The grounding line is the boundary between where a glacier sits on land and where it starts to float on water.

What is a grounding zone?

Grounding lines resemble a grounding zone as they cover a large area. The grounding zone delineates the gradual transition from the grounded ice sheet to the freely floating ice shelf typically spanning several kilometres.

Importantly, grounding lines are not static. The grounding lines move depending on climate variables and environmental processes. It is also the best area for monitoring the balance between outgoing ice fluxes and snow accumulation.

Why are grounding lines important?

Ice sheet-ocean interactions control and impact ice sheet dynamics and rates of melting and recession. 

Understanding and locating grounding lines allows scientists to use the information to inform ice sheet mass budget calculations, numerical modelling of ice sheet dynamics, ice-ocean interactions, ocean tides and subglacial environments. With this information, scientists can predict past, present and future ice shelf behaviour.

The transient features of grounding lines make them hard to map and EO plays a fundamental role in detecting subtle indicators of the grounding zone.

Mapping the grounding line

Scientists use SAR (Synthetic Aperture Radar) satellites to detect the location of grounding lines. They do this by measuring changes in surface elevation during the tidal cycle.

DinSAR directly measures the vertical motion of floating ice shelves in response to tides and atmospheric air pressure, with millimetre precision and oceanic processes.

SAR signals contain information about the strength and phase of a tide. Interferometer SAR (InSAR) takes two SAR observations taken in the same area from slightly different positions and extracts information about the Earth’s terrain.

Current grounding line retreat

Using these satellites measurements, scientists monitor ice sheet instability and grounding line retreat. Warm ocean currents reach the undersides of ice shelves and melt the grounded ice. This process leads to grounding line recession.

For example, scientists have been monitoring the rapid recession of Pine Island Glacier. A vast 31 km was lost between 1992 and 2011.

Scientists recognise previous locations of grounding lines by recognising distinct formations on the sea bed.

Net retreat of Antarctic glacier grounding lines

In West Antarctica, East Antarctica and at the Antarctica Peninsula, scientists Andy Shepherd and Anna Hogg have observed grounding lines retreating at rates faster than 25m per year. The team used ESA's CryoSat 2 to map grounding lines across 16,000 km of the coastline.

For more information about grounding lines, ice velocity and SAR , watch this lecture by Anna Hogg which forms part of the Imperative MOOC course: The Frozen Frontier: Monitoring The Greenland Ice Sheet From Space.