The strongest ocean current on Earth circles Antarctica. It’s the primary way water moves between the Atlantic, Pacific and Indian oceans, and helps regulate the global climate.

But a new study suggests the Antarctic Circumpolar Current (ACC), which until now has been extremely stable, might begin to slow down in the next 25 years, with potentially severe consequences for marine life, rising sea levels and the planet’s ability to absorb heat from the atmosphere.

In a scenario of high greenhouse gas emissions, mathematical models project that the current could slow down by up to 20% by 2050. Meltwater pouring off the Antarctic ice shelves is projected to dilute salty ocean water and disrupt the flow of one of the world’s most reliable and important ocean currents.

“The Southern Ocean is in a state of rapid change,” the study’s authors wrote. “Understanding the cause of this declining trend is important for predicting broader climatic and oceanic changes.”

The ACC is 100 times stronger than the Amazon River and helps distribute heat and nutrients throughout the world’s oceans. A disruption could impact the oceans’ ability to absorb excess heat and atmospheric carbon, undermining their ability to mitigate climate change.

“More than 90% of the extra heat from global warming has gone into the ocean,” Michael Meredith, an oceanographer specialized in polar oceans at the British Antarctic Survey, who wasn’t involved in the study, told Mongabay in a video interview. “If that process slows down, then all of a sudden we’ve lost one of the big buffers against global warming that we have.”

For decades, scientists have measured the ACC in the Drake Passage, a narrow bottleneck between South America and the Antarctic Peninsula, where its flow is easiest to track. The results have remained remarkably consistent.

But if the current begins to weaken, it would allow warmer water to seep in closer to the ice shelves, speeding up the melting of Antarctic ice. “Faster ice melting could then lead to further weakening of the current, commencing a vicious spiral of current slowdown,” the study’s authors wrote in The Conversation. Rapidly melting Antarctic ice would also further contribute to global sea-level rise.

No ACC slowdown has actually been observed yet in the real world, but the study shows a concerning trend.

Even a small slowdown would be a huge departure from how the current has functioned for as long as we’ve been able to measure it.

“The past is not necessarily a guide to the future,” Meredith said. “In the past, we’ve seen remarkable stability of this current, but that doesn’t mean that’s going to be the case going forward. And there’s good reasons to think it might not be.”

Banner image: Scientists on a British Antarctic Survey ship measure the conditions of the ACC. Image courtesy of Michael Meredith.