Oceanic Drivers of the Amundsen Sea Embayment
Ocean warming in the Amundsen Sea has reached a point where it will likely continue regardless of how much the world reduces greenhouse gas emissions. Researchers from the British Antarctic Survey found that the warming is driven by changes in ocean circulation and the warming of the Southern Ocean, which pushes warm water toward the ice sheet.
This shift focuses on the Amundsen Sea Embayment, where the West Antarctic Ice Sheet (WAIS) meets the ocean. The research indicates that the warming of the deep ocean is now a locked-in process. Once this warm water reaches the grounding lines—the point where the ice lifts off the seabed and begins to float—it melts the ice from below.
It looks like we’ve locked in this warming of theundsen Sea.
Kaitlin Naughten, Researcher at the British Antarctic Survey
The study suggests that the warming is not a direct result of current atmospheric temperatures but is instead a response to long-term changes in wind patterns and ocean currents. These changes are moving warmer Circumpolar Deep Water toward the coast of West Antarctica.
Mechanisms of Marine Ice Sheet Instability
The collapse of the WAIS is driven by a process called Marine Ice Sheet Instability (MISI). Most of the West Antarctic Ice Sheet sits on bedrock that is below sea level and slopes downward toward the interior of the continent.
When warm ocean water melts the grounding line, the ice retreats down this slope into deeper water. As the ice becomes thicker at the grounding line, the flow of ice into the ocean accelerates. This creates a feedback loop: the faster the ice flows, the more it thins, which allows the grounding line to retreat further, accelerating the process again.
A second mechanism, Marine Ice Cliff Instability (MICI), involves the collapse of vertical ice cliffs. When the floating ice shelves that act as buttresses for the ice sheet disappear, they leave behind towering cliffs of ice. These cliffs can become structurally unstable and collapse under their own weight, leading to a rapid discharge of ice into the sea.
The combination of MISI and MICI means that once the retreat begins in certain basins, it becomes self-sustaining. The physics of the slope and the weight of the ice drive the collapse independently of further atmospheric warming.
Projections for Four Metres of Sea Level Rise
The West Antarctic Ice Sheet contains enough ice to raise global sea levels by approximately 5.3 meters. Current projections focused on the collapse of the Amundsen Sea sector estimate a contribution of roughly four meters.
This figure differs from the projections for the East Antarctic Ice Sheet (EAIS), which is larger and generally more stable. While the EAIS contains significantly more ice, its bedrock is largely above sea level, making it less susceptible to the grounding-line retreat seen in the west.
The timeline for this rise is a subject of ongoing analysis. While the total collapse would not happen overnight, the tipping point marks the moment the process becomes irreversible. Scientific models indicate that the most rapid acceleration could occur over several centuries, though some sectors may respond more quickly.
The four-meter projection assumes the total loss of the WAIS. If only the Amundsen Sea sector collapses, the rise would be lower, but the process would still be irreversible.
Risks to Global Coastal Infrastructure
A four-meter rise in sea level would redefine global coastlines and render current flood defenses obsolete. Most existing sea walls and levee systems are designed for rises of less than one meter by 2100.
The impact would be most severe in low-lying megacities and river deltas. Cities such as Miami, Dhaka, and Ho Chi Minh City face permanent inundation of large urban areas. The loss of the WAIS would not be distributed evenly; gravitational effects mean that sea level rise is often higher in the Northern Hemisphere when Antarctic ice melts.
The lag time between the tipping point and the actual rise of the water creates a risk of under-preparation. Because the collapse takes centuries to fully realize, the immediate lack of water on the streets may lead to a false sense of security. However, the commitment to that rise is established the moment the grounding line retreats past the critical slope.
Current adaptation strategies focus on “managed retreat” and the construction of floating infrastructure. These measures are intended to mitigate the impact of the rise, but they cannot stop the physical collapse of the ice sheet once the thermal threshold in the Amundsen Sea is crossed.
