Byron R. Parizek

Effect of Sedimentation on Ice-Sheet Grounding-Line Stability

Sedimentation filling space beneath ice shelves helps to stabilize ice sheets against grounding-line retreat in response to a rise in relative sea level of at least several meters. Recent Antarctic changes thus cannot be attributed to sea-level rise, strengthening earlier interpretations that warming has driven ice-sheet mass loss. Large sea-level rise, such as the ≈100-meter rise at the end of the last ice age, may overwhelm the stabilizing feedback from sedimentation, but smaller sea-level changes are unlikely to have synchronized the behavior of ice sheets in the past.

Access of surface meltwater to beds of sub-freezing glaciers: preliminary insights

Abstract Sufficiently deep water-filled fractures can penetrate even cold ice-sheet ice, but glaciogenic stresses are typically smaller than needed to propagate water-filled fractures that are less than a few tens of meters deep, as shown by our simplified analytical treatment based on analogous models of magmatic processes. However, water-filled fractures are inferred to reach the bed of Greenland through >1 km of ice and then collapse to form moulins, which are observed. Supraglacial lakes appear especially important among possible crack ‘nucleation’ mechanisms, because lakes can warm ice, supply water, and increase the pressure driving water flow and ice cracking.

A numerical investigation of ice-lobe-permafrost interaction around the southern Laurentide ice sheet

Permafrost existed around and under marginal parts of the southern Laurentide ice sheet during the Last Glacial Maximum. The presence of permafrost was important in determining the extent, form and dynamics of ice lobes and the landforms they produced because of influences on resistance to basal motion and subglacial hydrology. We develop a two-dimensional time-dependent model of permafrost and glacier-ice dynamics along a flowline to examine: (i) the extent to which permafrost survives under an advancing ice lobe and how it influences landform development and hydrology, and (ii) the influence of permafrost on ice motion and surface profile. The model is applied to the Green Bay lobe, which terminated near Madison, Wisconsin, during the Last Glacial Maximum. Simulations of ice advance over permafrost indicate that the bed upstream of the ice-sheet margin was frozen for 60-200 km at the glacial maximum. Permafrost remained for centuries to a few thousand years under advancing ice, and penetrated sufficiently deep (tens of meters) into the underlying aquifer that drainage of basal meltwater became inefficient, likely resulting in water storage beneath the glacier. Our results highlight the influence of permafrost on subglacial conditions, even though uncertainties in boundary conditions such as climate exist.

A simple law for ice-shelf calving.

A major problem for ice-sheet models is that no physically based law for the calving process has been established. Comparison across a diverse set of ice shelves demonstrates that iceberg calving increases with the along-flow spreading rate of a shelf. This relation suggests that frictional buttressing loss, which increases spreading, also leads to shelf retreat, a process known to accelerate ice-sheet flow and contribute to sea-level rise.

Insights into spatial sensitivities of ice mass response to environmental change from the SeaRISE ice sheet modeling project I: Antarctica

Sophie Nowicki, Robert A. Bindschadler, Ayako Abe-Ouchi, Andy Aschwanden, Ed Bueler, Hyeungu Choi, Jim Fastook, Glen Granzow, Ralf Greve, Gail Gutowski, Ute Herzfeld, Charles Jackson, Jesse Johnson, Constantine Khroulev, Eric Larour, Anders Levermann, William H. Lipscomb, Maria A. Martin, Mathieu Morlighem, Byron R. Parizek, David Pollard, Stephen F. Price, Diandong Ren, Eric Rignot, Fuyuki Saito, Tatsuru Sato, Hakime Seddik, Helene Seroussi, Kunio Takahashi, Ryan Walker, and Wei Li Wang

Subglacial thermal balance permits ongoing grounding line retreat along the Siple Coast of West Antarctica

Abstract Changes in the discharge of West Antarctic ice streams are of potential concern with respect to global sea level. The six relatively thin, fast-flowing Ross ice streams are of interest as low-slope end-members among Antarctic ice streams. Extensive research has demonstrated that these “rivers of ice” have a history of relatively high-frequency , asynchronous discharge variations with evolving lateral boundaries. Amidst this variability, a ∼1300 km grounding-line retreat has occurred since the Last Glacial Maximum. Numerical studies of Ice Stream D (Parizek and others, 2002) indicate that a proposed thermal-regulation mechanism (Clarke and Marshall, 1998; Hulbe and MacAyeal, 1999; Tulaczyk and others, 2000a, b), which could buffer the West Antarctic ice sheet against complete collapse, may be over-ridden by latent-heat transport within melt-water from beneath inland ice. Extending these studies to Ice Stream A, Whillans Ice Stream and Ice Stream C suggests that further grounding-line retreat contributing to sea-level rise is possible thermodynamically However, the efficiency of basal water distribution may be a constraint on the system. Because local thermal deficits promote basal freeze-on (especially on topographic highs), observed short-term variability is likely to persist.

Influence of the Great Lakes on the dynamics of the southern Laurentide ice sheet: Numerical experiments

A time-dependent, flow-line ice-sheet model is used to explore interactions between last glacial climate, the Laurentide ice sheet, and the Great Lakes. We seek to understand perturbations in the ice-flow field that caused three neighboring lobes to leave different geomorphic and sedimentary records. Driven by reconstructed air- temperature variations from 65 to 18 ka, the simulated lobe dynamics are consistent with constraints from loess and till chronologies, till stratigraphy, and ice-surface profiles. Contrasts in lobe dynamics are best explained by the geometry of lakes along each flow line. As ice entered these lakes, calving influenced glacier mass balance. In particular, deep water in Lake Superior likely delayed ice advance into northern Wisconsin by promoting large calving losses, whereas lobes to the east that encountered shallower water were less affected by calving. The Driftless Area of Wisconsin may thus owe its existence, at least in part, to the presence of Lake Superior. Our results suggest that morainal-bank evolution should be treated in ice-sheet models in lacustrine and shallow-marine settings. Determining the sediment flux to the morainal bank remains a difficult task. For example, ice advance across central Lake Superior was probably sediment-flux limited. Otherwise, the Driftless Area would have been glaciated.

Toward a new generation of ice sheet models

Large ice sheets, such as those presently covering Greenland and Antarctica, are important in driving changes of global climate and sea level. Yet numerical models developed to predict climate change and ice sheet-driven sea level fluctuations have substantial limitations: Poorly represented physical processes in the ice sheet component likely lead to an underestimation of sea level rise forced by a warming climate. The resultant uncertainty in sea level projections, and the implications for climate policy, have been widely discussed since the publication of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) [IPCC, 2007]. The assessment report notes that current models do not include “the full effects of changes in ice sheet flow, because a basis in published literature is lacking.” The report also notes that the understanding of rapid dynamical changes in ice flow “is too limited to assess their likelihood or provide a best estimate or an upper bound for sea level rise.”

Sensitivity of Pine Island Glacier to observed ocean forcing

We present subannual observations (2009–2014) of a major West Antarctic glacier (Pine Island Glacier) and the neighboring ocean. Ongoing glacier retreat and accelerated ice flow were likely triggered a few decades ago by increased ocean-induced thinning, which may have initiated marine ice-sheet instability. Following a subsequent 60% drop in ocean heat content from early 2012 to late 2013, ice flow slowed, but by < 4%, with flow recovering as the ocean warmed to prior temperatures. During this cold-ocean period, the evolving glacier-bed/ice-shelf system was also in a geometry favorable to stabilization. However, despite a minor, temporary decrease in ice discharge, the basin-wide thinning signal did not change. Thus, as predicted by theory, once marine ice-sheet instability is underway, a single transient high-amplitude ocean cooling has only a relatively minor effect on ice flow. The long-term effects of ocean-temperature variability on ice flow, however, are not yet known.

Efficient flowline simulations of ice-shelf/ocean interactions: Sensitivity studies with a fully coupled model

AbstractThermodynamic flowline and plume models for the ice shelf–ocean system simplify the ice and ocean dynamics sufficiently to allow extensive exploration of parameters affecting ice-sheet stability while including key physical processes. Comparison between geophysically and laboratory-based treatments of ice–ocean interface thermodynamics shows reasonable agreement between calculated melt rates, except where steep basal slopes and relatively high ocean temperatures are present. Results are especially sensitive to the poorly known drag coefficient, highlighting the need for additional field experiments to constrain its value. These experiments also suggest that if the ice–ocean interface near the grounding line is steeper than some threshold, further steepening of the slope may drive higher entrainment that limits buoyancy, slowing the plume and reducing melting; if confirmed, this will provide a stabilizing feedback on ice sheets under some circumstances.