Scientists have long warned that melting ice in Antarctica could raise global sea levels and threaten coastal cities around the world.
But a new study suggests the danger may be even greater than current predictions because an important ocean process has largely been overlooked.
The research, led by scientists at the University of Maryland and published in Nature Geoscience, shows that melting Antarctic ice changes ocean circulation in ways that can cause even more melting.
According to the researchers, this hidden feedback loop may play a major role in future sea level rise.
The study’s lead author, Madeleine Youngs, says many current climate models do not fully account for this process.
Most models treat ice melting as a simple result of warming temperatures instead of something that can actively change the ocean itself.
The key process involves the interaction between cold and warm ocean water. Normally, very cold and dense water near Antarctica sinks toward the ocean floor and forms a protective barrier. This barrier helps block warmer deep-ocean water from reaching the underside of floating ice shelves.
But when glaciers and ice shelves melt, they release large amounts of freshwater into the ocean. Freshwater is less dense than salty seawater, so it weakens the cold barrier layer. Once that barrier weakens, warmer water from deeper in the ocean can move underneath the ice shelves and melt them from below.
This creates a dangerous cycle. More melting releases more freshwater, which weakens the barrier even further and allows even more warm water to reach the ice.
Youngs describes this as a positive feedback loop, meaning the process strengthens itself over time. If greenhouse gas emissions continue at high levels, the researchers warn that Antarctica could approach climate tipping points faster than expected.
The effects are not the same everywhere in Antarctica. In places such as the Weddell Sea, the feedback loop appears to strongly increase melting. Warm water can more easily move toward the ice shelves once the cold-water barrier breaks down.
However, the situation is more complicated in areas like the Amundsen Sea and the region around Thwaites Glacier, often called the “Doomsday Glacier.” In these locations, freshwater from upstream melting can temporarily form a colder surface layer that blocks warm water and slows melting for a short time.
Still, the researchers say this temporary protection depends on heavy melting happening elsewhere first, which already contributes to rising seas.
The study highlights what scientists call a major blind spot in current sea level projections. More than 680 million people worldwide live in low-lying coastal areas that are vulnerable to flooding and storm surges. Even a small increase beyond current estimates could worsen risks for cities such as Miami, Mumbai, and many others.
The research team is now building more detailed computer simulations to predict which Antarctic ice shelves are closest to reaching a point of no return before the year 2100.
