Response of Marine‐Terminating Glaciers to Forcing: Time Scales, Sensitivities, Instabilities, and Stochastic Dynamics

Recent observations indicate that many marine‐terminating glaciers in Greenland and Antarctica are currently retreating and thinning, potentially due to long‐term trends in climate forcing. In this study, we describe a simple two‐stage model that accurately emulates the response to external forcing of marine‐terminating glaciers simulated in a spatially extended model. The simplicity of the model permits derivation of analytical expressions describing the marine‐terminating glacier response to forcing. We find that there are two time scales that characterize the stable glacier response to external forcing, a fast time scale of decades to centuries, and a slow time scale of millennia. These two time scales become unstable at different thresholds of bed slope, indicating that there are distinct slow and fast forms of the marine ice sheet instability. We derive simple expressions for the approximate magnitude and transient evolution of the stable glacier response to external forcing, which depend on the equilibrium glacier state and the strength of nonlinearity in forcing processes. The slow response rate of marine‐terminating glaciers indicates that current changes at some glaciers are set to continue and accelerate in coming centuries in response to past climate forcing and that the current extent of change at these glaciers is likely a small fraction of the future committed change caused by past climate forcing. Finally, we find that changing the amplitude of natural fluctuations in some nonlinear forcing processes, such as ice shelf calving, changes the equilibrium glacier state

Englacial Pore Water Localizes Shear in Temperate Ice Stream Margins

The margins of fast‐moving ice streams are characterized by steep velocity gradients. Some of these gradients cannot be explained by a temperature‐dependent viscosity alone. Laboratory data suggest that water in the ice‐grain matrix decreases the ice viscosity; we propose that this causes the strong localization of shear in temperate ice stream margins. However, the magnitude of weakening and its consequences for ice stream dynamics are poorly understood. Here we investigate how the coupling between temperate ice properties, ice mechanics, and drainage of melt water from the ice stream margin alters the dynamics of ice streams. We consider the steady‐state ice flow, temperature, water content, and subglacial water drainage in an ice stream cross section. Temperate ice dynamics are modeled as a two‐phase flow, with gravity‐driven water transport in the pores of a viscously compacting and deforming ice matrix. We find that the dependence of ice viscosity on meltwater content focuses the temperate ice region and steepens the velocity gradients in the ice stream margin. It provides a possible explanation for the steep velocity gradients observed in some ice stream shear margins. This localizes heat dissipation there, which in turn increases the amount of meltwater delivered to the ice stream bed. This process is controlled by the permeability of the temperate ice and the sensitivity of ice viscosity to meltwater content, both of which are poorly constrained properties

The effect of buttressing on grounding line dynamics

Determining the position and stability of the grounding line of a marine ice sheet is a major challenge for ice-sheet models. Here, we investigate the role of lateral shear and ice-shelf buttressing in grounding line dynamics by extending an existing boundary layer theory to laterally confined marine ice sheets. We derive an analytic expression for the ice flux at the grounding line of confined marine ice sheets that depends on both local bed properties and non-local ice-shelf properties. Application of these results to a laterally confined version of the MISMIP 1a experiment shows that the boundary condition at the ice-shelf front (i.e. the calving law) is a major control on the location and stability of the grounding line in the presence of buttressing, allowing for both stable and unstable grounding line positions on downwards sloping beds. These results corroborate the findings of existing numerical studies that the stability of confined marine ice sheets is influenced by ice-shelf properties, in contrast to unconfined configurations where grounding line stability is solely determined by the local slope of the bed. Consequently, the marine ice-sheet instability hypothesis may not apply to buttressed marine ice sheets

The role of subtemperate slip in thermally driven ice stream margin migration

The amount of ice discharged by an ice stream depends on its width, and the widths of unconfined ice streams such as the Siple Coast ice streams in West Antarctica have been observed to evolve on decadal to centennial timescales. Thermally driven widening of ice streams provides a mechanism for this observed variability through melting of the frozen beds of adjacent ice ridges. This widening is driven by the heat dissipation in the ice stream margin, where strain rates are high, and at the bed of the ice ridge, where subtemperate sliding is possible. The inflow of cold ice from the neighboring ice ridges impedes ice stream widening. Determining the migration rate of the margin requires resolving conductive and advective heat transfer processes on very small scales in the ice stream margin, and these processes cannot be resolved by large-scale ice sheet models. Here, we exploit the thermal boundary layer structure in the ice stream margin to investigate how the migration rate depends on these different processes. We derive a parameterization of the migration rate in terms of parameters that can be estimated from observations or large-scale model outputs, including the lateral shear stress in the ice stream margin, the ice thickness of the stream, the influx of ice from the ridge, and the bed temperature of the ice ridge. This parameterization will allow the incorporation of ice stream margin migration into large-scale ice sheet models

A simple way to identify non-viable cells within living plant tissue using confocal microscopy.

BACKGROUND: Plant cell death is a normal process during plant development. Mutant plants may exhibit misregulation of this process, which can lead to severe growth defects. Simple ways of visualising cell death in living plant tissues can aid the study of plant development and physiology. RESULTS: Spectral variants of the fluorescent SYTOX dyes were tested for their usefulness for the detection of non-viable cells within plant embryos and roots using confocal laser-scanning microscopy. The dyes were selective for non-viable cells and showed very little background staining in living cells. Simultaneous detection of SYTOX dye and fluorescent protein (e.g. GFP) fluorescence was possible. CONCLUSION: The fluorescent SYTOX dyes are useful for an easy and quick first assay of plant cell viability in living plant samples using fluorescence and confocal laser-scanning microscopy.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Shaping orientation as development strategy to promote the further education in Vocational Education and Training (VET) - shown and examined on the example of VET-Net

In der Arbeit wird eine gestaltungsorientierte und vernetzte Fortbildung für Berufspädagogen als eine erstrebenswerte Entwicklungsstrategie im Prozess beruflicher Bildung in Sub-Sahara Afrika ausgewiesen. Dies geschieht am Beispiel des Projektes „Fortbildungs- und Forschungsnetzwerk für Berufspädagogen in Sub-Sahara Afrika“ (VET-Net), in dem die Verfasserin mitgewirkt hat. Eine berufswissenschaftliche Position wird als Grundlage für vernetztes Gestalten in der Fortbildung und auch für Lehren und Lernen in der beruflichen Bildung ausgewiesen. Gestalten wird als Grundkategorie, als ein spezielles Handeln beziehungsweise Tätig sein, dargestellt und begründet. In der Folge zeigen sich Gelingensbedingungen für ein erfolgreiches Gestalten. Es zeigt sich auch, dass Gestaltungskompetenz ein Schlüssel ist, mit dem Hochschullehrer im Zusammenwirken mit Lehrkräften in Schulen, Betrieben und anderen Berufsbildungseinrichtungen den Aufbau und die Nutzung eines Fortbildungssystems sinnvoll anstoßen und verfolgen können. Dies weit möglichst selbständig und unter den speziellen Bedingungen, die in Sub-Sahara Afrika gegeben sind. Dazu werden die Entwicklungen in dem Projekt VET-Net analysiert, das zunächst auf die Fortbildungssituationen in Universitäten in Maputo in Mosambik, in Jimma in Äthiopien und in Johannesburg in Südafrika abgestellt. Die empirische Untersuchung macht diese Entwicklung erkennbar. Dafür werden Methoden der qualitativen Sozialforschung eingesetzt: Die Gruppendiskussion in Kombination mit leitfadengestützten Experteninterviews. Durch eine kategorienbasierte inhaltsanalytische Auswertung der erhobenen Daten werden die zuvor gewonnenen Einsichten zur Gestaltungsorientierung als einer Entwicklungstrategie bestätigt und/ oder begründet modifiziert. Als Konsequenz aus den theoretischen Überlegungen und der empirischen Untersuchung wird schließlich ein Modell für ein Train-the-Trainer Fortbildungssystem (TtT-System) für Berufspädagogen vorgeschlagen.In the thesis a shaping oriented and networked further education for VET pedagogues is presented as a desirable development strategy in the process of VET in Sub-Sahara Africa. This is exemplified by the project „Further Education and Research Network for VET Professionals in Sub-Sahara Africa” (VET-Net), to which the author contributed. A scientific position based on vocational sciences is shown as a basis for networked shaping in VET further education as well as for teaching and learning in VET. Shaping is represented and justified as a base category, as a special action or activity. Conditions of success are demonstrated as a result for successful shaping. Also, it is shown that competency in shaping is a key for university lecturers, to initiate and pursue the development and use of a training system usefully - in cooperation with teachers in VET schools, companies and other vocational training institutions and under the specific conditions that are given in Sub-Sahara Africa. The developments in the VET-Net project will be analyzed, focusing on the continuing education situation in universities in Maputo, Mozambique, in Jimma, Ethiopia and in Johannesburg in South Africa. It becomes clear how professional VET pedagogues can develop a desirable shaping competence for the implementation of a further education network. With the help of the empirical investigation the development becomes recognizable. For this, methods of qualitative social research are used: the group discussion in combination with guided interviews with experts. Through a category-based content analysis of the collected data the previously gained insights into the shaping orientation have been confirmed or reasonable modified. As a consequence of the theoretical considerations and the empirical investigation, a shaping and competence oriented further education model for a Train-the-Trainer system (TtT-System) for professional pedagogues is finally proposed

Kann man Wahlaussagen verstehen? : Über die Sprache der Parteiprogramme zur Bundestagswahl 2009

Die Sprache der Politik hat nicht den besten Ruf. Als Ergebnis von innerparteilichen Expertenrunden entstehen oft Texte in einer von Bürokratismen durchzogenen Fachsprache. Unverständlich vor allem für die, an die sie gerichtet sind: die Wähler. Wie ist es konkret um die Verständlichkeit der Wahlprogramme 2009 bestellt? Eine Analyse

Oscillatory subglacial drainage in the absence of surface melt

The presence of strong diurnal cycling in basal water pressure records obtained during the melt season is well established for many glaciers. The behaviour of the drainage system outside the melt season is less well understood. Here we present borehole observations from a surge-type valley glacier in the St Elias Mountains, Yukon Territory, Canada. Our data indicate the onset of strongly correlated multi-day oscillations in water pressure in multiple boreholes straddling a main drainage axis, starting several weeks after the disappearance of a dominant diurnal mode in August 2011 and persisting until at least January 2012, when multiple data loggers suffered power failure. Jökulhlaups provide a template for understanding spontaneous water pressure oscillations not driven by external supply variability. Using a subglacial drainage model, we show that water pressure oscillations can also be driven on a much smaller scale by the interaction between conduit growth and distributed water storage in smaller water pockets, basal crevasses and moulins, and that oscillations can be triggered when water supply drops below a critical value. We suggest this in combination with a steady background supply of water from ground water or englacial drainage as a possible explanation for the observed wintertime pressure oscillations