Increased mass loss from the Greenland Ice Sheet due to rapid changes in tidewater outlet glacier dynamics has the potential to substantially increase sea level within this century, yet the external factors triggering these changes and the internal controls that govern the glaciers' dynamic response are poorly understood. The observational and numerical modeling studies presented herein focus on the tidewater outlet glaciers that drain the Greenland Ice Sheet with the aim of improving the current understanding of regional variability in dynamic change.
Using remotely-sensed observations of glacier surface elevations and speeds, we find that dynamic thinning initiated near the termini of the majority of the glaciers draining the northwestern portion of the Greenland Ice sheet prior to the detection of regional mass loss acceleration by GRACE and GPS in ~2005. The timing and magnitude of thinning varied widely between glaciers such that no clear regional signal could be discerned. The lag in the detection of regional mass loss acceleration is likely due to the time required for dynamic changes to propagate out of the narrow outlets and across the broad ice sheet interior.
Although it is likely that dynamic acceleration of tidewater outlet glacier throughout Greenland are triggered by changes in oceanographic conditions, temporal changes in submarine melt rates beneath the floating tongues of 13 glaciers located throughout Greenland were uncorrelated with dynamic changes observed between 2000 and 2010. Our first-order estimates indicate, however, that submarine melt rates reached values of up to ~3 m/d and accounted for 5-85% of the volume lost from the floating ice tongues during the study period.
In order to determine whether variations in glacier shape can at least partially explain the observed variability in glacier dynamics, we simulate the transient behavior of 9 glaciers with idealized geometries using a width- and depth-integrated numerical ice flow model (i.e., 1D flowline model). The modeling results indicate that for glaciers draining the same interior catchment, wider glaciers and those that overlie deeper basal depressions are more likely to undergo rapid, unstable retreat years after the onset of an applied perturbation. These results not only indicate that shape differences may help explain intra-regional variability in glacier behavior but also that uncertainty in glacier shape can strongly influence predictions of future dynamic change. Confidence in prognostic ice flow modeling is further limited by the influence of parameter uncertainty on predictions of future dynamic change. Model simulations performed using a non-unique combination of ice rheology and basal sliding parameter values can reproduce similar steady-state glacier configurations; however, once perturbed, the response of the simulated glaciers varies widely.
Taken together, the results of our studies indicate that although the observed dynamic changes are likely triggered by changes in external forcing, the dynamic response of each glacier is largely determined by internal controls (i.e., shape, rheology, basal sliding, etc.). Thus, uncertainty in the parameterizations of these internal controlling factors will strongly limit our ability to confidently predict future dynamic change in the absence of improved observational constraints.