A cross-section showing an ice shelf and inward-sloping bedrock. Kelvinsong via Wikimedia, CC BY-SA

Danger Of Thwaites Glacier Collapse Less Likely Than Feared — For Now

August 23, 2024August 23, 20244 months ago Steve Hanley 0 Comments

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The Thwaites Glacier in Antarctica has been called the “Doomsday Glacier” because if it melts, it will lead to an increase in sea levels large enough to drown many coastal regions. Those coastal areas are home to hundreds of millions of people and dozens of major cities. The enormity of the Thwaites Glacier is hard for people to grasp. In total it is 74,000 square miles (192,000 square kilometers) in size. It is larger than Florida, England, Wales, and Northern Ireland combined. But that’s only part of the picture. It is between 2600 and 4000 feet (800 and 1200 meters) high at its grounding line, the place where Antarctica ends and the ocean surrounding it begins.

Glaciologists — the people who study glaciers — have long speculated that because of how high the Thwaites Glacier is, once destabilization occurs at the grounding line, the glacier will proceed to disassemble itself in a rapid progression of cascading failures. 4000 feet of ice weighs a lot — hundreds of millions of pounds. It is held together by nothing more than the bonds of one frozen water molecule to the frozen molecule next to it. If rapid decomposition of the Thwaites Glacier happens, the increase in sea levels around the world would be so rapid that there would not be time to protect coastal cities from being overwhelmed by flooding. If you think such things can’t happen, consider that a few days ago, the Bayesian, a 184 foot long sailboat, sank in less than 60 seconds while anchored off the coast of Sicily. We ignore the power of the sea at our peril.

On August 21, 2024, a team of scientists led by Mathieu Morlighem published new research in the journal Science Advances that suggests the breakup of the Thwaites Glacier may not happen as quickly as feared and probably won’t happen in this century at all. Is that comforting news? That depends on how old you are. Most of us may not care, but our grandchildren very well might. Here’s what the researchers have to say:

“Among all sources of uncertainty in future sea level rise, the dynamic response of the Greenland and Antarctic ice sheets remains the largest contributor. In its latest assessment, the Intergovernmental Panel on Climate Change (IPCC) added a high-end scenario that includes a ‘low likelihood, high impact storyline.’ This scenario predicts twice as much global mean sea level rise by 2100 compared to all other projections. Sea level rise under this scenario would exceed 15 meters by 2300, three times more than other projections, due to the collapse of the West Antarctic Ice Sheet and parts of the East Antarctic Ice Sheet. The mass loss is more extreme than others considered because it includes the potential for a runaway process known as marine ice cliff instability (MICI).

“According to the MICI hypothesis, tall and steep ice cliffs could be exposed if the floating ice shelves that fringe ice sheets collapse rapidly through a process such as hydrofracture. Above a threshold height, stresses at the cliff exceed the shear strength of ice, causing structural failure of ice and rapid retreat through calving. This process could become self-sustaining if exposed cliffs remain above the threshold height but could be halted if changes in ice geometry reduce the terminal cliff below the threshold. Sea level projections that include MICI have so far been based on a single ice sheet model, using a fairly coarse resolution and a simple parameterization implemented as a vertical ‘wastage’ term. Currently, MICI is still not widely accepted or implemented in ice sheet models because it has yet to be directly observed.”

Thwaites Glacier Instability

Writing in The Conversation, lead author Mathieu Morlighem, says “What we are seeing with Thwaites Glacier right now is a disaster in slow motion. The bedrock under Thwaites Glacier sits below sea level and slopes downward going inland, so the glacier gets deeper toward the interior of the ice sheet. Once the glacier begins losing more ice than it gains from new snowfall and starts to retreat, it’s very hard to slow it down because of this slope. And Thwaites is already retreating at an accelerating rate as the climate warms.”

He says the Thwaites Glacier holds enough ice to raise global sea level by more than 2 feet (0.65 meters). Once Thwaites starts to destabilize, it also will destabilize neighboring glaciers. What happens to Thwaites affects all of the West Antarctic Ice Sheet, and that affects sea level rise along coastlines everywhere. Marine ice cliff instability is a relatively new concept proposed by scientists in the past decade. Many of the glaciers around Antarctica have huge floating extensions called ice shelves that buttress the glacier and slow its ice flow into the ocean. With the climate warming, we have seen some of these floating extensions collapse, sometimes very rapidly, in the span of a few weeks or months.

If Thwaites’ ice shelf was to collapse, it would expose a very tall ice cliff facing the ocean along its 75 mile (120 kilometer) front. There is only so much force that ice can sustain, so if the cliff is too tall, it will collapse into the ocean. Once that happens, a new ice cliff farther back would be exposed, and the new cliff would be even taller because it is farther inland. The theory of marine ice cliff instability suggests that if the cliffs collapse quickly enough, that could have a domino effect of ever-higher ice cliffs collapsing one after the other.

“However, no one has observed marine ice cliff instability in action. We don’t know if it will happen, because a lot depends on how quickly the ice collapses. When the theory of marine ice cliff instability was first introduced, it used a rough approximation of how ice cliffs might collapse once the ice shelf was gone. Studies since then have determined that ice cliffs won’t fail systematically until the ice is about 442 feet (135 meters) high. Even at that point, they would fail more slowly than projected until they became much taller,” he says.

Three Thwaites Glacier Models Instead Of One

Morlighem says his team used three high resolution models to explore what this new physical understanding of ice cliff instability would mean for Thwaites Glacier this century. The results show that if Thwaites’ entire ice shelf collapsed today, its ice front would not rapidly retreat inland due to marine ice cliff instability alone. Without the ice shelf, the glacier’s ice would flow much faster toward the ocean, thinning the front of the glacier. As a result, the ice cliffs wouldn’t be as high. This new research suggests the Thwaites Glacier would remain fairly stable at least through 2100. When the researchers simulated an ice shelf collapse in 50 years, when the glacier’s grounding line would have retreated deeper inland, they found that marine ice cliff instability alone would not cause a rapid retreat.

The results call into question some recent estimates of just how fast Thwaites might collapse, Morlighem says. That includes a worst-case scenario that the Intergovernmental Panel on Climate Change mentioned in its latest assessment report but labeled as being a low likelihood scenario. “Thwaites is the glacier everyone is worried about. If you model the entire ice sheet, this is where marine ice cliff instability starts and where it propagates far inland. So, if Thwaites isn’t as vulnerable to ice cliff failure as we thought, that’s a good sign for the entire ice sheet.”

But marine ice cliff instability is only one mechanism of ice loss. This finding doesn’t mean Thwaites is stable. There are many processes that make the Antarctic ice sheet unstable, some of them very well understood. Ice/ocean interactions explain most of the recent ice mass loss so far. Antarctica is a very cold place, so atmospheric warming isn’t having a large effect yet. But warm ocean currents are getting under the ice shelves, and they are thinning the ice from below, which weakens the ice shelves. When that happens, the ice streams flow faster because there is less resistance. Over the past few decades, the Amundsen Sea sector, where Thwaites and Pine Island glaciers are located, has seen an intrusion of warm water from the Antarctic Circumpolar Current, which has been melting the ice from below.

The Takeaway

What this research makes clear is that the Thwaites Glacier is collapsing, which will lead to higher sea levels. The reason for the collapse may not be the one scientists worry about most — fracturing of the glacier from above — but rather melting of the glacier from below. In the final analysis, the end result is the same — drowned cities and hundreds of millions of people dispossessed from their homes. Which reminds me of a poem by Robert Frost that goes like this:

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.