Retreat of Pine Island Glacier controlled by marine ice-sheet instability

Abstract

Currently Pine Island Glacier (PIG) is responsible for 20% of the total ice loss offdischarge from the West Antarctic Ice Sheet (WAIS) ([22]; [30]). The accelerated thinning observed since the 1980s has essentially been attributed to enhanced sub-ice shelf melting [21] induced by the recent alteration of Circumpolar Deep Water circulation [10]. This has reduced the buttressing exerted by the ice shelf, leading to the acceleration of the ice stream and the ongoing retreat of the grounding line (GL) along the glacier's trunk observed since 1992 [17]. Today the GL lies over bedrock that has a steep retrograde slope [29] (Figure 1c) raising the possibility that PIG may already be engaged in an irrevocable retreat. ProvidedAssuming that ice flow is dominated * durand@lgge.obs.ujf-grenoble.fr 2 by basal sliding and lateral variation can be ignored, grounding lines located on retrograde slopes are always unstable [24,3], but in realistic, three-dimensional geometries lateral drag and buttressing in the ice shelf can act to prevent unstable retreat [9]. Assessing the stability of PIG therefore requires numerical models that accurately represent these additional forces.Models designed to study the evolution of PIG have been reported, though limited to flowline geometries [7] or extreme forcings [11]. Overall, the short-term behaviour of PIG is not well understood, and projections vary wildly, ranging from modest retreat to almost full collapse of the main trunk within a century [11,7].Here, we evaluate the potential instability of PIG and its short-term contribution to sea-level rise (SLR) using state-of-the-art ice flow models. To decide whether PIG is subject to Marine Ice Sheet Instability (MISI) at present, we must answer two questions: (i) to what extent is the dynamic response of PIG to changes in its ice shelf dictated by the bedrock topography rather than the type and amplitude of the perturbation, and, (ii) can the GL be stabilized on the retrograde slope? Confidence in the answers we propose is of course affected by the accuracy of both the physics implemented in the models that we use and our estimates of poorly constrained parameters. We addressed these questions using three different ice-flow models: the full Stokes For all three models, the geometry is relaxed over 15 years to remove unphysical surface undulations induced by remaining uncertainties in the model initial conditions [6]. Surface accumu-3 lation is given by the regional atmospheric model RACMO (1980RACMO ( -2004) and sub-ice shelf melting is imposed as a piecewise linear function of the lower surface elevationwater depth with a maximum melting rate of 100 m a −1 below -800 m depth, linearly decreasing to no melt above -400 m. This melt-rate parametrisation, which we will refer to as m0 control, is in reason- The recent retreat of PIG is now firmly attributed to acceleration of the glacier in response to sub-ice shelf melting. To evaluate the consequences of melting on PIG dynamics, fivefour different melt-rate perturbations are tested. These ar...

Recent Progress in Understanding and Projecting Regional and Global Mean Sea Level Change

“…Upstream of the 2011 grounding line positions, there are no major bed obstacles that would prevent irreversible retreat in this sector of the West Antarctic ice sheet [78]. Ice sheet model experiments suggest that current retreat of Pine Island Glacier (PIG) [79] and Thwaites Glacier (TG) [80] may be irreversible.…”

Glacial retreat in the Amundsen Sea sector, West Antarctica – first cosmogenic evidence from central Pine Island Bay and the Kohler Range

“…This unique configuration makes the WAIS a marine-based ice-sheet, and thereby especially vulnerable to changes in ocean temperature and prone to collapsing (e.g. Mercer, 1978;Joughin and Alley, 2011;Favier et al, 2014). In addition, the influence on the ice-sheet dynamics of underlying tectonic structures, seen in bathymetry images (e.g.…”

“…Pritchard et al, 2009;Joughin et al, 2010). Not only have the past two decades been marked by accelerated thinning, recent modeling studies suggest that Pine Island Glacier in particular may already be in an unstable condition (Favier et al, 2014). Those studies indicate that the average mass loss could multiply five times over the next 20 years, reaching approximately 100 Gt/yr.…”

Small-scale topographically-controlled glacier flow switching in an expanding proglacial lake at Breiðamerkurjökull, SE Iceland

“…Weertman, 1974;Thomas, 1979;Conway and others, 2002;Pfeffer, 2007;Favier and others, 2014;Aschwanden and others, 2016). Flow switching, where the trajectory of ice flow significantly alters in time, has been inferred in modern ice sheets (e.g.…”