Mengel, M. and Levermann, A. Ice plug prevents irreversible discharge from East Antarctica. Nature Clim. Change 4, 451-455 (2014). doi:10.1038/nclimate2226
Sea-level rise due to the melting of unstable marine ice in Antarctica remains an understudied topic. Recent attention has been given to the Amundsen Sea sector of the West Antarctic ice sheet (see Rignot et al., 2014) where findings suggest irreversible ice sheet retreat regardless of mitigating greenhouse gas emissions. Marine ice sheets are particularly prone to instability when the inland topography of the continent slopes below sea-level. This sub-glacial topography has been observed in the Wilkes Basin of the East Antarctic ice sheet, as well as in better-studied portions of West Antarctica. The Wilkes Basin topography below the overlying ice sheet is composed of a series of below sea level troughs that were cut by streams, probably during a warmer climate interval when ice sheets were much smaller (Fig 1). These troughs were then widened by successive advances of glacial ice. In the event of ice sheet melt due to a warming climate, glacial ice would begin to retreat inland. Consequentially, the position of the grounding line (the location where marine glacial ice attaches to the continent) would also be forced inland and away from the ocean. Since the inland topography is composed of a series of below sea-level troughs, the grounding line would fall below sea-level, a configuration which is inherently unstable. Instability is much like tipping that first precariously placed domino. In the case of ice sheets, melting becomes a runaway process. As the grounding line retreats into the inland troughs, seawater would begin to infiltrate the deeper troughs causing ice sheet floatation and enhanced melting.
Evidence of previous ice sheet instability in the Wilkes Basin has been found during the mid to late Pliocene (4.8 – 3.5 million years ago). During this period of climatic warming, grounding lines receded very rapidly based on evidence found in sediment cores. With CO2 levels and climatic warming during the Pliocene serving as an analogue to modern day warming, the authors of the cited paper chose to explore the stability of the Wilkes Basin using the Parallel Ice Sheet Model (PISM).
PISM is a computer model that can be used to simulate the movement of the grounding line by changing various parameters that influence grounding line motion. For example, stable ice sheets with stationary grounding lines are subjected to measurable parameters such as friction and ocean temperature, which preserve an equilibrium state (where average annual ice mass remains constant). By changing these parameters, scientists can simulate forcing an ice sheet into an unstable state. The researchers conducted a series of modeling experiments, which subjected the bottom of stable ice sheets to warm water pulses 1 – 2.5 °C above equilibrium values. These experiments were simulated through time, with warm water pulses lasting for 200 and 800 years.
The stability of marine ice is largely dependent on the persistence or depletion of a feature called the “ice plug”. The ice plug is a wedge shaped mass of ice that interfaces with the ocean and the continent below sea-level (Fig 2). In the event that the ice plug was lost, the grounding line would retreat into the deep inland troughs, leading to enhanced ice melt.
The results of the experiment suggest that the rate of warm water pulses dictates how the Wilkes Basin responds. For example, weak warming of 1.2 °C over an 800-year pulse allows for the ice sheet to stabilize despite ice loss through melting. Experiments modeling a stronger 1.8 °C over a shorter 400-year pulse depletes the ice plug, forcing the ice sheet into instability, in which the rate of ice loss due to melting is too rapid for the ice sheet to stabilize (Fig 3). The authors note that the calculable heat flux to drive the Wilkes Basin into instability is not an unrealistic estimate, with locations in West Antarctica presently experiencing a similar supply of heat. The specific topographic configuration of an ice plug resting on a ridge affront a large basin makes the Wilkes Basin vulnerable to instability. Due to a similar topographic configuration, the Pine Island Glacier of the West Antarctic Ice Sheet should serve as a case study for the Wilkes Basin, as it became detached in the 1970s and has been in retreat ever since. The authors are clear that although the Wilkes Basin will contribute to sea-level rise at a rate twice that of Antarctica’s present contribution, the duration of forcing occurs on timescales of centuries and the experiments show full retreat over tens of millennia.
This study enhances our understanding of ice sheet dynamics in East Antarctica, a region historically characterized as stable. Although the Wilkes Basin will probably not significantly contribute to sea-level rise in the short term, East Antarctica should be considered a future contributor. With recent research suggesting irreversible glacial retreat in the Amundsen sector of the West Antarctic ice sheet and potentially unstudied basins in East Antarctica, small contributions to sea-level rise from multiple sources will sum to much more significant sea-level rise in centuries to come.
I am a recent graduate (Dec. 2015) from the University of Rhode Island Graduate School of Oceanography, with a M.S. in Oceanography. My research interests include the use of geophysical mapping techniques in continental shelf, nearshore and coastal environments, paleoceanography, sea-level reconstructions and climate change.