Ice world: Antarctica’s riskiest glacier is attacked from below and loses its grip

When you fly over Antarctica, it’s hard to see what it’s all about. Like a giant wedding cake, the icing of snow on the world’s largest ice sheet looks smooth and immaculate, beautiful and perfectly white. Small swirls of snow dunes cover the surface.

But as you approach the edge of the ice sheet, there is a sense of tremendous underlying power. Cracks appear in the surface, sometimes organized like a washboard, and sometimes a complete chaos of spires and ridges, revealing the pale blue crystalline heart of the ice below.

As the plane flies lower, the size of these breaks steadily grows. These aren’t just any cracks, but ravines big enough to swallow a jet plane, or spires the size of monuments. Cliffs and tears, rips in the white blanket appear, indicating a force that can fling city blocks of ice like so many wrecked cars in an agglomeration. It’s a twisted, torn, twisted landscape. There is also a sense of movement, in a way that no ice-free part of the Earth can convey – the whole landscape is in motion and doesn’t seem very happy about it.

Broken ice where the Thwaites Glacier enters the sea.
Ted Scambos

Antarctica is a continent made up of several large islands, one of which is the size of Australia, all buried beneath a 10,000 feet thick layer of ice† The ice contains enough fresh water to raise sea levels by nearly 200 feet.

The glaciers have always been in motion, but beneath the ice, changes are taking place that deep effects about the future of the ice sheet – and about the future of coastal communities around the world.

Breaking, thinning, melting, collapsing

Antarctica is where I work. Like a polar scientist I have visited most parts of the ice sheet on over 20 trips to the continent, using sensors and weather stations, trekking over glaciers or measuring the speed, thickness and structure of the ice.

I am currently the US coordinating scientist for a major international research effort on Antarctica’s most risky glacier – more on that later. I’ve carefully crossed crevices, tread carefully on hard blue-blown ice, and drove for days over the most monotonous landscape imaginable.

Mountains direct the flow of glaciers to the sea.
66 North via Unsplash

For most of the past few centuries, the ice sheet has been stable, as far as polar science can tell. Our ability to track how much ice flows out each year and how much snow falls on top goes back only a handful of decadesbut what we see is an ice cap that was almost in equilibrium as recently as the 1980s

In the beginning, changes in the ice occurred slowly. Icebergs would break, but the ice was replaced by new outflow. Total snowfall hadn’t changed much in ages – we knew this from looking at ice cores – and in general the ice flow and the height of the ice sheet seemed so constant that a main goal of early ice research in Antarctica was to find a place, any place, that had changed drastically.

Deep cracks leave jagged ice columns with a layer of snow on top, ready to tip into the sea.
Ice breaks off the front of a glacier in Antarctica.
66 North via Unsplash

A map of the ice sheet showing faster flowing ice on the ice shelves and especially around the edges of West Antarctica.

A map of Antarctica seen from above, most of the ice sheet, shows the speed of the ice stream ice. Thwaites Glacier is on the left.
NASA’s Goddard Space Flight Center Scientific Visualization Studio

But now, as the surrounding air and ocean warm, areas of the Antarctic ice sheet that had been stable for thousands of years are breaking, thinning, meltingor in some cases on a lot of collapse† As these edges of the ice react, they send a powerful reminder: If even a small portion of the ice sheet crumbled completely into the sea, the consequences for the world’s coasts would be dire.

Like many geoscientists, I think about what the Earth looks like under the part we can see. For Antarctica, that means thinking about the landscape beneath the ice. What does the buried continent look like — and how does that rocky basement shape the ice’s future in a warming world?

Visualizing the world under the ice

recent attempts to combine data from hundreds of aircraft and ground studies have us kind of map of the continent under the ice. It reveals two very different landscapes, separated by the Transantarctic Mountains.

In East Antarctica, the part closer to Australia, the continent is rough and furrowed, with several small mountain ranges† Some of these have alpine valleys intersected by the very first glaciers formed in Antarctica 30 million years ago, when the climate resembled that of Alberta or Patagonia. Most of the rock of East Antarctica is above sea level. This is where the city-sized Conger Ice Shelf collapsed amid an unusually intense heat wave in March 2022.

A view of Antarctica's bedrock beneath today's ice shows islands to the west and more oversea bedrock to the east.

Beneath the ice, recent studies have mapped the bottom of Antarctica and show that much of the western side is below sea level.
bed folder2; Fretwell 2013

In West Antarctica, the bedrock is very different, with parts much deeper. This area was once the ocean floor, an area where the continent was stretched and divided into smaller blocks with a deep sea floor in between. Large islands of volcanic mountain ranges are connected by a thick blanket of ice. But the ice here is warmer and moves faster.

Only 120,000 years agothis area was probably an open ocean – and certainly so in the past 2 million years† This is important because our climate today is rapidly approaching temperatures like that of a few million years ago.

The realization that the West Antarctic Ice Sheet had disappeared in the past is of great concern in the era of global warming.

Early Stages of a Large-Scale Retreat

Off the coast of West Antarctica is a large ice field called Thwaites Glacier† This is the widest glacier in the world, at 70 miles across, draining an area nearly the size of Idaho.

Satellite data tell us that it is in the early stages of a large-scale retreat† The elevation of the surface drops by as much as 3 feet each year. Huge cracks have formed on the coast and many large icebergs have drifted adrift. The glacier flows at more than a mile a year, and this rate has nearly doubled in the past three decades.

Two decades of satellite data show the fastest ice loss near Thwaites Glacier. nasa.

A view over the ice from an airplane with many fractures.

From above, fractures are evident in the Thwaites Glacier.
Ted Scambos

This area was noted early on as a place where the ice could lose its grip on the rock† The region became the “weak underbelly” of the ice sheet

Some of the first measurements of the ice depth, using radio echo sound, showed that the center of West Antarctica had bedrock up to a mile and a half below sea level. The coastal area was shallower, with a few mountains and some elevated ground; but close to the coast lay a wide gulf between the mountains. This is where the Thwaites Glacier meets the sea.

This pattern, with deeper ice piled high near the center of an ice sheet, and shallower but still low bedrock near the coast, is a recipe for disaster – albeit a very slow one.

Ice flows under its own weight – something we learned in high school earth science, but now think about it. With very high and very deep ice near the center of Antarctica, there is enormous potential for faster currents. Being shallower at the edges stops the flow — scouring the bedrock as it tries to leave, and with a shorter column of ice on the coast pushing it out.

An Antarctic glacier flows between mountains.  Lines in ice show it's flowing.
An Antarctic glacier flows to the sea.
Erin Pettiti

The warmer water undermines the glacier.

If the ice stepped back far enough, the retreating front of “thin” ice would go – still nearly 3,000 feet thick – to thicker ice towards the center of the continent. At the receding edge, the ice would flow faster, because the ice is now thicker. By flowing faster, the glacier pulls the ice behind it, allowing it to float, causing more retreat. This is what is known as a positive feedback loop – retreat leads to thicker ice at the front of the glacier, which makes for a faster flow, leading to more retreat.

Warming water: the attack from below

But how would this retreat begin? Until recently, Thwaites hadn’t changed much since it was mapped first in the forties. Early on, scientists thought a retreat would be the result of warmer air and surface melting. But the cause of the changes at Thwaites seen in satellite data is not so easy to see from the surface.

under the iceHowever, at the point where the ice sheet first lifts off the continent and begins to rise above the ocean like a floating ice shelf, the cause of the retreat becomes clear. Here ocean water is well above the melting point eroding the base of the iceerase if an ice cube were to float in a glass of water.

An illustration of an ice shelf and glacier with water flowing under the ice shelf and eroding it to the sea floor

Warming water reaches beneath the ice shelf and erodes it from below.
Scambos et al 2017

Water capable of melting as much as 50 to 100 feet of ice annually meets the edge of the ice sheet here. This erosion allows the ice to flow faster and push against the floating ice shelf.

The ice shelf is one of the limiting forces holding back the ice sheet. But the pressure of the land ice is slowly break this ice shelf† Like a plank that shatters under too much weight, it develops huge cracks. When it succumbs – and mapping the fractures and the flow rate suggests this will only be a few more years – it will be another step that will allow the ice to flow faster, feeding the feedback loop.

Up to 10 feet of sea level rise

Looking back at the ice-covered continent from our camp this year is a sobering sight. A huge glacier, which flows to the coast and stretches from horizon to horizon, rises to the center of the West Antarctic Ice Sheet. There is a palpable feeling that the ice is sinking on the shore.

Ice is still ice – it doesn’t move as fast no matter what floats it; but this massive area called West Antarctica could soon begin a multi-year decline that would add up to 10 feet to sea level. In the process, the rate of sea level rise would increase several times, posing major challenges for those with an interest in coastal cities. What we all really are.

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