Grounding lines are the boundaries where glaciers and ice sheets transition from resting on solid ground to floating on seawater. Grounding lines influence the flow of ice into the ocean and are central to understanding ice sheet stability and sea-level rise.
What are grounding lines?
Grounding lines form where ice, bedrock, and ocean meet. Researchers use what is known as the flotation criterion: whether the ice is thick enough to be grounded or too thin to float. Ice thickness and density, seawater density, and bedrock elevation are all features that determine the location of grounding lines.
The topography of the bedrock underlying the glacier is very important to grounding line stability. Glaciers with retrograde beds-that is, bedrock sloping downward as it extends inland-are particularly sensitive to retreat. As grounding lines retreat inland, more ice is exposed to warm ocean currents that encourage melting and further retreat. This can lead to feedback loops which accelerate the flow of ice into the ocean and further destabilize ice sheets.
Why grounding lines are critical for ice sheet stability
The grounding line serves as the starting control point for the flow of ice from glaciers into the ocean. Most of the ice loss occurs around these boundaries due to warm ocean water that melts the ice from underneath.
Warm ocean water reduces the stability of the glacier and causes the retreat of the grounding line, starting a feedback that accelerates the loss of ice: The ice shelves lose mass and can no longer hold back the glaciers behind them. Without that support, the glaciers accelerate toward the ocean and thin as they move, causing further retreat of the grounding line, which promotes more acceleration and thinning.
Retreating grounding lines
West Antarctic Ice Sheet
The Amundsen Sea sector of the West Antarctic Ice Sheet is one region where grounding lines have retreated dramatically. Researchers have analyzed radar data collected in the Amundsen Sea region from European Earth Remote Sensing (ERS-1 and -2) satellites over decades to map the retreat of grounding lines in this area. Analyzing data for Pine Island between 1992 to 2011 revealed the retreat of the grounding line of that glacier by 19 miles (31 kilometers). Thwaites glaciers retreated by 9 miles (14 kilometers) during the same time period.
Denman Glacier, East Antarctica
According to a study published in Geophysical Research Letters, researchers at NASA’s Jet Propulsion Laboratory and the University of California, Irvine (UCI) found that between 1996 and 2018, the grounding line on the western flank of Denman Glacier retreated about 5.4 kilometers (3.4 miles). Denman Glacier’s retrograde bed on the western flank makes it particular vulnerable to accelerated retreat due to warmer ocean waters being bale to penetrate further inland. Researchers calculate that a collapse of this glacier would contribute to a 1.5 meter rise in global sea levels.
Monitoring and research of grounding lines
The study of the grounding lines has improved with advances in earth observation technology. Satellite radar interferometry-through ESA’s Sentinel-1 and earlier European Remote Sensing satellites-allows scientists to create maps showing changes in the position of grounding lines in unprecedented detail. NASA’s Operation IceBridge has further refined knowledge of the thickness of the ice and basal melting through airborne radar campaigns.
Other sources of insight into the processes operating beneath the ice shelves include robotic submersibles and satellite altimetry. These, for example, have revealed high-resolution, detailed evidence of the extremely critical rapid thinning in glaciers like Smith and hundreds of meters of loss in ice thickness at places.
References
Brancato, V., Rignot, E., Milillo, P., Morlighem, M., Mouginot, J., An, L., … & Prats‐Iraola, P. (2020). Grounding line retreat of Denman Glacier, East Antarctica, measured with COSMO‐SkyMed radar interferometry data. Geophysical Research Letters, 47(7), e2019GL086291.
Lynch, P. (2014, May 12). The ‘unstable’ west Antarctic ice sheet: A primer. NASA Jet Propulsion Laboratory (JPL).
