Mark Fahnestock

The Link Between Climate Warming and Break-Up of Ice Shelves in the Antarctic Peninsula

A review of in situ and remote-sensing data covering the ice shelves of the Antarctic Peninsula provides a series of characteristics closely associated with rapid shelf retreat: deeply embayed ice fronts; calving of myriad small elongate bergs in punctuated events; increasing flow speed; and the presence of melt ponds on the ice-shelf surface in the vicinity of the break-ups. As climate has warmed in the Antarctic Peninsula region, melt-season duration and the extent of ponding have increased. Most break-up events have occurred during longer melt seasons, suggesting that meltwater itself, not just warming, is responsible. Regions that show melting without pond formation are relatively unchanged. Melt ponds thus appear to be a robust harbinger of ice-shelf retreat. We use these observations to guide a model of ice-shelf flow and the effects of meltwater. Crevasses present in a region of surface ponding will likely fill to the brim with water. We hypothesize (building on Weertman (1973), Hughes (1983) and Van der Veen (1998)) that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat. A thermodynamic finite-element model is used to evaluate ice flow and the strain field, and simple extensions of this model are used to investigate crack propagation by meltwater. The model results support the hypothesis.

Large subglacial lakes in East Antarctica at the onset of fast-flowing ice streams

Water plays a crucial role in ice-sheet stability and the onset of ice streams. Subglacial lake water moves between lakes and rapidly drains, causing catastrophic floods. The exact mechanisms by which subglacial lakes influence ice-sheet dynamics are unknown, however, and large subglacial lakes have not been closely associated with rapidly flowing ice streams. Here we use satellite imagery and ice-surface elevations to identify a region of subglacial lakes, similar in total area to Lake Vostok, at the onset region of the Recovery Glacier ice stream in East Antarctica and predicted by ice-sheet models. We define four lakes through extensive, flat, featureless regions of ice surface bounded by upstream troughs and downstream ridges. Using ice velocities determined using interferometric synthetic aperture radar (InSAR), we find the onset of rapid flow (moving at 20 to 30 m yr-1) of the tributaries to the Recovery Glacier ice stream in a 280-km-wide segment at the downslope margins of these four subglacial lakes. We conclude that the subglacial lakes initiate and maintain rapid ice flow through either active modification of the basal thermal regime of the ice sheet by lake accretion or through scouring bedrock channels in periodic drainage events. We suggest that the role of subglacial lakes needs to be considered in ice-sheet mass balance assessments.

Interferometric estimation of three-dimensional ice-flow using ascending and descending passes

Satellite radar interferometry (SRI) provides an important new tool for determining ice-flow velocity. Interferometric measurements made from a single-track direction are sensitive only to a single component of the three-component velocity vector. Observations from along three different track directions would allow the full velocity vector to be determined. A north/south-looking synthetic aperture radar (SAR) could provide these observations over large portions of the globe, but not over large areas of the polar ice sheets. The authors develop and demonstrate a technique that allows the three-component velocity vector to be estimated from data acquired along two track directions (ascending and descending) under a surface-parallel flow assumption. This technique requires that there are accurate estimates of the surface slope, which are also determined interferometrically. To demonstrate the technique, the authors estimate the three-component velocity field for the Ryder Glacier, Greenland. Their results are promising, although they do not have yet ground-truth data with which to determine the accuracy of their estimates.

Synchronous retreat and acceleration of southeast Greenland outlet glaciers 2000-06 : ice dynamics and coupling to climate

A large portion of the recent increase in the rate of mass loss from the Greenland ice sheet is from increased outlet glacier discharge along its southeastern margin. While previous investigations of the region’s two largest glaciers suggest that acceleration is a dynamic response to thinning and retreat of the calving front, it is unknown whether this mechanism can explain regional acceleration and what forcing is responsible for initiating rapid thinning and retreat. We examine seasonal and interannual changes in ice-front position, surface elevation and flow speed for 32 glaciers along the southeastern coast between 2000 and 2006. While substantial seasonality in front position and speed is apparent, nearly all the observed glaciers show net retreat, thinning and acceleration, with speed-up corresponding to retreat. The ratio of retreat to the along-flow stress-coupling length is proportional to the relative increase in speed, consistent with typical ice-flow and sliding laws. This affirms that speed-up results from loss of resistive stress at the front during retreat, which leads to along-flow stress transfer. Large retreats were often preceded by the formation of a flat or reverse-sloped surface near the front, indicating that subsequent retreats were influenced by the reversed bed slope. Many retreats began with an increase in thinning rates near the front in the summer of 2003, a year of record high coastal-air and sea-surface temperatures. This anomaly was driven in part by recent warming, suggesting that episodes of speed-up and retreat may become more common in a warmer climate.

Ice sheet motion and topography from radar interferometry

Both topography and motion information are present in repeat pass ERS-1 interferograms over ice sheets. The authors demonstrate that the topography is separable from the surface displacement field when a sequence of radar images are available. If the velocity field is constant over the time span of observation, the topography can be derived from differential interferograms formed from sequential observations. With this measurement, a pure displacement field can then be obtained by removal of the topographic contribution to the interferometric phase at each pixel. Further, they discuss how the vertical and horizontal components of displacement affect the interferometrically-derived motion field. They illustrate their approach with four successive (3-day repeat) ERS-1 images of a flow feature in northeastern Greenland.

Catastrophic ice-shelf break-up by an ice-shelf-fragment-capsize mechanism

Two disintegration events leading to the loss of Larsen A and B ice shelves, Antarctic Peninsula, in 1995 and 2002, respectively, proceeded with extreme rapidity (order of several days) and reduced an extensive, seemingly integrated ice shelf to a jumble of small fragments. These events strongly correlate with warming regional climate and accumulation of surface meltwater, supporting a hypothesis that meltwater-induced propagation of pre-existing surface crevasses may have initiated ice-shelf fragmentation. We address here an additional, subsequent mechanism that may sustain and accelerate the ice-shelf break-up once it begins. The proposed mechanism involves the coherent capsize of narrow (less than thickness) ice-shelf fragments by rolling 90° in a direction toward, or away from, the ice front. Fragment capsize liberates gravitational potential energy, forces open ice-shelf rifts and contributes to further fragmentation of the surrounding ice shelf.

Ice-front variation and tidewater behavior on Helheim and Kangerdlugssuaq Glaciers, Greenland

We used satellite images to examine the calving behavior of Helheim and Kangerdlugssuaq Glaciers, Greenland, from 2001 to 2006, a period in which they retreated and sped up. These data show that many large iceberg-calving episodes coincided with teleseismically detected glacial earthquakes, suggesting that calving-related processes are the source of the seismicity. For each of several events for which we have observations, the ice front calved back to a large, pre-existing rift. These rifts form where the ice has thinned to near flotation as the ice front retreats down the back side of a bathymetric high, which agrees well with earlier theoretical predictions. In addition to the recent retreat in a period of higher temperatures, analysis of several images shows that Helheim retreated in the 20th Century during a warmer period and then re-advanced during a subsequent cooler period. This apparent sensitivity to warming suggests that higher temperatures may promote an initial retreat off a bathymetric high that is then sustained by tidewater dynamics as the ice front retreats into deeper water. The cycle of frontal advance and retreat in less than a century indicates that tidewater glaciers in Greenland can advance rapidly. Greenlands larger reservoir of inland ice and conditions that favor the formation of ice shelves likely contribute to the rapid rates of advance.

Estimation of ice-sheet motion using satellite radar interferometry: method and error analysis with application to Humboldt Glacier, Greenland

Satellite radar interferometry provides glaciologists with an important new tool for determining the motion and topography of large ice sheets. We examine the sources of error in interferometrically derived ice-motion measurements, including those errors due to inaccurate estimates of the interferometric baseline. Several simulations are used to assess baseline accuracy in terms of tie-point error and the number and distribution of tie points. These results give insight into how best to select tie points, and also demonstrate the level of accuracy that can be achieved. Examination of two representative cases likely to occur in mapping ice-sheet motion leads to the conclusion that with adequate tie-point information ice velocity can be measured accurately to within a few meters per year. A method to correct horizontal velocity estimates for the effect of vertical displacement using surface slopes is also developed. Finally, we estimate the single-component velocity field for an area on Humboldt Glacier, northern Greenland, using interferograms formed from ERS-I SAR images. We estimate that these velocity measurements are accurate to within 2.3 m year -1 .

Measurement of ice-sheet topography using satellite-radar interferometry

Detailed digital elevation models (DEMs) do not exist for much of the Greenland and Antarctic ice sheets. Radar altimetry is at present the primary, in many cases the only, source of topographic data over the ice sheets, but the horizontal resolution of such data is coarse. Satellite-radar interferometry uses the phase difference between pairs of synthetic aperture radar (SAR) images to measure both ice-sheet topography and surface displacement. We have applied this technique using ERS-1 SAR data to make detailed (i.e. 80 m horizontal resolution) maps of surface topography in a 100 km by 300 km strip in West Greenland. extending northward from just above Jakobshavns Isbrae. Comparison with a 76 km long line of airborne laser-altimeter data shows that we have achieved a relative accuracy of 2.5m along the profile. These observations provide a detailed view of dynamically supported topography near the margin of an ice sheet. In the final section we compare our estimate of topography with phase contours due to motion, and confirm our earlier analysis concerning vertical ice-sheet motion and complexity in ERS-1 SAR interferograms.

Mechanical and hydrologic basis for the rapid motion of a large tidewater glacier: 1. Observations

The data presented in part 1 of this paper (Meier et al., this issue) are here used to assess the role of water input/output, water storage, and basal water pressure in the rapid movement of Columbia Glacier, Alaska. Consistently high basal water pressures, mostly in the range from 300 kPa below to 100 kPa above the ice overburden pressure, are responsible in an overall way for the high glacier flow velocities (3.5–9 m d^−1), which are due mainly to rapid basal sliding caused by the high water pressure. Diurnal fluctuation in basal water pressure is accompanied by fluctuation in sliding velocity in what appears to be a direct causal relation at the upglacier observation site. The water pressure fluctuation tracks the time-integrated water input (less a steady withdrawal), as expected for the diurnally fluctuating storage of water in the glacier far from the terminus. At the downglacier site, the situation is more complex. Diurnal peaks in water level, which are directly related to intraglacial water storage as well as to basal water pressure, are shifted forward in time by 4 hours, probably as a result of the effect of diurnal fluctuation in water output from the glacier, which affects the local water storage fluctuations near the terminus. Because of the forward shift in the basal water pressure peaks, which at the downglacier site lead the velocity peaks by 6 hours, a mechanical connection between water pressure and sliding there would have to involve a 6-hour (quarter period) delay. However, the nearly identical nature of the diurnal fluctuations in velocity at the two sites argues for a single, consistent control mechanism at both sites. The velocity variations in nondiurnal “speed-up events” caused by extra input of water on the longer timescale of several days are only obscurely if at all correlated with variations in basal water pressure but correlate well with water storage in the glacier. It appears that small variations in water pressure (≤100 kPa) sufficient to produce the observed velocity variations (15–30%) are mostly masked by pressure fluctuations caused by spontaneous local reorganizations of the basal water conduit system on a spatial scale much smaller than the longitudinal coupling length over which basal water pressure is effectively averaged in determining the sliding velocity. At the achieved level of observation the clearest (though not complication free) control variable for the sliding velocity variations is basal water storage by cavitation at the glacier bed.