By In Antarctica, the warm ocean is stealthily attacking a major glacier through a previously unknown route — undermining its foundation on a daily basis.
As each rising tide lifts the coastal terminus of the southern continent’s Thwaites Glacier a tiny bit off the seafloor, warm salty water squeezes in underneath, satellite measurements reveal. This inrush of seawater forces its way many kilometers inland as it melts the ice from beneath. The melt water and seawater are then flushed back out as the tide falls, researchers report May 20 in the Proceedings of the National Academy of Sciences.
”That is going to greatly accelerate the retreat” of the ice in some places, says Theodore Scambos, a glaciologist at the University of Colorado Boulder who was not part of the study.
The West Antarctic Ice Sheet, where Thwaites Glacier resides, is a fortress besieged by enemies. This dome of ice the size of Alaska sits in a bowl-shaped ocean basin. The edges of the ice sheet, where it rises off the seafloor, are constantly assaulted by warm, dense, salty ocean currents that pour across the seafloor like invading armies.
The thermal assault is especially ferocious along the section of coastline where Thwaites, a rapidly moving corridor of ice 120 kilometers across, empties into the ocean (SN: 2/15/23). Thwaites is currently hemorrhaging 75 billion tons of ice per year — accounting for roughly half the ice lost from all of Antarctica.
A glacier’s underbelly
Thwaites Glacier and most of the West Antarctic Ice Sheet sit on a gravelly bed that is hundreds of meters below sea level — making the ice vulnerable to warm, salty ocean currents that hug the seafloor. Thwaites is especially vulnerable because parts of its grounding zone (where it lifts off its bed and floats on the ocean) sit as far as 1 kilometer below sea level, exposing it to the warmest water.
Anatomy of a glacier
Much of this loss stems from the melting and retreat of the glacier’s grounding line, where its outer wall rests on the seafloor. As Thwaites’ grounding line recedes, it reduces friction at the glacier’s bed. This allows the glacier to slide and dump its ice into the ocean more quickly, contributing to sea level rise, says Eric Rignot, a glaciologist at University of California, Irvine.
For two decades, Rignot had used sporadic satellite radar measurements to monitor Thwaites’ grounding line, which is currently retreating roughly half a kilometer per year. But in 2023, he and his colleagues used a new set of satellites, called ICEYE, to capture these measurements three times a day.
Those more frequent measurements revealed “some big surprises,” Rignot says. “We see a dynamic that we’ve never seen before.”
At each high tide, a thin layer of seawater 10 to 70 centimeters thick pushed under the edge of the ice sheet and migrated six to 12 kilometers inland. The researchers could detect this because the satellite radar showed the top surface of the ice rising and falling as the water moved beneath it. Across the entire glacier, that amounts to 200 million cubic meters of seawater rushing in and out each day — about 400 times the volume of the world’s largest type of oil tanker.
That seawater is just 3.6 degrees Celsius above the ice’s melting point — but it packs a surprising punch. “The glacier ice hates the salt” because it accelerates melting, Rignot says.
His team estimates that during the intrusions, this water injects 150 million kilowatts of thermal power into the ice, similar to the heat output of 10 million kitchen ovens. They estimate that this could melt 20 meters off the bottom of the ice each year — roughly the height of a five-story building.
Rignot noticed that the seawater intruded in blobby, irregular patterns, rushing far inland in some places but not others. To explain this, he turned to Christine Dow, a glaciologist at the University of Waterloo in Canada who has mapped several subglacial freshwater rivers that flow out from beneath Thwaites Glacier and pour into the ocean.
Dow found that the seawater preferentially migrates under the ice in the broad areas between the rivers, where it can flow across level or downward sloping terrain, and the ice is most easily lifted due to differences in pressure.
This kind of saltwater invasion could double the overall rate of ice loss in some glaciers, recent simulations suggest. The new results might explain why the grounding lines of two nearby glaciers, Pope and Smith, retreated two to four kilometers in a single year.
This tidal intrusion “is a phenomenon we just haven’t known to look for,” Dow says. “I suspect it’s very widespread.” Scambos notes that it will be important to include this new phenomenon in computer simulations that predict future ice loss and sea level rise. “I’m really interested to see how this will affect the retreat rates.”
