Warm, deep water is shifting closer to Antarctica, threatening ice shelves and altering global ocean circulation, with implications for sea level and climate.
A long-term analysis of ocean data has revealed that heat stored deep in the ocean is moving closer to Antarctica, raising concerns about the stability of the ice shelves that surround the continent.
The research, led by the University of Cambridge in collaboration with the University of California and published in Communications Earth & Environment, brought together decades of measurements from research vessels and robotic floats. The team found that a body of warm water known as circumpolar deep water has both expanded and shifted toward the Antarctic continental shelf over the past 20 years.
Until now, scientists lacked enough consistent observations to confirm this warming trend. “It’s concerning because this warm water can flow beneath Antarctic ice shelves, melting them from below and destabilizing them,” said Joshua Lanham, lead author of the study from Cambridge’s Department of Earth Sciences.
Antarctic Ice Shelves and Sea Level Threat
Ice shelves act as barriers that slow the flow of glaciers and ice sheets into the ocean. Together, these ice reserves contain enough freshwater to raise global sea levels by about 58 meters (about 190 feet).
Lanham noted that this is the first clear evidence of deep ocean heat shifting across the Southern Ocean. “It’s something that had been predicted by climate models due to global warming, but we hadn’t seen it in data.”
Historically, observations in the Southern Ocean relied on ship-based surveys conducted about once every ten years. These surveys provided detailed snapshots of temperature, salinity, and nutrients, but gaps between measurements made it difficult to track long-term changes in heat distribution.
Filling Data Gaps With Floats and Machine Learning
To improve coverage, researchers added data from a global network of autonomous Argo floats, which drift through the upper ocean and collect continuous measurements. Although these floats offer more frequent data, they have not been operating as long as ship-based surveys.
Argo floats, pictured here, are robotic devices that drift through the upper water column collecting real-time ocean data. A global network of these instruments provides continuous snapshots of the ocean, but the programme hasn’t been running as long as ships have been collecting detailed hydrographic sections. Credit: Scripps Institution of Oceanography, UC San Diego
The team used machine learning to merge float data with long-term ship observations, creating a detailed monthly record spanning the past 40 years. This approach revealed the gradual movement of warm water toward Antarctica.
“In the past, the ice sheets were protected by a bath of cold water, preventing them from melting. Now it looks like the ocean’s circulation has changed, and it’s almost like someone turned on the hot tap and now the bath is getting warmer!” said Professor Sarah Purkey, one of the senior authors of the study from Scripps Institution of Oceanography. She added that the expansion of this warm water is expected, since more than 90 percent of excess heat from global warming is stored in the ocean, with the Southern Ocean absorbing a large share.
Global Climate and Ocean Circulation Impacts
The shift in ocean heat has implications beyond Antarctica, said Professor Ali Mashayek from Cambridge. “The Southern Ocean plays a key role in regulating global heat and carbon storage, so changes in heat distribution here have wider implications for the global climate system.”
In polar regions, freezing and dense water sinks to the deep ocean, helping drive a global circulation system often described as a conveyor belt. This system includes the Atlantic Meridional Overturning Circulation (AMOC), which moves water through the Atlantic Ocean.
Climate models used by the IPCC suggest that rising air temperatures and increased freshwater from melting ice are reducing the formation of this dense water in the North Atlantic. This process could weaken the AMOC.
Evidence Climate Predictions Are Already Happening
Similar patterns are now expected in the Southern Ocean. Models have predicted that less cold, dense water will form near Antarctica, allowing warmer circumpolar deep water to move in and fill the gap.
“We can now see this scenario is already emerging in the observations,” said Lanham. “This isn’t just a possible future scenario suggested by models; it’s something that is happening now, bringing wider implications for how carbon, nutrients, and heat are cycled through the global ocean.”
Reference: “Poleward migration of warm Circumpolar Deep Water towards Antarctica” by Joshua Lanham, Sarah Purkey, Kaushik Srinivasan, Matthew Mazloff, Laura Cimoli and Ali Mashayek, 28 April 2026, Communications Earth & Environment.
DOI: 10.1038/s43247-026-03426-x
Never miss a breakthrough: Join the SciTechDaily newsletter.
Follow us on Google and Google News.