Thwaites Glacier, one of the largest and fastest-changing glaciers in Antarctica, has long concerned scientists due to its potential to dramatically raise global sea levels if it continues retreating rapidly. New satellite radar data reveals that processes melting the glacier from below may be even more extensive than previously thought, with extensive seawater intrusions penetrating deep inland underneath the glacier’s grounded ice. These findings have major implications both for our understanding of Antarctica’s glaciers and for future projections of sea level rise.

Thwaites Glacier drains a massive basin in West Antarctica equivalent to a 65 cm rise in global sea levels if the entire glacier was lost to the ocean. Over recent decades, it has been thinning and retreating at an increasing rate, shedding around 50 billion tons of ice per year. Its stability is critical, as once Thwaites is gone, adjacent glaciers would also likely accelerate their melting, triggering multi-meter sea level rise over centuries. However, exactly what is driving Thwaites’ rapid changes has remained uncertain.

Traditionally, scientists have modeled glaciers like Thwaites as having a fixed boundary called the “grounding line” between where the ice sits firmly on bedrock and where it lifts up to float on the ocean. In reality, new data shows this boundary is better described as a kilometers-wide “grounding zone” that migrates up and down with ocean tides. Within this zone, warming ocean waters can more readily access and eat away at the glacier’s vulnerable underside.

Going a step further, a University of California research team led by glaciologist Eric Rignot has now used a unique new set of daily satellite radar images to detect irregular seawater intrusions extending even beyond the grounding zone underneath Thwaites’ grounded ice. Working with data from the ICEYE constellation of small satellites, which provided unprecedented daily coverage in early 2023, the team mapped out in detail how Thwaites’ grounding zone boundary moves each tidal cycle.

To their surprise, they found evidence that during high tides, seawater can rush inland as far as 12 kilometers underneath the glacier, penetrating deep into areas that models assume should be protected on land. By tracking subtle changes in the elevation of Thwaites’ surface, they detected certain locations bulging upward in sync with rising tides – a telltale sign of seawater being squeezed up from below like a bladder filling with fluid. These intrusions took the form of circular “bull’s eyes” up to 10 centimeters thick centered over known subsurface depressions.

The team attributes the inland bulging to seawater flowing at speeds over 50 centimeters per second along the glacier’s bed. This rapid subglacial transport system allows warm ocean water extensive access to melt vulnerable grounded ice. Even brief high tide intrusions could have outsized impacts, as the heat required to melt just 3 centimeters of overlying ice would be enough to warm an entire meter-deep column of incoming seawater by 3°C – resetting its melting potential for the next tide cycle.

The findings shed new light on how ocean heat may be melting Thwaites from within, even far beyond where models assume ice is solidly pinned to bedrock. Century-old concepts of glaciers having a fixed transition to floating ice may need revising, with implications for ice sheet predictability. The researchers note that including mechanisms like widespread subglacial flooding could allow models to better reproduce Antarctica’s recent rapid changes.

To gauge the potential melting from the detected intrusions, the team ran calculations based on the tide-driven changes in seawater volume within Thwaites’ widest measured 6 km grounding zone. Even taking conservative assumptions, they estimate melting rates up to 65 meters per year – enough to hollow out the glacier from within on timescales that concern climate projections.

The exact timing and pathways of seawater transport are influenced by Antarctica’s complex subglacial hydrologic system, as modeled by the University of Waterloo’s Céline Dow using available bed topographic data. Her Glacier Drainage System (GlaDS) model predicted a high-pressure distributed drainage network, with intense subglacial water flowing between lower-pressure channels aligned with troughs in the glacier bed.

Strikingly, the satellites detected the bull’s eyes of maximum seawater intrusion extent clustered right between two such major channels – explaining how seawater could propagate far inland along high-pressure zones. The GlaDS outputs align seawater access directly with regions most vulnerable to hydraulic jacking from below as the tide rises and falls. Farther inland, persistent bull’s eyes were attributed to subglacial water cycling within the subglacial system, rather than direct ocean intrusion.

Thwaites currently sits at a transient tipping point – retreating against a rising bed slope that temporarily slows its collapse, yet melting from extensive ocean access continues unabated. Once the glacier slides backward past an upcoming ridge called “Mouginot Ridge” within the coming decade, steeply declining bed topography will afford no brakes to an accelerating runaway retreat. Its fate and that of neighboring glaciers will determine if West Antarctica undergoes a marine ice sheet instability sucking several meters of sea level rise out of the ocean over centuries.

The new evidence of oceans surreptitiously infiltrating far inland suggests Thwaites and Antarctica’s marine glaciers may already be losing more ice than models project. As the researchers conclude, including the physics of widespread basal flooding in ice sheet simulations could be key to unlocking why past changes outpaced model expectations. Updating our view of how ocean heat interacts with Antarctica’s vulnerable underbelly may in turn allow improving projections of its still uncertain but potentially enormous future contribution to sea level as global warming continues relentlessly on.

Reference(s)

1. Rignot, Eric (2024). Widespread seawater intrusions beneath the grounded ice of Thwaites Glacier, West Antarctica [Dataset]. Dryad. https://doi.org/10.5061/dryad.3ffbg79rm

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