter data from the Ice, Cloud and land Elevation Satellite (ICESat) to map the grounding zone (GZ) of the Filchner-Ronne Ice Shelf, Antarctica. Ice flexure in the GZ occurs as the ice shelf responds to ocean-height changes due primarily to tides. We have identified three 'ice plains', regions of low surface slope near the GZ where the ice is close to hydrostatic equilibrium: one on Institute Ice Stream; another to its east; and another west of Foundation Ice Stream. The vertical information from repeated ICESat tracks enables us to study the topography, state of flotation and flexure characteristics across these features. In regions of ephemeral grounding, tidal migration of the grounding line allows us to estimate bed slope (similar to 1-2 x 10(-3)). From these studies we develop a classification scheme for ice plains, expressed in terms of the evolution, or 'life cycle', of these features. A lightly grounded ice plain progresses to a state of ephemeral grounding as the ice sheet thins near the GZ. Once sufficient thinning has occurred, the ice plain becomes a fully floating, relict ice plain with an undulated surface topography similar to that of lightly grounded ice; we expect viscous relaxation to a smooth ice-shelf surface to occur over a timescale of decades. Our improved insight into ice-plain evolution suggests added complexity in modeling ice in the vicinity of the GZ, and a role for ice-plain observations as a guide to relatively rapid changes in ice-sheet mass balance.

Bindschadler, R, Choi H, Wichlacz A, Bingham R, Bohlander J, Brunt K, Corr H, Drews R, Fricker H, Hall M, Hindmarsh R, Kohler J, Padman L, Rack W, Rotschky G, Urbini S, Vornberger P, Young N.  2011.  Getting around Antarctica: new high-resolution mappings of the grounded and freely-floating boundaries of the Antarctic ice sheet created for the International Polar Year. Cryosphere. 5:569-588.   10.5194/tc-5-569-2011   AbstractWebsite

Two ice-dynamic transitions of the Antarctic ice sheet - the boundary of grounded ice features and the freely-floating boundary - are mapped at 15-m resolution by participants of the International Polar Year project ASAID using customized software combining Landsat-7 imagery and ICESat/GLAS laser altimetry. The grounded ice boundary is 53 610 km long; 74% abuts to floating ice shelves or outlet glaciers, 19% is adjacent to open or sea-ice covered ocean, and 7% of the boundary ice terminates on land. The freely-floating boundary, called here the hydrostatic line, is the most landward position on ice shelves that expresses the full amplitude of oscillating ocean tides. It extends 27 521 km and is discontinuous. Positional (one-sigma) accuracies of the grounded ice boundary vary an order of magnitude ranging from +/-52 m for the land and open-ocean terminating segments to +/-502 m for the outlet glaciers. The hydrostatic line is less well positioned with errors over 2 km. Elevations along each line are selected from 6 candidate digital elevation models based on their agreement with ICESat elevation values and surface shape inferred from the Landsat imagery. Elevations along the hydrostatic line are converted to ice thicknesses by applying a firn-correction factor and a flotation criterion. BEDMAP-compiled data and other airborne data are compared to the ASAID elevations and ice thicknesses to arrive at quantitative (one-sigma) uncertainties of surface elevations of +/-3.6, +/-9.6, +/-11.4, +/-30 and +/-100 m for five ASAID-assigned confidence levels. Over one-half of the surface elevations along the grounded ice boundary and over one-third of the hydrostatic line elevations are ranked in the highest two confidence categories. A comparison between ASAID-calculated ice shelf thicknesses and BEDMAP-compiled data indicate a thin-ice bias of 41.2+/-71.3m for the ASAID ice thicknesses. The relationship between the seaward offset of the hydrostatic line from the grounded ice boundary only weakly matches a prediction based on beam theory. The mapped products along with the customized software to generate them and a variety of intermediate products are available from the National Snow and Ice Data Center.

Massom, RA, Giles AB, Fricker HA, Warner RC, Legresy B, Hyland G, Young N, Fraser AD.  2010.  Examining the interaction between multi-year landfast sea ice and the Mertz Glacier Tongue, East Antarctica: Another factor in ice sheet stability? Journal of Geophysical Research-Oceans. 115   10.1029/2009jc006083   AbstractWebsite

The Mertz Glacier tongue (MGT), East Antarctica, has a large area of multi-year fast sea ice (MYFI) attached to its eastern edge. We use various satellite data sets to study the extent, age, and thickness of the MYFI and how it interacts with the MGT. We estimate its age to be at least 25 years and its thickness to be 10-55 m; this is an order of magnitude thicker than the average regional sea-ice thickness and too thick to be formed through sea-ice growth alone. We speculate that the most plausible process for its growth after initial formation is marine (frazil) ice accretion. The satellite data provide two types of evidence for strong mechanical coupling between the two types of ice: The MYFI moves with the MGT, and persistent rifts that originate in the MGT continue to propagate for large distances into the MYFI. The area of MYFI decreased by 50% following the departure of two large tabular icebergs that acted as pinning points and protective barriers. Future MYFI extent will be affected by subsequent icebergs from the Ninnis Glacier and the imminent calving of the MGT. Fast ice is vulnerable to changing atmospheric and oceanic conditions, and its disappearance may have an influence on ice tongue/ice shelf stability. Understanding the influence of thick MYFI on floating ice tongues/ice shelves may be significant to understanding the processes that control their evolution and how these respond to climate change, and thus to predicting the future of the Antarctic Ice Sheet.

Allison, I, Alley RB, Fricker HA, Thomas RH, Warner RC.  2009.  Ice sheet mass balance and sea level. Antarctic Science. 21:413-426.   10.1017/s0954102009990137   AbstractWebsite

Determining the mass balance of the Greenland and Antarctic ice sheets (GIS and AIS) has long been a major challenge for polar science. But until recent advances in measurement technology, the uncertainty in ice sheet mass balance estimates was greater than any net contribution to sea level change. The Fourth Assessment Report of the Intergovernmental Panel on Climate Change (AR4) was able, for the first time, to conclude that, taken together, the GIS and AIS have probably been contributing to sea level rise over the period 1993-2003 at an average rate estimated at 0.4 nine yr(-1). Since the cut-off date for work included in AR4, a number of further studies of the mass balance of GIS and AIS have been made using satellite altimetry, satellite gravity measurements and estimates of mass influx and discharge using a variety of techniques. Overall, these studies reinforce the conclusion that the ice sheets are contributing to present sea level rise, and suggest that the rate of loss from GIS has recently increased. The largest unknown in the projections of sea level rise over the next century is the potential for rapid dynamic collapse of ice sheets.

King, MA, Coleman R, Freemantle AJ, Fricker HA, Hurd RS, Legresy B, Padman L, Warner R.  2009.  A 4-decade record of elevation change of the Amery Ice Shelf, East Antarctica. Journal of Geophysical Research-Earth Surface. 114   10.1029/2008jf001094   AbstractWebsite

We report on long-term surface elevation changes of the central Amery Ice Shelf (AIS) by comparing elevation records spanning 4 decades (1968-2007). We use elevation records acquired with the following methods: optical leveling (1968-1969); ERS radar altimetry (1992-2003); GPS (1995-2006); and Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry (2003-2007). We compute multidecadal elevation trend (dh/dt) values at crossovers between the leveling route and each of the GPS and ICESat tracks as well as shorter-period dh/dt at ERS-ERS, GPS-GPS, and ICESat-ICESat crossovers. At GPS-leveling crossovers the mean long-term dh/dt is -0.003 m a(-1), and at ICESat-leveling crossovers the mean dh/dt is +0.013 m a(-1); neither trend is significantly different from zero. The data do, however, exhibit variable trends: near-zero change between 1991 and mid-1996, then thickening to similar to 2003, followed by thinning similar to 2003-2007, with 5 year dh/dt averages exceeding similar to +/- 0.1 m a(-1). The changes in dh/dt pattern in mid-1996 and again in 2003 occur with unexpected speed. The ice shelf exhibits different dh/dt patterns than does the surrounding grounded ice, suggesting that surface mass balance variations or longer-term variations in firn densification processes are unlikely to be major causes. We conclude that these observed multiyear elevation changes must be due to currently unexplained or presently poorly quantified phenomena involving surface or basal processes and/or ice dynamics. With the multidecadal stability of the AIS established, the short-term fluctuations that we observe suggests that for other ice shelves, observed strong dh/dt signals over short time periods do not necessarily indicate ice shelf instability.