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Gradual reduction of a small ice shelf in the Pine Island Bay area is measured using eleven Landsat images spanning 1972 to 2003. Measurements of Ice shelf area indicate that it expanded slightly during the first two decades of observations from approximately 6.19 km 2 measured on December 7, 1972 to a maximum of about 6.82 km2 observed in 1986. This maximum was followed by a nearly continuous decrease in area and ultimate disappearance of the ice shelf by January 17, 2003. No ice shelf has reappeared since 2003 as observed in subsequent Landsat images. Ten of the eleven Landsat images were co-registered and warped to one of a pair of 2003 geographic reference images before area measurement. Individual study team members made independent measurements of the ice shelf area apparent in each image. The average of these measurements had a standard deviation of 0.14 km2 . The specific cause of this ice shelf disappearance is unknown, but is probably related to increased basal melting by warmer ocean waters reaching Pine Island Bay. Intrusions of warm “Circumpolar Deep Water” are related to ice shelf and outlet glacier thinning and retreat as reported throughout the Amundsen Sea region. This is the first report of complete ice shelf loss so far south or in the Amundsen Bay region. This small, previously unnamed ice shelf formerly occupied what is now known as the Elizabeth City State University Bay.

Satellite radar interferometry (SRI) is used to provide new information on grounding zones in areas of the eastern Ross Ice Shelf and the Filchner Ice Shelf, Antarctica. At the times of the RADARSAT SRI passes, separated by 24 days, a tidal model predicts a change in vertical displacement of the freely floating ice of >1 m in both areas. The change in vertical position occurs over a 5–10 km flexure zone adjacent to the grounding line and would lead to a relatively high interferometric phase fringe rate. This was observed in some areas, and suitable imagery has been used to map the grounding-zone position to an estimated accuracy of 1–2 km. Results for the ice-plain area upstream of the Crary Ice Rise are consistent with the tidal model and improve the previous grounding-line estimates based on field surveys and Système Probatoire pour l’Observation de la Terre (SPOT) data. The results support the suggestion of increased ice grounding in this area, and show that a sub-ice-shelf water channel around the southern end of the Crary Ice Rise is unlikely. Results for the Filchner Ice Shelf also show that existing maps of the grounding zone can be refined. In particular, we identify a large ice rise close to the mouth of the Bailey Ice Stream.

Nature 541, 77–80 (2017); doi:10.1038/nature20136 In this Letter, on page 78, the sentence beginning “At this ‘null zone’20” should have cited ref. 18, instead of ref. 20. The original Letter has been corrected online.

A digital elevation model (DEM) produced from airborne interferometric synthetic aperture radar (TOPSAR) data of a 5 km/spl times/65 km region of the Greenland Ice Sheet is compared with multiple airborne laser altimeter profiles of the same surface. TOPSAR-derived elevation values were calculated on a 10 m/spl times/10 m grid. Surface topography at wavelengths greater than one kilometer is accurately reproduced. A planar adjustment of -8 m with along-track and cross-track tilts of 0.17 and 2.4 m/km, respectively, is required to match the two elevation data sets. Residual noise of the TOPSAR data at subkilometer wavelengths is less than two meters. This study underscores the need for independent elevation data for controlling TOPSAR elevation surveys.

[1] A dense grid of ice-penetrating radar sections acquired over Pine Island Glacier, West Antarctica has revealed a network of sinuous subglacial channels, typically 500 m to 3 km wide, and up to 200 m high, in the ice-shelf base. These subglacial channels develop while the ice is floating and result from melting at the base of the ice shelf. Above the apex of most channels, the radar shows isolated reflections from within the ice shelf. Comparison of the radar data with acoustic data obtained using an autonomous submersible, confirms that these echoes arise from open basal crevasses 50–100 m wide aligned with the subglacial channels and penetrating up to 1/3 of the ice thickness. Analogous sets of surface crevasses appear on the ridges between the basal channels. We suggest that both sets of crevasses were formed during the melting of the subglacial channels as a response to vertical flexing of the ice shelf toward the hydrostatic condition. Finite element modeling of stresses produced after the formation of idealized basal channels indicates that the stresses generated have the correct pattern and, if the channels were formed sufficiently rapidly, would have sufficient magnitude to explain the formation of the observed basal and surface crevasse sets. We conclude that ice-shelf basal melting plays a role in determining patterns of surface and basal crevassing. Increased delivery of warm ocean water into the sub-ice shelf cavity may therefore cause not only thinning but also structural weakening of the ice shelf, perhaps, as a prelude to eventual collapse.

Ice cores collected on dynamic ice sheets cannot be correctly interpreted without accounting for how the ice-sheet shape and flow have changed. By being the last ice sheet of the type occupying primarily a marine basin, the West Antarctic ice sheet has a highly variable behavior. For the past quarter century, researchers have sought a quantitative prediction of its future behavior, including its potential for collapse, and the corresponding effect on sea level. Observations confirm changes on timescales from the Quaternary to less than a minute. The dynamics of the ice sheet involve the complex interaction of kilometers-thick ice that is warm at its base and cold along the margins of ice streams, subglacial till reaches a level where it is composed of a combination of marine sediment and eroded sedimentary rocks, and water that moves primarily between the ice and bed, but whose direction can differ from the direction of ice motion. The pressure of the water system is often sufficient to float the ice sheet locally and small changes in the amount of water in the till can cause it to rapidly switch from very weak to very stiff.

The results of the 1984-85 post-GISP campaigns in central Greenland are presented. Eight ice cores were obtained, some spanning up to 360 years. We present: I. Geographical positions and elevations at the drill sites, II. Density and temperature in the bore holes, III. Filtered δ18O profiles and accumulation-rate variations along the ice cores. The δ18O and accumulation profiles, along with those from the 400 m Crête ice core obtained in 1974, are compared. The accumulation-rate series over a large region have a very high coherence. This indicates that a single bore hole in this region would give a representative accumulation time series over a long period. Further, it appears that for this region there is a linear relation between the measured annual accumulation rate and the mean annual δ18O values. Thus the area between Crête and Summit, just west of the ice divide, seems to be favorable for a deep-drilling operation.

Measurement of geometry, motion, and mass balance from Variegated Glacier, Alaska portray conditions in this surge-type glacier close to the mid-point of its 20 year surge cycle. Comparison of longitudinal profiles of ice depth, surface slope, and surface speed indicate that the motion occurs largely by internal deformation assuming the ice deforms according to the experimental law of Glen. Surface speed is not noticeably affected by local surface slope on the scale of the ice thickness or smaller, but correlates well with slope determined on a longitudinal averaging scale about one order of magnitude larger than the ice depth. The rate of motion on Variegated Glacier agrees well with rates on non-surge type temperate glaciers which have similar depth and slope. Although the (low regime at the time of the measurements is apparently typical of temperate glaciers, a large discrepancy between the balance flux needed for steady state and the actual flux is indicative of a rapidly changing surface elevation profile and internal stress distribution.

The results of the 1984-85 post-GISP campaigns in central Greenland are presented. Eight ice cores were obtained, some spanning up to 360 years. We present: I.Geographical positions and elevations at the drill sites,II.Density and temperature in the bore holes,III.Filtered δ18O profiles and accumulation-rate variations along the ice cores.The δ18O and accumulation profiles, along with those from the 400 m Crête ice core obtained in 1974, are compared.The accumulation-rate series over a large region have a very high coherence. This indicates that a single bore hole in this region would give a representative accumulation time series over a long period. Further, it appears that for this region there is a linear relation between the measured annual accumulation rate and the mean annual δ18O values. Thus the area between Crête and Summit, just west of the ice divide, seems to be favorable for a deep-drilling operation.