A major new study based on decades of ocean data has found clear evidence that heat from the deep ocean is moving to Antarctica. This change poses an increasing threat to the delicate ice shelves that help stabilize ice sheets along the continent’s coasts.
Researchers led by the University of Cambridge, in collaboration with colleagues at the University of California, analyzed long-term measurements collected by research vessels and robotic ocean instruments. Their findings indicate that a relatively warm body of water known as “circumpolar deep water” has expanded over the past two decades, gradually moving closer to the Antarctic continental shelf.
First clear evidence of long-predicted changes
Until now, scientists lacked sufficient continuous data to confirm this warming trend. “This is concerning because this warm water could flow beneath Antarctic ice shelves, melting them from below and destabilizing them,” said Joshua Lanham, lead author of the study from the Cambridge Institute of Geosciences.
Ice shelves act as a barrier to Antarctica’s inland glaciers and ice sheets. Together, these frozen reserves contain enough water to raise global sea levels by about 58 meters.
This is the first time researchers have directly observed deep-sea heat transfer throughout the Southern Ocean, Lanham said. “This was predicted by climate models due to global warming, but was not confirmed in the data.”
Combination of ship recording and floating sensors
Historically, scientists relied on surveys of the Southern Ocean by ships orbiting Antarctica. These surveys were typically conducted about once every 10 years and provided detailed snapshots of temperature, salinity, and nutrient levels. But gaps between measurements have made it difficult to track long-term changes in how heat moves through the ocean.
To improve the situation, the research team combined the records of these ships with data from a global network of autonomous floats. These drifting instruments, known as Argo floats, continuously measure conditions in the surface ocean and provide more frequent observations despite their short operational lives.
Using machine learning techniques, the team integrated the float data with patterns identified in vessel measurements. This approach allowed us to reconstruct a detailed monthly record of ocean conditions over the past 40 years, revealing that ocean temperatures have been steadily rising.
Antarctica’s cooling ability is weakening
“Previously, ice sheets were protected by a bath of cold water, keeping them from melting. Now, it seems like the ocean circulation has changed, and it’s like someone turned on a faucet and the bathtub got warmer,” said Professor Sarah Purkey, one of the study’s lead authors, from the Scripps Institution of Oceanography.
Parkey noted that this expansion of warm water is consistent with what scientists expect in a warming world. More than 90 percent of the extra heat from global warming is absorbed by the oceans, and the Southern Ocean absorbs most of that heat.
Global climate impacts beyond Antarctica
The impact extends beyond ice melting. “The Southern Ocean plays a key role in regulating the Earth’s heat and carbon stores, so changes in heat distribution here have far-reaching effects on the Earth’s climate system,” said Professor Ali Mashaek, from Cambridge Geosciences, another senior author.
Near the poles, extremely cold and dense water forms and sinks into the deep ocean. As it sinks, heat, carbon, and nutrients are pulled downwards, helping to propel the global ocean current system, sometimes described as a conveyor belt. This system includes the Atlantic Meridional Overturning Circulation (AMOC), which moves water across the Atlantic Ocean.
Climate models, including those used by the IPCC, suggest that this thick water formation in the North Atlantic is already decreasing due to rising temperatures and increased freshwater from melting ice. This process could weaken AMOC.
Change is already underway
Similar changes are expected in the Southern Ocean. Models show that cold, less dense water forms around Antarctica, allowing warmer circumpolar deep water to approach the continent and fill the gap.
“Observations show that this scenario is already emerging,” Lanham said. “This isn’t just a possible future scenario suggested by models; it’s happening now, and it has broader implications for how carbon, nutrients and heat circulate through Earth’s oceans.”
