The glacier is already moving. The question is how far.
Beneath the Antarctic ice, a glacier the size of Great Britain is quietly unraveling — and with it, the stability of coastlines that billions of people call home. Thwaites glacier, already responsible for four percent of global sea-level rise, is losing its restraining ice shelf to a web of cracks that scientists say could give way within a decade. What follows may be a slow reckoning or a cascading catastrophe, but either way, the glacier is already in motion — and the world's great coastal cities are already in its path.
- The eastern ice shelf of Thwaites is fracturing rapidly, and glaciologists warn it could fully collapse within ten years, removing the only brake on the glacier's seaward surge.
- Warming ocean water is eating the shelf from below, pushing the glacier's anchor point deeper into a downward-sloping seabed that accelerates its own retreat in a self-reinforcing spiral.
- Scientists fear a chain reaction called Marine Ice Cliff Instability, in which Thwaites' collapse exposes towering ice cliffs that crumble and destabilize neighboring glaciers across the Amundsen Sea Basin.
- The potential consequences range from 65 centimeters of sea-level rise from Thwaites alone to several meters if the broader West Antarctic ice sheet unravels — enough to erase island nations and flood global megacities.
- Debate continues among researchers about whether sea ice and calved debris could buffer the worst outcomes, but no credible voice disputes that Thwaites is already contributing to rising seas and will continue to do so.
Somewhere beneath the Antarctic ice, a frozen river the size of Great Britain is coming apart. The Thwaites glacier holds enough ice to raise every ocean on Earth by 65 centimeters — a figure that translates, in human terms, to the inundation of Shanghai, New York, Miami, and Tokyo, and the outright erasure of island nations like Kiribati, Tuvalu, and the Maldives.
Thwaites already accounts for roughly four percent of current global sea-level rise. Since 2000, it has shed more than a trillion tons of ice, and its flow speed has doubled over three decades. But the deeper alarm lies in what restrains it: a floating ice shelf acting as a cork in a bottle. That cork is now cracking. Glaciologist Erin Pettit of Oregon State University warns the eastern shelf, now webbed with fissures, could collapse within a decade.
Without that restraint, Thwaites would discharge its remaining ice into the ocean over decades or centuries. Worse, it sits alongside other weakening glaciers in the Amundsen Sea Basin. Scientists fear a mechanism called Marine Ice Cliff Instability — a chain reaction in which one glacier's collapse exposes unstable ice cliffs that crumble and trigger neighbors to follow. That scenario could produce several meters of sea-level rise, not merely the 65 centimeters Thwaites holds alone.
The mechanism driving all of this is warming ocean water undercutting the ice shelves from below, thinning them from within, and pushing the anchoring point backward along a seabed that slopes ever downward — a geometry that accelerates the very retreat it enables. Some researchers hold that sea ice and calved debris may yet buffer the worst outcomes, and the science of cascading collapse remains unsettled. But the glacier is already moving. The question is no longer whether Thwaites will contribute to rising seas — only how far, and how fast.
Somewhere beneath the Antarctic ice, a frozen river the size of Great Britain is coming apart. The Thwaites glacier, locked in West Antarctica, holds enough ice to raise every ocean on Earth by 65 centimeters if it melted completely. That may not sound like much until you consider what it means: the inundation of Shanghai, New York, Miami, Tokyo, and Mumbai. The erasure of island nations like Kiribati, Tuvalu, and the Maldives. Coastlines redrawn across the planet. And the worry among scientists is that Thwaites may not collapse alone.
The glacier already contributes roughly 4 percent of the world's current sea-level rise. Since 2000, it has shed more than a trillion tons of ice, a loss that has accelerated steadily over three decades. Its flow speed has doubled in that time, meaning the glacier now discharges twice as much ice into the ocean as it did in the 1990s. But the real alarm comes from what holds it back. Thwaites is restrained by a floating platform of ice called an ice shelf—a kind of cork in a bottle. Recent research has confirmed that this cork is cracking. The eastern section of the ice shelf is now webbed with fissures, and according to glaciologist Erin Pettit at Oregon State University, it could collapse within a decade.
What happens next is where the catastrophe multiplies. If the ice shelf fails, Thwaites would lose the thing that slows its seaward march. The glacier would then discharge all its remaining ice into the ocean over the following decades or centuries. But that's not the only danger. Thwaites sits in the Amundsen Sea Basin alongside other glaciers—the Pine Island glacier among them—all held back by ice shelves that are themselves weakening. Scientists worry about a mechanism called Marine Ice Cliff Instability, or MICI, in which the collapse of one glacier exposes tall, unstable cliffs of ice that crumble into the sea, triggering a regional chain reaction. If that happens, several meters of sea-level rise could follow, not just the 65 centimeters from Thwaites alone.
The culprit is warming ocean water. As the seas heat up, they undercut the ice shelves from below, melting them from underneath and thinning them from within. This subsurface melting weakens the ice, allowing the cracks observed at the surface to develop and spread. It also pushes backward the point where the ice shelf anchors to the seafloor. In the Amundsen Sea, the seabed slopes downward, which means as the anchoring point retreats, the glacier loses its footing and can accelerate rapidly backward—a process that feeds on itself.
Yet uncertainty persists about how this will unfold. Not all scientists believe a wholesale collapse is inevitable. Some research suggests that sea ice and chunks of ice breaking away from a collapsing shelf might provide enough restraint to prevent the chain reaction from consuming the entire West Antarctic ice sheet. The science of MICI is still being worked out, and questions remain about the future of the region. But one thing is certain: Thwaites will keep rising, and it will keep adding to sea levels for years to come, whether catastrophically or incrementally. The glacier is already moving. The question now is how far it will go.
Notable Quotes
The eastern ice shelf now has cracks crisscrossing its surface and could collapse within ten years— Erin Pettit, glaciologist at Oregon State University
The Hearth Conversation Another angle on the story
Why does this glacier matter more than any other?
Because of where it sits and what it holds back. Thwaites is like a keystone in an arch—if it goes, the whole structure becomes unstable. It's also already moving faster than it used to, which tells us something is changing.
The ice shelf could collapse in ten years. Is that a certainty?
It's a strong possibility based on what we're seeing now—the cracks, the thinning. But ten years is also the edge of what we can predict with confidence. The real point is that it's destabilizing faster than models suggested it would.
You mentioned a chain reaction. How does one glacier's collapse trigger another's?
When Thwaites' ice shelf fails, it stops acting as a brake. The glacier accelerates and dumps more ice into the ocean. That ice can physically push against neighboring glaciers, or it can expose cliff faces that are inherently unstable and prone to collapse. It's a domino effect.
What would several meters of sea-level rise actually look like for a city like New York?
Permanent flooding of lower Manhattan. Saltwater intrusion into aquifers and soil. Neighborhoods that are home to millions becoming uninhabitable. It's not a distant abstraction—it's infrastructure, property, lives.
Is there any scenario where this doesn't happen?
Yes. Some research suggests that broken ice from the collapse might actually slow things down, acting as a buffer. But that's speculative. What we know is that the glacier is warming, the ice shelf is cracking, and the ocean is getting warmer. Those are facts.
So what do we do?
We keep watching. We keep measuring. We prepare for what's coming, because even in the best-case scenario, sea levels are rising. The question is how much, and how fast.