Modelling ice dynamic contributions to sea level rise from the Antarctic Peninsula

The future ice dynamical contribution to sea-level rise (SLR) from 210 ice shelf nourishing drainage basins of the Antarctic Peninsula Ice Sheet (APIS) is simulated, using the British Antarctic Survey Antarctic Peninsula Ice Sheet Model. Simulations of the grounded ice sheet include response to ice-shelf collapse, estimated by tracking thermal ice shelf viability limits in 14 IPCC Global Climate Model ensemble temperature projections. Grounding line retreat in response to ice shelf collapse is parameterized with a new multivariate linear regression model utilizing a range of glaciological and geometric predictor variables. Multi-model means project SLR up to 9.4 mm sea-level equivalent (SLE) by 2200, and up to 19 mm SLE by 2300. Rates of SLR from individual drainage basins throughout the peninsula are similar to 2100, yet diverge between 2100 and 2300 due to individual basin characteristics. Major contributors to SLR are the outlet glaciers feeding southern George VI Ice Shelf, accounting for >75% of total SLR in some model runs. Ice sheet thinning induced by ice-shelf removal is large (up to ∼500 m), especially in Palmer Land in the Southern Antarctic Peninsula, and may propagate as far as 135 km inland. These results emphasize the importance of the ice dynamical contribution to future sea level of the APIS on decadal to centennial timescales

Hysteretic evolution of ice rises and ice rumples with variations in sea level

Ice rises and ice rumples are locally grounded features found in coastal Antarctica and are surrounded by otherwise freely floating ice shelves. An ice rise has an independent flow regime, whereas the flow regime of an ice rumple conforms to that of the ice shelf and merely slows the flow of ice. In both cases, local highs in the bathymetry are in contact with the ice shelf from below, thereby regulating the large-scale ice flow, with implications for the upstream continental grounding line position. This buttressing effect, paired with the suitability of ice rises as a climate archive, necessitates a better understanding of the transition between ice rise and ice rumple, their evolution in response to a change in sea level, and their dynamic interaction with the surrounding ice shelf. We investigate this behaviour using a three-dimensional full Stokes ice flow model. The simulations span end-member basal friction scenarios of almost stagnant and fully sliding ice at the ice-bed interface. We analyse the coupling with the surrounding ice shelf by comparing the deviations between the non-local full Stokes surface velocities and the local shallow ice approximation (SIA). Deviations are generally high at the ice divides and small on the lee sides. On the stoss side, where ice rise and ice shelf have opposing flow directions, deviations can be significant. Differences are negligible in the absence of basal sliding where the corresponding steady state ice rise is larger and develops a fully independent flow regime that is well described by SIA. When sea level is increased and a transition from ice rise to ice rumple is approached, the divide migration is more abrupt the higher the basal friction. In each scenario, the transition occurs after the stoss side grounding line has moved over the bed high and is positioned on a retrograde slope. We identify a hysteretic response of ice rises and ice rumples to changes in sea level, with grounded area being larger in a sea level increase scenario than in a sea level decrease scenario. This hysteresis not only shows irreversibility following an equal increase and subsequent decrease in sea level, but also has important implications for ice flow model initialisation. The initial grounded area needs to be carefully considered, as this will determine the formation of either an ice rise or an ice rumple, thereby causing different buttressing effects

Dynamic response of Antarctic Peninsula Ice Sheet to potential collapse of Larsen C and George VI ice shelves

Ice shelf break-up and disintegration events over the past 5 decades have led to speed-up, thinning, and retreat of upstream tributary glaciers and increases to rates of global sea-level rise. The southward progression of these episodes indicates a climatic cause and in turn suggests that the larger Larsen C and George VI ice shelves may undergo a similar collapse in the future. However, the extent to which removal of the Larsen C and George VI ice shelves will affect upstream tributary glaciers and add to global sea levels is unknown. Here we apply numerical ice-sheet models of varying complexity to show that the centennial sea-level commitment of Larsen C embayment glaciers following immediate shelf collapse is low ( < 2.5 mm to 2100,  < 4.2 mm to 2300). Despite its large size, Larsen C does not provide strong buttressing forces to upstream basins and its collapse does not result in large additional discharge from its tributary glaciers in any of our model scenarios. In contrast, the response of inland glaciers to a collapse of the George VI Ice Shelf may add up to 8mm to global sea levels by 2100 and 22mm by 2300 due in part to the mechanism of marine ice sheet instability. Our results demonstrate the varying and relative importance to sea level of the large Antarctic Peninsula ice shelves considered to present a risk of collapse

Sensitivity of Heinrich-type ice-sheet surge characteristics to boundary forcing perturbations

Heinrich-type ice-sheet surges are one of the prominent signals of glacial climate variability. They are characterised as abrupt, quasi-periodic episodes of ice-sheet instabilities during which large numbers of icebergs are released from the Laurentide ice sheet. The mechanisms controlling the timing and occurrence of Heinrich-type ice-sheet surges remain poorly constrained to this day. Here, we use a coupled ice sheet–solid Earth model to identify and quantify the importance of boundary forcing for the surge cycle length of Heinrich-type ice-sheet surges for two prominent ice streams of the Laurentide ice sheet – the land-terminating Mackenzie ice stream and the marine-terminating Hudson ice stream. Both ice streams show responses of similar magnitude to surface mass balance and geothermal heat flux perturbations, but Mackenzie ice stream is more sensitive to ice surface temperature perturbations, a fact likely caused by the warmer climate in this region. Ocean and sea-level forcing as well as different frequencies of the same forcing have a negligible effect on the surge cycle length. The simulations also highlight the fact that only a certain parameter space exists under which ice-sheet oscillations can be maintained. Transitioning from an oscillatory state to a persistent ice streaming state can result in an ice volume loss of up to 30 % for the respective ice stream drainage basin under otherwise constant climate conditions. We show that Mackenzie ice stream is susceptible to undergoing such a transition in response to all tested positive climate perturbations. This underlines the potential of the Mackenzie region to have contributed to prominent abrupt climate change events of the last deglaciation.</p

Quantifying the effect of ocean bed properties on ice sheet geometry over 40 000 years with a full-Stokes model

Simulations of ice sheet evolution over glacial cycles require integration of observational constraints using ensemble studies with fast ice sheet models. These include physical parameterisations with uncertainties, for example, relating to grounding-line migration. More complete ice dynamic models are slow and have thus far only be applied for  50 % under almost equal forcing. Grounding-line positions differ by up to 49 km, show significant hysteresis, and migrate non-steadily in both scenarios with long quiescent phases disrupted by leaps of rapid migration. The simulations quantify the evolution of two different ice sheet geometries (namely thick and slow vs. thin and fast), triggered by the variable grounding-line migration over the differing ocean beds. Our study extends the timescales of 3D full-Stokes by an order of magnitude compared to previous studies with the help of parallelisation. The extended time frame for full-Stokes models is a first step towards better understanding other processes such as erosion and sediment redistribution in the ice shelf cavity impacting the entire catchment geometry

Fully-coupled 3D modelling of Halvfarryggen Ice Rise, Dronning Maud Land, East Antarctica

Antarctica is fringed by floating ice shelves through which more than 80% of the overall ice is discharged. These ice shelves provide the main interface between the Antarctic Ice Sheet and the surrounding ocean. Virtually all ice shelves are either laterally constrained by embayments or locally reground on topographic highs causing the formation of ice rises. In both cases the locally enhanced friction is transmitted upstream, resulting in a restraining force that decelerates ice discharge and controls rates of sea-level rise. As ice rises are typically on the order of ten sof kilometers in diameter, they are usually not resolved, both in the observations and the physical approximations,in large-scale ice-sheet models. In addition to their stabilising influence, ice rises also provide a proxy for stable ice-sheet condition in the past as they archive their own evolution in their stratigraphy, providing the opportunity to derive a Pan-Antarctic archive for the deglaciation history. To adequately simulate ice rise evolution, the full stress balance needs to be considered including the full coupling of ice sheet, ice shelf, and ice rise. Here, we use the Full-Stokes ice-sheet model Elmer/Ice for the Ekström Ice Shelf embayment in Dronning Maud Land, East Antarctica, to study the effect of ice rises on the overall stability of the ice sheet. We initialise the model for prognostic simulations using today’s surface velocity to invert for basal drag and ice-shelf rigidity in full Stokes. To account for inconsistencies in the input data we relax the initial geometry over 10 years resulting in a quasi-steady state which stays close to today’s observations. As a first application of this 3D model including the fully coupled system with a dynamic grounding line, we derive erosion rates for the outlet glaciers of the Ekström Ice Shelf embayment, revealing moderate rates of up to∼0.75 mm/yr, using published sliding-erosion ratios from others areas. This will be compared to sedimentary structures derived from seismic measurements. Our perturbation experiments of the Ekström Ice Shelf embayment will investigate the effect of changing atmospheric/oceanic conditions on ice-rise evolution and divide migration using for the first time a fully coupled 3D ice-sheet model. This approach will permit to unambiguously show if and how much changes in external forcing influence divide position and internal stratigraphy, a proxy that has been widely used to deduce stable ice-flow and grounding-line conditions, but from studies that either use simplified model physics or omit the coupling between ice rise and ice shelf. Our novel modelling set-up will help to unravel the importance of ice rises for the past and future timing of sea-level rise, and represents a first step towards using ice rises as a Pan-Antarctic archive to constrain paleo ice-sheet simulations

Relevance of field observations as boundary conditions for understanding ice-sheet-ocean interactions

The direct contact of warm ocean water with the front and base of ice shelves is the main driver for accelerated mass loss of the Antarctic ice sheet. We present a compilation of observations from various projects and methodological approaches applied over the last decade along the Dronning Maud Land coast and highlight their importance for understanding the ice-ocean interactions. With a focus on the Ekström ice shelf, these include spatially continuous seismic observations in combination with airborne gravity inversion to yield sub-shelf bathymetry and geomorphological evidence of past ice-flow activity; ice-dynamic numerical modelling to investigate the role of seafloor/subglacial substrate characteristics to enhance or reduce ice-sheet extent and advance/retreat rates; sub-shelf CTD measurements to determine ocean properties driving basal melting; satellitebased remote sensing to determine ice-shelf height changes and spatially-distributed basal melting; and point measurements of basal melt with surface-based phase-sensitive radar to determine ocean-driven melt and validate remote-sensing products. As the Dronning Maud Land coast plays a critical role in preconditioning the water mass of the coastal current before it enters the Filcher ice-shelf cavity, we argue that a coordinated inter- and transdisciplinary observational network is required to facilitate monitoring a potential ice-sheet mass loss in this part of Antarctica

Short term effects of exercise training on exercise capacity and quality of life in patients with pulmonary arterial hypertension: protocol for a randomised controlled trial

<p>Abstract</p> <p>Background</p> <p>Advances in the understanding and management of pulmonary arterial hypertension have enabled earlier diagnosis and improved prognosis. However, despite best available therapy, symptoms of exertional dyspnoea and fatigue are commonly reported and result in a reduced capacity to perform daily activities and impaired quality of life. Exercise training has demonstrated efficacy in individuals with other respiratory and cardiovascular diseases. Historically, however, exercise training has not been utilised as a form of therapy in pulmonary arterial hypertension due to the perceived risk of sudden cardiac death and the theoretical possibility that exercise would lead to worsening pulmonary vascular haemodynamics and deterioration in right heart function. Now, with the advances in pharmaceutical management, determining the safety and benefits of exercise training in this population has become more relevant. Only three studies of supervised exercise training in pulmonary arterial hypertension have been published. These studies demonstrated improvements in exercise capacity and quality of life, in the absence of adverse events or clinical deterioration. However, these studies have not utilised an outpatient-based, whole body exercise training program, the most common format for exercise programs within Australia. It is uncertain whether this form of training is beneficial and capable of producing sustained benefits in exercise capacity and quality of life in this population.</p> <p>Design/Methods</p> <p>This randomised controlled trial will determine whether a 12 week, outpatient-based, supervised, whole body exercise training program, followed by a home-based exercise program, is safe and improves exercise capacity and quality of life in individuals with pulmonary arterial hypertension. This study aims to recruit 34 subjects who will be randomly allocated to the exercise group (supervised exercise training 3 times a week for 12 weeks, followed by 3 sessions per week of home exercise for 12 weeks) or the control group (usual medical care). Subjects will be assessed at baseline, 12 weeks and 24 weeks.</p> <p>Discussion</p> <p>This study will determine whether outpatient-based, whole body exercise training is beneficial and safe in individuals with pulmonary arterial hypertension. Additionally, this study will contribute to clinical practice guidelines for this patient population.</p> <p>Trial registration</p> <p>Australia and New Zealand Clinical Trials Register (ANZCTR): <a href="http://www.anzctr.org.au/ACTRN12609000502235.aspx">ACTRN12609000502235</a></p

Nerve growth factor gene therapy using adeno-associated viral vectors prevents cardiomyopathy in type 1 diabetic mice

Diabetes is a cause of cardiac dysfunction, reduced myocardial perfusion, and ultimately heart failure. Nerve growth factor (NGF) exerts protective effects on the cardiovascular system. This study investigated whether NGF gene transfer can prevent diabetic cardiomyopathy in mice. We worked with mice with streptozotocin-induced type 1 diabetes and with nondiabetic control mice. After having established that diabetes reduces cardiac NGF mRNA expression, we tested NGF gene therapies with adeno-associated viral vectors (AAVs) for the capacity to protect the diabetic mouse heart. To this aim, after 2 weeks of diabetes, cardiac expression of human NGF or \u3b2-Gal (control) genes was induced by either intramyocardial injection of AAV serotype 2 (AAV2) or systemic delivery of AAV serotype 9 (AAV9). Nondiabetic mice were given AAV2-\u3b2-Gal or AAV9-\u3b2-Gal. We found that the diabetic mice receiving NGF gene transfer via either AAV2 or AAV9 were spared the progressive deterioration of cardiac function and left ventricular chamber dilatation observed in \u3b2-Gal-injected diabetic mice. Moreover, they were additionally protected from myocardial microvascular rarefaction, hypoperfusion, increased deposition of interstitial fibrosis, and increased apoptosis of endothelial cells and cardiomyocytes, which afflicted the \u3b2-Gal-injected diabetic control mice. Our data suggest therapeutic potential of NGF for the prevention of cardiomyopathy in diabetic subjects