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.

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

“…processes of calving, grounding line retreat and submarine melting), it has a robust treatment of grounding line migration 28 and calving 29 , and reproduces the current observed dynamical behaviour of several narrow marine outlet glaciers well 13,14,21 .…”

“…Petermann Glacier in north Greenland has been flowing steadily 11 , terminating in a relatively long (~50 km) and wide (~20 km) floating ice tongue, under which high rates of submarine melt occur 12 . The break off of two substantial icebergs in 2010 (~270 km 2 ) and July 2012 (~120 km 2 ) raised concerns about this glacier's stability, but did not cause major flow acceleration 13 Various mechanisms related to atmospheric and ocean forcing have been proposed to explain the recent behaviour of the major outlet glaciers, but large uncertainties in their relative importance remain 14,15 . A warmer ocean can melt submarine ice and thereby cause the grounding line to retreat, especially when subglacial meltwater produces more vigorous buoyancy-driven circulation 16 .…”

The sensitivity of flowline models of tidewater glaciers to parameter uncertainty

“…Width-and depth-integrated (1-D) numerical ice flow models (i.e., flowline models) have been used to simulate the dynamic behavior of narrow, fast-flowing tidewater glaciers in a number of geographic settings, including Greenland (e.g., Nick et al, 2009Nick et al, , 2012Nick et al, , 2013Vieli and Nick, 2011), Antarctica (e.g., Gladstone et al, 2012;Jamieson et al, 2012), Iceland (Nick et al, 2007a), and Alaska (Nick et al, 2007b;Colgan et al, 2012). These models simulate ice flow by balancing the gravitational driving stress with basal and lateral resistive stresses and along-flow longitudinal stress gradients.…”

“…These models simulate ice flow by balancing the gravitational driving stress with basal and lateral resistive stresses and along-flow longitudinal stress gradients. The parameterization of basal and lateral resistive stresses vary for each model: basal resistance is commonly described by a Weertman-type sliding law that assumes an effective-pressure dependency (e.g., Nick et al, 2009;Vieli and Nick, 2011;Jamieson et al, 2012) and lateral resistance is parameterized by integrating the horizontal shear stress over the channel width (van der Veen and Whillans, 1996) or by multiplying the driving stress by a shape factor that accounts for differences in the cross-sectional area of the glacier along flow (Cuffey and Paterson, 2010, pp. 342).…”

Dynamic changes in outlet glaciers in northern Greenland from 1948 to 2015

“…Alongside thinning and acceleration, terminus retreat has been widespread since the 1990s across the ice sheet (e.g. Box and Decker, 2011;Carr et al, 2017b;Jensen et al, 2016;Moon 5 and Joughin, 2008) and several studies have identified terminus retreat as a key control on inland ice flow acceleration and dynamic surface thinning (Howat et al, 2005;Joughin et al, 2004Joughin et al, , 2010Nick et al, 2009;Thomas, 2004;Vieli and Nick, 2011).…”