Ice-stream stability on a reverse bed slope

Marine-based ice streams whose beds deepen inland are thought to be inherently unstable. This instability is of particular concern because significant portions of the marine-based West Antarctic and Greenland ice sheets are losing mass and their retreat could contribute significantly to future sea-level rise. However, the present understanding of ice-stream stability is limited by observational records that are too short to resolve multi-decadal to millennial-scale behaviour or to validate numerical models8. Here we present a dynamic numerical simulation of Antarctic ice-stream retreat since the Last Glacial Maximum (LGM), constrained by geophysical data, whose behaviour is consistent with the geomorphological record. We find that retreat of Marguerite Bay Ice Stream following the LGM was highly nonlinear and was interrupted by stabilizations on a reverse-sloping bed, where theory predicts rapid unstable retreat. We demonstrate that these transient stabilizations were caused by enhanced lateral drag as the ice stream narrowed. We conclude that, as well as bed topography, ice-stream width and long-term retreat history are crucial for understanding decadal- to centennial-scale ice-stream behaviour and marine ice-sheet vulnerability

A decade of detailed observations (2008-2018) in steep bedrock permafrost at the Matterhorn Hörnligrat (Zermatt, CH)

The PermaSense project is an ongoing interdisciplinary effort between geo-science and engineering disciplines and started in 2006 with the goals of realizing observations that previously have not been possible. Specifically, the aims are to obtain measurements in unprecedented quantity and quality based on technological advances. This paper describes a unique >10-year data record obtained from in situ measurements in steep bedrock permafrost in an Alpine environment on the Matterhorn Hörnligrat, Zermatt, Switzerland, at 3500ma:s:l. Through the utilization of state-of-the-art wireless sensor technology it was possible to obtain more data of higher quality, make these data available in near real time and tightly monitor and control the running experiments. This data set (https://doi.org/10.1594/PANGAEA.897640,Weber et al., 2019a) constitutes the longest, densest and most diverse data record in the history of mountain permafrost research worldwide with 17 different sensor types used at 29 distinct sensor locations consisting of over 114.5 million data points captured over a period of 10 or more years. By documenting and sharing these data in this form we contribute to making our past research reproducible and facilitate future research based on these data, e.g., in the areas of analysis methodology, comparative studies, assessment of change in the environment, natural hazard warning and the development of process models. Finally, the cross-validation of four different data types clearly indicates the dominance of thawing-related kinematics

Grounding line migration in an adaptive mesh ice sheet model

Grounding line migration is a key process affecting the stability of marine ice sheets such as the West Antarctic ice sheet. Recent studies have shown that ice sheet models employing a fixed spatial grid (such as are commonly used for whole ice sheet simulations) cannot be used to solve this problem in a robust manner. We have developed a one-dimensional (vertically integrated) “shelfy stream” ice sheet model that employs the adaptive mesh refinement (AMR) technique to bring higher resolution to spatially and temporally evolving subregions of the model domain. A higher-order solver, the piecewise parabolic method (PPM), is used to compute the thickness evolution. Both AMR and PPM extend readily to greater than one dimension and could be used in full ice sheet simulations. We demonstrate that this approach can bring improvements in terms of accuracy and consistency in both grounded ice sheet and ice stream/ice shelf simulations, given the appropriate choice of refinement criteria. In particular, we demonstrate that AMR, in conjunction with a parameterization for subgrid scale grounding line position, can produce predictions of grounding line migration

Late Quaternary glaciation in the Hebrides sector of the continental shelf: was St Kilda overrun by the British-Irish Ice Sheet?

Until recently, the British-Irish Ice Sheet (BIIS) was thought to have reached no farther than a mid-continental shelf position in the Hebrides Sector, NW Britain, during the last glaciation (traditional model). However, recent discovery of widespread shelf-edge moraines in this sector has led to a suggestion of much more extensive ice (Atlantic Shelf model). The position of the St Kilda archipelago, approximately mid-way between the Outer Hebrides and the continental shelf edge, makes it ideal as an onshore location to test which of the two competing models is more viable. To this end, we (i) reassessed the characteristics, stratigraphy and morphology of the Quaternary sediments exposed on the largest island (Hirta), and (ii) applied time-dependent 2D numerical modelling of possible glacier formation on Hirta. Instead of three glaciations (as previously suggested), we identified evidence of only two, including one of entirely local derivation. The numerical model supports the view that this glaciation was in the form of two short glaciers occupying the two valleys that dominate Hirta. The good state of preservation of the glacial sediments and associated moraine of this local glaciation indicate relatively recent formation. In view of the low inferred equilibrium line altitude of the glacier associated with the best morphological evidence (∼120 m), considerable thickness of slope deposits outside the glacial limits and evidence of only one rather than two tills, a Late Devensian rather than Younger Dryas age is preferred for this glaciation. Re-examination of the submarine moraine pattern from available bathymetry suggests that the ice sheet was forced to flow around St Kilda, implying that the ice was of insufficient thickness to overrun the islands. Accepting this leaves open the possibility that a St Kilda nunatak supported local ice while the ice sheet extended to the continental shelf edge

Causes of pre-collapse changes of the Larsen B ice shelf: Numerical modelling and assimilation of satellite observations

Satellite observations revealed that beside a rapid thinning, the Larsen B ice shelf (LBIS) was undergoing a significant acceleration before its collapse in 2002. This paper investigates the ice shelf acceleration between 1995 and 1999 using a combination of data assimilation and numerical modelling. Based on a flow model adjusted to the 1995 InSAR velocities, perturbation experiments are performed, such as ice front retreat, thinning, increase in tributary flow and rheological weakening. Furthermore, an inversion for ice shelf rheology and tributary flow velocity is performed for both the 1995 and the 1999 InSAR velocities. The perturbation experiments together with the inversion strongly suggest that the acceleration cannot solely be explained by the retreat of the ice shelf front but relies on a further significant rheological weakening of the already weak shear zones within the LBIS. Minor tributary acceleration is found to be an effect rather than a cause of the ice shelf acceleration. Furthermore, the observed acceleration cannot be explained by the observed recent thinning. We conclude that for smaller ice shelves such as the LBIS, such weak shear margins play a crucial role in controlling their dynamics and are the key to understand changes in the future. Finally, we compare the dynamic thinning likely to be associated with the observed acceleration with the observed thinning. For the ice shelf as a whole, this thinning accounts for 20% of the observed value, which implies that factors such as enhanced basal melt were the primary cause of the observed thinning

Glacier change: Dynamic projections

Mountain glaciers around the world are in decay. According to a modelling study that — unusually — includes full ice flow physics, those in Western Canada will largely be restricted to the coastal region by the year 2100

Future sea-level rise from Greenland's main outlet glaciers in a warming climate.

Over the past decade, ice loss from the Greenland Ice Sheet increased as a result of both increased surface melting and ice discharge to the ocean. The latter is controlled by the acceleration of ice flow and subsequent thinning of fast-flowing marine-terminating outlet glaciers. Quantifying the future dynamic contribution of such glaciers to sea-level rise (SLR) remains a major challenge because outlet glacier dynamics are poorly understood. Here we present a glacier flow model that includes a fully dynamic treatment of marine termini. We use this model to simulate behaviour of four major marine-terminating outlet glaciers, which collectively drain about 22 per cent of the Greenland Ice Sheet. Using atmospheric and oceanic forcing from a mid-range future warming scenario that predicts warming by 2.8 degrees Celsius by 2100, we project a contribution of 19 to 30 millimetres to SLR from these glaciers by 2200. This contribution is largely (80 per cent) dynamic in origin and is caused by several episodic retreats past overdeepenings in outlet glacier troughs. After initial increases, however, dynamic losses from these four outlets remain relatively constant and contribute to SLR individually at rates of about 0.01 to 0.06 millimetres per year. These rates correspond to ice fluxes that are less than twice those of the late 1990s, well below previous upper bounds. For a more extreme future warming scenario (warming by 4.5 degrees Celsius by 2100), the projected losses increase by more than 50 per cent, producing a cumulative SLR of 29 to 49 millimetres by 2200.Journal ArticleResearch Support, Non-U.S. Gov'tResearch Support, U.S. Gov't, Non-P.H.S.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Palaeoclimate science: Pulsating ice sheet

During the last ice age, huge numbers of icebergs were episodically discharged from an ice sheet that covered North America. Numerical modelling suggests that these events resulted from a conceptually simple feedback cycle