Scientists have long feared that warming could cause a breakdown of ocean circulation in the North Atlantic. But new research finds the real risk lies in Antarctica’s waters, where melting could disrupt currents in the next few decades, with profound impacts on global climate.

A new analysis by Australian and American researchers, using new and more detailed modeling of the oceans, predicts that the long-feared turn-off of the circulation will likely occur in the Southern Ocean, as billions of tons of ice melt on the land mass of Antarctica.

The ocean circulation system, often called the global conveyor, follows a regular path through the Earth’s oceans and stirs their waters from top to bottom.

The conveyor is driven by the descent of cold, salty water to the ocean floor in just two places: in the far North Atlantic near Greenland and in the Southern Ocean around Antarctica.

Since the 1990s, measurements taken from ships have shown that the water on the ocean floor, below 13,000 feet in depth, has warmed and freshened, with the trend strongest in the Southern Ocean.

The slowing of ocean circulation will “Profoundly alter the ocean overturning of heat, fresh water, oxygen, carbon, and nutrients, with impacts felt throughout the global ocean for centuries to come,” concludes lead author Qian Li, an oceanographer at the Massachusetts Institute of Technology.