Thwaites Glacier earned the nickname “Doomsday Glacier” because its complete collapse could raise global sea levels by roughly 10 feet, enough to submerge vast stretches of coastline and entire island nations. At approximately 74,100 square miles, it’s about the size of Florida, and it’s losing ice at an accelerating rate. But the real reason for the dramatic name isn’t just the glacier itself. Thwaites acts as a physical barrier holding back much of the broader West Antarctic Ice Sheet, which means its fate could trigger far larger consequences.
How Much Sea Level Rise Thwaites Could Cause
If Thwaites melted entirely, it would add about 3 meters (just under 10 feet) to global sea levels. To put that in perspective, even 1 meter of rise would flood major portions of Miami, Mumbai, Shanghai, and dozens of other coastal cities. Ten feet would redraw coastlines worldwide.
But the threat extends beyond the glacier alone. Thwaites and neighboring Pine Island Glacier are key structural pieces of the West Antarctic Ice Sheet. Together, they function like a cork in a bottle. If Thwaites destabilizes and retreats far enough, it could open pathways for the ice behind it to flow into the ocean. The full West Antarctic Ice Sheet holds enough ice to raise sea levels by up to 10 feet on its own, potentially doubling the impact. That cascading risk is the core of the “doomsday” concern.
Why the Glacier Is Melting From Below
Thwaites isn’t primarily melting from warm air above. The main driver is warm ocean water attacking it from underneath. A deep, relatively warm current called Circumpolar Deep Water flows onto the continental shelf and reaches the base of the glacier through channels carved into the seafloor. Researchers have identified at least three deep troughs at the glacier’s northern ice front that funnel this warm, salty water directly into the cavity beneath the floating ice shelf.
One of the more surprising discoveries came from an autonomous underwater vehicle that mapped conditions beneath the ice. Scientists found that warm deep water enters the cavity from two different directions: one pathway runs along Thwaites Trough itself, while a previously unknown eastern branch carries warm water from neighboring Pine Island Bay. Both streams converge beneath the ice shelf, attacking it from multiple angles. The water reaching the glacier’s underside has been measured at temperatures above 1°C, which is warm enough to steadily erode ice that sits well below freezing.
The Grounding Line Problem
The grounding line is where the glacier transitions from resting on solid bedrock to floating on the ocean. This boundary is critical because once ice starts floating, it melts and breaks apart much faster. Over the past several decades, Thwaites’ grounding line has been retreating inland, exposing more and more of the glacier’s underside to warm seawater.
What makes this especially dangerous is the shape of the ground beneath the glacier. The bedrock slopes downward as it goes inland, like a bowl tilting away from the coast. As the grounding line retreats into deeper terrain, even more ice gets exposed to warm water, which accelerates the retreat further. This creates a self-reinforcing cycle: retreat exposes deeper ice, which melts faster, which causes more retreat. The topography of the seafloor controls where the grounding line retreats, while ocean temperatures control how fast it happens. And the influence of ocean warming grows stronger as the grounding line moves further inland into deeper basins.
Ice Cliff Instability: The Worst-Case Scenario
Beyond steady melting, scientists have debated whether Thwaites could experience something far more sudden. The hypothesis is called marine ice cliff instability. The idea is straightforward: if the floating ice shelves that buttress the glacier collapse, they could leave behind towering ice cliffs at the glacier’s edge. If those cliffs are tall enough, the sheer weight of the ice exceeds its own structural strength, and the cliff face fails, calving massive chunks into the ocean. If each collapse exposes another cliff that’s equally tall, the process becomes self-sustaining, like dominoes falling.
This scenario would produce rapid, dramatic sea level rise rather than the gradual increase from steady melting. However, recent modeling work has offered some cautious reassurance. Simulations that forced Thwaites’ grounding line to retreat into deeper basins, where cliffs would be tallest, found that rapid thinning and changes in ice flow actually reduced the calving rate, stabilizing the cliff before runaway collapse could take hold. The conclusion from the International Thwaites Glacier Collaboration’s research through 2024 is that while ice cliff failure is a real physical process, it has proven difficult to sustain in model scenarios. Concerns remain, though, particularly about how damaged or fractured ice might lower the threshold at which cliffs begin to fail.
How Fast Ice Is Disappearing
Thwaites is already losing substantial amounts of ice. Between 2009 and 2017, the glacier discharged an average of about 117 billion tonnes of ice per year. By 2017, the net loss had reached roughly 37 billion tonnes annually after accounting for new snowfall. These numbers have been climbing for decades. Four decades of satellite measurements show a clear acceleration in ice loss across West Antarctica, with Thwaites as one of the largest individual contributors.
The glacier’s eastern ice shelf, a floating extension that helps brace the glacier and slow its flow into the sea, has shown alarming signs of structural weakness. Satellite imagery has revealed expanding fractures and rifts across the shelf. One analysis estimated that at the rate those weaknesses were growing, the eastern ice shelf could collapse in as few as five years, though the researchers cautioned that extrapolating nonlinear processes is inherently uncertain. Losing that ice shelf wouldn’t directly raise sea levels much, since it’s already floating, but it would remove a critical restraint on the glacier behind it, allowing land-based ice to flow into the ocean faster.
What Makes Thwaites Uniquely Dangerous
Antarctica has many glaciers losing ice, but Thwaites stands out for a combination of reasons. Its sheer size means it holds an enormous volume of ice. Its bed slopes inland, making it vulnerable to accelerating retreat. Warm ocean currents have direct access to its underside through deep seafloor channels. And its position as a structural keystone for the broader West Antarctic Ice Sheet means its collapse wouldn’t be an isolated event.
Undersea “storms,” intense pulses of warm water driven onto the continental shelf, add another layer of concern. These events can produce bursts of rapid melting at the glacier’s base. As sea ice coverage in the surrounding ocean declines with warming temperatures, these pulses are expected to become more frequent and more intense, further accelerating the process. The combination of steady warm-water intrusion, episodic intense melting events, unfavorable bedrock geometry, and the glacier’s role as a linchpin for the entire ice sheet is what makes Thwaites the single most watched glacier on Earth.