Ice Sheet Dynamics
The present-day ice sheets contain the equivalent of about 65 meters of additional sea level, locked in the form of ice. Consequently, even relatively minor changes in the imbalance of the ice sheets have global significance. Two decades of satellite measurements in Greenland and Antarctica reveal snapshots of ongoing ice-sheet change, which imply significant sea-level rise in the next centuries. In order to better understand and constrain the projections of how climate will alter ice sheet evolution and how changes in the ice sheets will influence global climate change in the coming centuries, we need to understand the dynamics of ice sheets and the way ice is discharged from the ice-sheets? interior to their margins — into the surrounding oceans.
Ice sheets flow under their own weight. On timescales larger than a few days, their flow is similar to that of a viscous fluid whose viscosity is non-Newtonian. That means it changes with applied stress; and it also depends on ice temperature. The rate of ice-sheet flow is determined by the balance between the effects of gravity, basal resistance opposing the ice sheets, and internal deformation of ice. Depending on basal resistance, which is determined by the nature of underlying strata (e.g., hard rock, deformable sediments, etc), ice flow can be dominated by either basal sliding (as happens in ice streams) or vertical shear (as happens in parts of ice sheets where ice is frozen to the bed).
Ice streams are regions on ice sheets that move significantly faster than the surrounding ice. They act as ways for ice to discharge from the interior of the ice sheets to the margins, at rates controlled by conditions at the ice-bed interface. The basal resistance of ice streams is highly variable, but the causes of this heterogeneity are not well understood. Mostly, it is attributed to changes in the underlying geology, such as variations in the sediment thickness.
Direct observations at the glaciers? bed over extensive areas to understand the basal mechanics of ice flow is logistically infeasible. An alternative approach is to employ inverse methods and utilize surface observations to infer the spatial distributions of basal resistance of ice streams. GFDL scientists have applied inverse models together with high-resolution data sets of satellite and airborne collected observations to two Antarctic ice streams, Pine Island and Thwaites. Both are currently losing mass at an accelerated rate. This work has revealed that the basal traction underneath these glaciers has organized spatial patterns such that narrow, rib-like structures with very high basal shear stress are embedded in much larger areas with zero basal shear. Such an organized spatial pattern implies that it arises from the complex interactions of glacier flow, subglacial water and deformable sediments.
The basal traction ribs most likely arise from the long-term evolution of dynamic instabilities, manifesting as bed-friction variation arising from changes in the effective pressure in space (the difference between ice overburden and subglacial water pressure). The present spatial configuration of basal resistance in these ice streams is not immutable and can potentially change over decades to centuries in response to changes to ice-sheet geometry or water input. Changes in basal shear stress distributions in the vicinity of the grounding line inevitably cause variations in ice flow and its flux through the grounding line ?(the boundary between floating ice and ice grounded on its bed). This triggers its migration, with consequent changes in ice discharge to the ocean, and the glaciers’ contribution to sea level.
A spectacular collapse of Larsen A and B ice shelves (Antarctic Peninsula) in 1995 and 2002, respectively, introduced a completely new timescale to glaciology. Larsen B Ice Shelf lost an area of 3,600 square kilometers (approximately the size of Rhode Island) that disintegrated into numerous icebergs within three weeks. In contrast, large tabular icebergs originated from the Filchner-Ronne and Ross ice shelves usually calve twice a century.
Both Larsen A and B ice shelves experienced strong surface melting during several austral summers prior to their disintegrations. A number of surface lakes and melt ponds have been detected on satellite images. However, several thousands of these lakes synchronously drained a few days prior to the Larsen B collapse. This synchronous drainage of surface meltwater and specific spatial scale of fractures that resulted in small icebergs, (which consequently capsized) require a unique triggering mechanism that could lead to such events. Considerations of ice-shelf deformation in response to the presence (filling) and the absence (drainage) of supraglacial lakes suggest that extensional stresses induced at the ice-shelf surface in the immediate vicinity of a filled lake and in the center of a drained lake are sufficiently large to initiate fractures and crevasses. Upward propagating fractures from the ice shelf base meet downward propagating fractures from the ice shelf surface, leading to through-cutting rifts.
The spacing of the fractures is determined by the spatial distribution and depths of lakes. Analysis of the lakes? geometries show that the spacing of these fractures caused a large proportion of the Larsen B Ice Shelf fragments to have aspect ratios that were unstable to capsize. Moreover, lake drainage?the event that leads to lake-induced fractures?can be self-catalyzed and self-sustaining and can cooperatively occur across widely varying flow and stress regimes of the Larsen B Ice Shelf.
- Sergienko, Olga V., T T Creyts, and R C A Hindmarsh, June 2014: Similarity
of organized patterns in driving and basal stresses of Antarctic and
Greenland ice sheets beneath extensive areas of basal sliding. Geophysical Research Letters, 41(11), DOI:10.1002/2014GL059976.
- Sergienko, Olga V., April 2014: A vertically integrated treatment of ice stream and ice shelf thermodynamics. Journal of Geophysical Research, 119(4), DOI:10.1002/2013JF002908.
- Sergienko O. V. and R. C. A. Hindmarsh. 2013. Regular patterns in frictional resistance of ice-stream beds seen by surface data inversion, Science, 342(6162), 1086-1089, DOI: 10.1126/science.1243903
- Banwell A. F., D. R. MacAyeal and O. V. Sergienko. 2013. Breakup of the Larsen B Ice Shelf triggered by chain reaction of supraglacial lakes, Geophys. Res. Lett., 40, 5872- 5876, DOI:10.1002/2013GL057694