At Thwaites Glacier in West Antarctica, seawater reaches beneath the glacier, well past the point where the ice starts to float. This seawater can cause the glacier to melt from below. We use a combination of radar, tiltmeters, GPS, borehole images, and sediment samples to show that seawater likely extends over a kilometer inland of the grounding line. In one borehole, we see a pocket of seawater under grounded ice, and sediments at the site are salty. To understand how this water gets there, we used a model that simulates how the ice flexes in response to tides. The model shows that although the grounding line itself doesn’t move very much with the tides, the ice just inland is only lightly grounded and is connected to the ocean. This suggests that ocean water can move inland without needing the grounding line to shift.

To improve future sea-level projections, we argue that models need to better capture this kind of ocean access beneath grounded ice. As one step toward that goal, we’re developing a physical flume experiment to study grounding line behavior in the lab.