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The largest uncertainty in future sea-level rise is loss of ice from the Greenland and Antarctic Ice Sheets. Ice shelves, freely floating platforms of ice that fringe the ice sheets, play a crucial role in restraining discharge of grounded ice into the ocean through buttressing. However, since the 1990s, several ice shelves have thinned, retreated, and collapsed. If this pattern continues, it could expose thick cliffs that become structurally unstable and collapse in a process called marine ice cliff instability (MICI). However, the feedbacks between calving, retreat, and other forcings are not well understood. Here we review observed modes of calving from ice shelves and marine-terminating glaciers, and their relation to environmental forces. We show that the primary driver of calving is long-term internal glaciological stress, but as ice shelves thin they may become more vulnerable to environmental forcing. This vulnerability—and the potential for MICI—comes from a combination of the distribution of preexisting flaws within the ice and regions where the stress is large enough to initiate fracture. Although significant progress has been made modeling these processes, theories must now be tested against a wide range of environmental and glaciological conditions in both modern and paleo conditions. Ice shelves, floating platforms of ice fed by ice sheets, shed mass in a near-instantaneous fashion through iceberg calving. Most ice shelves exhibit a stable cycle of calving front advance and retreat that is insensitive to small changes in environmental conditions. Some ice shelves have retreated or collapsed completely, and in the future this could expose thick cliffs that could become structurally unstable called ice cliff instability. The potential for ice shelf and ice cliff instability is controlled by the presence and evolution of flaws or fractures within the ice. [ABSTRACT FROM AUTHOR]

Ice shelves limit the flux of grounded ice into the ocean by buttressing the discharge of land-based ice upstream. Ice shelf weakening and collapse can lead to decreased buttressing and observations increasingly show that some ice shelves have experienced increased melt and increased calving, with recent hypotheses suggesting that increased melt leads to increased fracturing. However, the specific processes that control this correlation are not yet understood, with mechanisms other than melt affecting fracturing. Here we use the topography of the ice shelf base from BedMachine to investigate how basal melting and ice deformation contribute to crevasse and melt channel formation and evolution on the Pine Island Ice Shelf in West Antarctica. We find that high basal melt rates and high first principal strain rates lead to substantial roughening of the ice shelf through a collection of features, including melt channels and crevasses. Critically, melt channels and crevasses are the deepest in all directions at locations where the highest rates of melting and straining occur simultaneously. This suggests that the combination of melt rates and strain rates work in tandem to excavate and seed the deepest melt channels and crevasses on ice shelves. These features then may form lines of weakness that transform into rifts and, ultimately, the detachment boundary for calving events. This implies that melt and fracture play an important role in controlling the dynamics of ice shelves. Plain Language Summary Future sea level rise is tied to floating Antarctic ice shelves which limit the flow of ice from the continent. Ice shelf collapse could trigger an acceleration of land-based ice into the ocean due to the loss of support, leading to future sea level rise. It has been shown that ice shelves with a higher melt have more crevasses, are more fractured, and are rough. However, the specific processes driving the potential connection between melt and fracture are not well understood. Here we use ice shelf topography to study the relationship between roughness and the size and spacing of geometric features on the Pine Island Ice Shelf. We find that high melt rates and high strain rates result in deeper and wider features on the ice shelf. We also find that, in contrast to the results of previous studies, both melt and fracture contribute to the deepest features. This suggests that these processes contribute to enhancing the depth of features. We hypothesize that this amplifying interaction may then promote rifting and iceberg calving, hinting that it is the combination of melt and fracture that is important in controlling the dynamics of ice shelves. [ABSTRACT FROM AUTHOR]

The Brunt Ice Shelf, Antarctica, accelerated rapidly from a velocity of 900 to 1500 m a -1 during 6 months, following calving of a 1500 km 2 iceberg on 22 January 2023. The immediate response to calving was observed as a change to the rate of acceleration and not to velocity directly. Acceleration increased by a factor of 10, with a second, smaller calving at the end of June 2023, leading to further tripling of acceleration. The acceleration was caused by the reduction of buttressing at the McDonald Ice Rumples, leading to high localised strain rates, which reduce the strength of the remaining ice shelf. [ABSTRACT FROM AUTHOR]

We combined an ensemble of satellite altimetry, interferometry, and gravimetry data sets using common geographical regions, time intervals, and models of surface mass balance and glacial isostatic adjustment to estimate the mass balance of Earth's polar ice sheets. We find that there is good agreement between different satellite methods--especially in Greenland and West Antarctica--and that combining satellite data sets leads to greater certainty. Between 1992 and 2011, the ice sheets of Greenland, East Antarctica, West Antarctica, and the Antarctic Peninsula changed in mass by -142 ± 49, +14 ± 43, -65 ± 26, and -20 ± 14 gigatonnes year(-1), respectively. Since 1992, the polar ice sheets have contributed, on average, 0.59 ± 0.20 millimeter year(-1) to the rate of global sea-level rise.

The Brunt Ice Shelf, Antarctica, accelerated rapidly from 900 m a-1 to 1500 m a-1 during six months following the calving of a 1500 km2 iceberg on 22nd January 2023. Initially, the rate of acceleration increased by a factor of ten, with a second, smaller calving at the end of June 2023 leading to further tripling of acceleration. The acceleration is caused by reduction of buttressing at the McDonald Ice Rumples due to loss of contact with the sea floor and has led to high strain rates to the south, with potential consequences for the stability of the remaining ice shelf. [ABSTRACT FROM AUTHOR]

It is commonly asserted that there are two distinct classes of glacier surges: slow, long-duration 'Svalbard-type' surges, triggered by a transition from cold- to warm-based conditions (thermal switching), and fast, shorter-duration 'Alaska-type' surges triggered by a reorganisation of the basal drainage system (hydraulic switching). This classification, however, reflects neither the diversity of surges in Svalbard and Alaska (and other regions), nor the fundamental dynamic processes underlying all surges. We argue that enthalpy balance theory offers a framework for understanding the spectrum of glacier surging behaviours while emphasising their essential dynamic unity. In this paper, we summarise enthalpy balance theory, illustrate its potential to explain so-called 'Svalbard-type' and 'Alaska-type' surges using a single set of principles, and show examples of a much wider range of glacier surge behaviour than previously observed. We then identify some future directions for research, including strategies for testing predictions of the theory against field and remote sensing data, and priorities for numerical model development. [ABSTRACT FROM AUTHOR]

Outlet glaciers of ice sheets are the primary means of transporting ice from the interior to the oceans, and their flow velocity is one control that determines the mass balance of ice sheets. Estimates of ice velocities, particularly for remote areas, are commonly based on satellite remote-sensing data. As radar-based systems emit energy at frequencies high enough to penetrate clouds, they can record backscatter signals from surfaces throughout the year. Radar data, which are collected with repeat-pass spaceborne platforms, are therefore good for extracting ice velocities using interferometry for slow-moving ice and the cross-correlation techniques for fast-moving ice. Here, we present an improvement on the cross-correlation technique that enables independent quality checks and enhances the spatial extent of the derived velocity fields. Filtering of ESA European Remote Sensing satellite scenes, based on a Butterworth high-pass spatial-frequency filter, focuses the cross-correlation technique on smaller, movable surface features, instead of large fixed features. It also allows a nonregional culling based on the signal-to-noise ratio of the velocities, with the effect of increasing the coverage of robust velocity estimates. Index Terms--Correlation, Greenland, Helheim, high-pass filters, radar velocity measurement, tracking.

Multibaseline European Remote Sensing (ERS) interferometric synthetic aperture radar coherence images from Cardiff, U.K. are investigated with respect to urban form. A model of spatial coherence, taking into account the vertical distribution of scatterers, is inverted to allow urban building height variance to be retrieved. Sixty-nine coherence maps are employed, generated from 20 ERS images. No a priori information is required in the analysis. However, realistic vertical scatterer distributions are retrieved, and Cardiff's central business district is automatically identified. This analysis demonstrates the utility of the urban decorrelation models employed. Index Terms--Decorrelation, interferometric synthetic aperture radar (InSAR), interferometry, surface roughness.

Two image-to-image patch offset techniques for estimating feature motion between satellite synthetic aperture radar (SAR) images are discussed. Intensity tracking, based on patch intensity cross-correlation optimization, and coherence tracking, based on patch coherence optimization, are used to estimate the movement of glacier surfaces between two SAR images in both slant-range and azimuth direction. The accuracy and application range of the two methods are examined in the case of the surge of Monacobreen in Northern Svalbard between 1992 and 1996. Offset-tracking procedures of SAR images are an alternative to differential SAR interferometry for the estimation of glacier motion when differential SAR interferometry is limited by loss of coherence, i.e., in the case of rapid and incoherent flow and of large acquisition time intervals between the two SAR images. In addition, an offset-tracking procedure in the azimuth direction may be combined with differential SAR interferometry in the slant-range direction in order to retrieve a two-dimensional displacement map when SAR data of only one orbit configuration are available. Index Terms--Glacier-motion estimation, offset tracking, SAR, SAR interferometry, surge-type glacier.

The radiometric correction of synthetic aperture radar (SAR) imagery for topographically induced changes in the pixel scattering area is examined. The relative significance of ground slope in the range and azimuth directions is compared, and a widely used correction factor, based on the sin of the local radar incidence angle, is shown to be sufficiently accurate. The importance of performing this correction in slant range, rather than ground range, is demonstrated. Index Terms - AIRSAR, forest, scattering area normalization, synthetic aperture radar (SAR).