Understanding and Modelling Rapid Dynamic Changes of Tidewater Outlet Glaciers: Issues and Implications

Abstract

Recent dramatic acceleration, thinning and retreat of tidewater outlet glaciers in Greenland raises concern regarding their contribution to future sea-level rise. These dynamic changes seem to be parallel to oceanic and climatic warming but the linking mechanisms and forcings are poorly understood and, furthermore, large-scale ice sheet models are currently unable to realistically simulate such changes which provides a major limitation in our ability to predict dynamic mass losses. In this paper we apply a specifically designed numerical flowband model to Jakobshavn Isbrae (JIB), a major marine outlet glacier of the Greenland ice sheet, and we explore and discuss the basic concepts and emerging issues in our understanding and modelling ability of the dynamics of tidewater outlet glaciers. The modelling demonstrates that enhanced ocean melt is able to trigger the observed dynamic changes of JIB but it heavily relies on the feedback between calving and terminus retreat and therefore the loss of buttressing. Through the same feedback, other forcings such as reduced winter sea-ice duration can produce similar rapid retreat. This highlights the need for a robust representation of the calving process and for improvements in the understanding and implementation of forcings at the marine boundary in predictive ice sheet models. Furthermore, the modelling uncovers high sensitivity and rapid adjustment of marine outlet glaciers to perturbations at their marine boundary implying that care should be taken in interpreting or extrapolating such rapid dynamic changes as recently observed in Greenland.

The sensitivity of the Greenland Ice Sheet to glacial–interglacial oceanic forcing

“…Various numerical models have been used to simulate current submarine melt rates (Jenkins, 2011;Rignot et al, 2016;Sciascia et al, 2013;Xu et al, 2012Xu et al, , 2013 and dynamic retreat Vieli and Nick, 2011) of the GrIS marine-terminating glaciers, as well as ice-dynamic future projections of the whole GrIS (Fürst et al, 2015;Nowicki et al, 2013), due to changes in the oceanic temperatures. However, how this thermal forcing affected the past GrIS configuration has not been explored from a modelling perspective so far.…”

The response of Petermann Glacier, Greenland, to large calving events, and its future stability in the context of atmospheric and oceanic warming

“…An increasing number of studies have shown that the thinning and acceleration take place in all sections along the flowline, although the emerging consensus view is that they are mainly initiated at the glacier front (Howat and others, 2007;Holland and others, 2008;Nick and others, 2009;Murray and others, 2010). Various forcing mechanisms have been proposed, but large uncertainties remain in their relative importance (Vieli and Nick, 2011).…”

Dynamics of Greenland�s glacial fjords

“…The other half of the mass loss comes from changes in ice discharge -glaciers that drain the ice sheet have accelerated and retreated -but this dynamic component of mass loss is more complicated and poorly understood. The synchronous acceleration of many outlet glaciers originated at their marine termini (Nick et al, 2009;Vieli and Nick, 2011;Howat et al, 2007) and coincided with warming ocean temperatures around Greenland (Holland et al, 2008a; the only time over the past century when glaciers in southeast and west Greenland retreated as much as in the present day was in the 1930s, consistent with the North Atlantic warming. These include the reconstruction of frontal positions of glaciers in southeast and North Atlantic during a negative N has been used to explain the synch ocean heat content anomalies of th the 1950s to 2000s 67,68,32 (Fig.…”

“…Ocean warming has been implicated as a driver of glacier acceleration and retreat, with enhanced submarine melting as the presumed connection (Holland et al, 2008a;Murray et al, 2010;Vieli and Nick, 2011;Straneo and Heimbach, 2013). In Greenland, however, we have no direct measurements of this melting and only limited understanding of the circulation in fjords where glaciers meet the ocean.…”

“…Enhanced submarine melting of outlet glaciers has been identified as a plausible trigger for part of the Greenland Ice Sheet's accelerated mass loss (Thomas, 2004;Holland et al, 2008a;Vieli and Nick, 2011), which currently accounts for a quarter of global sea level rise (Shepherd et al, 2012).…”