The most consequential boundary on Thwaites Glacier is hidden beneath hundreds of meters of Antarctic ice. It is the grounding line, where ice resting on the seabed becomes buoyant and begins to float. As that boundary retreats inland, grounded ice is exposed to ocean influence over a larger area, and the glacier can discharge more ice toward the Amundsen Sea.
“Line” is an imperfect description. Tides lift and lower the ice, moving the transition across a grounding zone several kilometers wide. Recent radar observations have detected seawater traveling even farther upstream beneath grounded ice during high tides. The position mapped on one day is therefore part of a moving interface, while the multiyear migration of the whole zone reveals true retreat.
A Boundary Already Farther Inland
Satellite radar shows that the central grounding line retreated about 14 kilometers between 1992 and 2011, with different amounts along the glacier’s flanks. More detailed observations from 2010 to 2022 found sustained local retreat rates as high as roughly 0.7 kilometer per year where channels beneath the floating shelf meet the grounding zone.
The present rate is not the glacier’s fastest possible behavior. Ridges preserved in sediment on the seabed record a past episode when the grounding zone withdrew at more than 2.1 kilometers per year for about five and a half months. Those closely spaced ridges formed as daily tides lifted and lowered ice against the bed. They show that Thwaites has previously retreated in short, rapid bursts when it became detached from a stabilizing seabed feature.
This evidence is a warning about capacity, not proof that the same rate will begin immediately. The modern glacier encounters a different bed, ice thickness, ocean state, and pattern of pinning points. Retreat can accelerate over an unfavorable slope, pause on a ridge, or vary sharply across the glacier’s approximately 120-kilometer-wide front.
Warm Water Works Through Channels
Thwaites is vulnerable because relatively warm Circumpolar Deep Water reaches the continental shelf and circulates beneath its floating ice. The water remains only a few degrees above the local freezing point, but that is sufficient to melt ice from below. Basal channels focus circulation and create deeply incised pathways in the underside of the shelf.
Where these channels intersect the grounding zone, melt rates can be exceptionally high. Some local estimates approach 250 meters per year, although such values apply to restricted features rather than the entire glacier base. Melting thins the ice until buoyancy allows it to lift from the seabed, shifting the grounding boundary inland.
Subglacial water arriving from beneath the ice sheet may strengthen this channelized pattern. Discharge can change salinity, drive circulation, and deliver heat to particular parts of the grounding zone. The result is a landscape of concentrated melt beside areas where a relatively flat ice base melts more slowly.
Tides Push the Ocean Farther Upstream
High-resolution interferometric radar has revealed a tidally controlled grounding zone about two to six kilometers wide. Thin layers of seawater can intrude an additional several kilometers inland during spring tides, occupying low spots beyond a bedrock ridge. Each tidal cycle can pump warmer saline water into contact with ice that models once treated as securely grounded.
This does not mean that a large open ocean cavity exists permanently beneath the glacier. The intrusions can be only centimeters thick and highly irregular. Their importance lies in repeated heat exchange and water pressure. Both can promote basal melting, alter friction, and broaden the region that behaves as a grounding zone.
Mapping the interface remains difficult. Radar detects tiny changes in surface motion caused by tidal flexing, while autonomous vehicles and instruments lowered through boreholes sample only narrow corridors beneath the shelf. Seismic surveys and airborne radar are needed to map the bed, because an unseen ridge or trough may determine whether retreat slows or continues.
Why Bed Geometry Controls the Next Stage
Much of West Antarctica rests on rock below sea level. In several sectors behind Thwaites, the bed deepens inland. Once retreat begins down such a reverse slope, thicker ice reaches the grounding line. That thicker ice can flow more rapidly and require greater shelf resistance to remain stable, creating the possibility of self-sustaining retreat.
Real terrain is more complicated than a smooth slope. Ridges can provide temporary anchoring, and rough bed material can increase resistance. The eastern floating shelf has also been partly stabilized by contact with a seabed high, while fractures have weakened its ability to restrain the glacier. Forecasts depend on when those controls are lost and what features lie behind them.
Thwaites alone contains enough grounded ice to raise global sea level by roughly 65 centimeters, although its loss would unfold over a long period rather than as a sudden wave. Its wider importance is that retreat could alter neighboring parts of the West Antarctic Ice Sheet. The grounding line is where that long-term change begins: a mobile, tidally active boundary being reshaped from below by the ocean and from above by the weight and motion of the ice.