A layer of water that shields the deep ocean from surface heat has been turning saltier, denser and sitting deeper since Antarctic sea ice began its sharp decline in 2016, according to research published Wednesday. The reversal of a decades-long freshening trend could allow more heat to reach deeper waters, with consequences for the world's climate that scientists say are unknown.
Researchers analysed 20 years of data from the Argo fleet of robotic floats, building three independent versions of the ocean record and checking them against satellite sea-ice observations and separate studies of ocean conditions from 2004 to 2024. Every version showed Antarctic intermediate water growing fresher and lighter through 2015, then rapidly becoming saltier, denser and deeper from 2016 onward. The change was strongest in the South Atlantic.
Antarctic intermediate water forms each southern spring and summer when melting sea ice injects fresh water into the ocean. The water sinks and spreads northward into the Atlantic, Pacific and Indian oceans at depths of about 700 to 1,200 metres, forming a barrier that limits how much heat absorbed at the surface can move into the deep ocean. The paper describes a measured salinity increase in a layer extending from about 2,300 to 4,000 feet deep and encircling Antarctica.
The pattern of slow freshening had held from at least the 1970s through 2015, the researchers write. The 2016 collapse in sea ice cut off the seasonal meltwater supply and reversed the trend within a few years. Similar reversals appear in ocean sediment records from the ends of past ice ages, but those shifts unfolded over centuries or longer, rather than a single decade.
The implications may extend well beyond the Southern Ocean. The layer carries heat, carbon, oxygen and nutrients toward the tropics and even across the equator, said co-author Bishakhdatta Gayen, a researcher at the University of Melbourne and the Centre for Atmospheric and Oceanic Sciences in Bengaluru, India. Other parts of the global ocean circulation, such as the Atlantic's overturning circulation, are also showing signs of change, and the ocean already holds more than 90 percent of the excess heat added to the climate system by greenhouse gases.
The researchers say they do not know whether the reversal marks a lasting shift; the freshening trend could return. They say continued measurements from programmes such as Argo will be essential for tracking whether the change is temporary or persistent, and for seeing how far the saltier water spreads as it moves north and whether it influences how the ocean exchanges heat with the atmosphere. In the bigger picture, they say, the changes underline the need for rapid, sustained cuts in greenhouse gas emissions to limit further warming and give Antarctic sea ice a chance of recovery.
Analysis
Why This Matters
- The ocean absorbs more than 90 percent of the excess heat added to the climate system by greenhouse gases; where that heat ends up will shape how climate changes over coming decades.
- Antarctic intermediate water spreads into the Atlantic, Pacific and Indian oceans, carrying heat, carbon, oxygen and nutrients toward the tropics and even across the equator.
- A shift that took centuries in past geological records has happened within a decade, pointing to ocean change faster than expected.
Background
Antarctic intermediate water forms when Antarctic sea ice melts each spring and summer, freshening surface water that sinks and spreads north. It sits about 700 to 1,200 metres beneath the surface and acts as a barrier limiting how much heat from surface waters can move into the deep ocean. For decades, from at least the 1970s through 2015, this layer grew slightly fresher and lighter each year. After Antarctic sea ice began a sharp decline in 2016, the researchers found that trend reversed within a few years. The layer is part of the wider system of global ocean circulation, which also includes the Atlantic overturning circulation, itself showing signs of change.
Key Perspectives
The research team: says the reversal is confirmed across three independent versions of Argo data and could allow more heat to reach deeper ocean waters, with unknown climate consequences.
Co-author Bishakhdatta Gayen: says the shift's implications extend beyond Antarctica because the water layer carries heat, carbon, oxygen and nutrients toward the tropics and even across the equator.
The researchers: caution it is unknown whether the change is permanent; the freshening trend could return, and more observations are needed.
What to Watch
- Whether the saltier, denser layer persists or freshening resumes in coming seasons.
- How far the changed water spreads northward into the subtropical oceans and whether it alters heat exchange with the atmosphere.
- The fate of Antarctic sea ice and the wider ocean circulation, including the Atlantic overturning circulation, both of which show signs of change.
Checked
12 claims in this story checked against 2 sources: 12 supported.
Show each claim
- SupportedThe decline in Antarctic sea ice turned Antarctic Intermediate Water saltier.Science - Ars Technica: “The rapid decline of Antarctic sea ice since 2016 has triggered a dramatic shift in a crucial part of Earth’s climate system, scientists say.”
- SupportedThe shift reverses decades of freshening in Antarctic Intermediate Water.Science - Ars Technica: “The shift reverses decades of freshening”
- SupportedThe shift may let more heat reach deeper waters.The Conversation – Articles (AU): “As a result, more heat may reach the deeper waters – with unknown consequences for the world’s climate.”
- SupportedThe research was published Wednesday.Science - Ars Technica: “In a paper published Wednesday, researchers documented a sharp salinity increase”
- SupportedScientists say the climate consequences are unknown.The Conversation – Articles (AU): “As a result, more heat may reach the deeper waters – with unknown consequences for the world’s climate.”
- SupportedResearchers used 20 years of Argo data.The Conversation – Articles (AU): “To make sure the observed changes were real, we drew on 20 years of data from the Argo programme.”
- SupportedResearchers built three independent versions of the ocean record.The Conversation – Articles (AU): “We built three independent versions of the ocean record from the Argo data.”
- SupportedThe three records were checked against satellite sea-ice observations and separate ocean studies covering 2004 to 2024.The Conversation – Articles (AU): “Then we checked all three against satellite sea-ice observations and separate studies of ocean conditions from 2004 to 2024.”
- SupportedEvery version showed freshening through 2015, followed by a reversal to saltier, denser, deeper water from 2016 onward.The Conversation – Articles (AU): “Every version showed the freshening through 2015, then a sharp reversal to saltier, denser, deeper water from 2016 onward.”
- SupportedThe change was strongest in the South Atlantic.The Conversation – Articles (AU): “The change was strongest in the South Atlantic.”
- SupportedThe salinity increase was measured in a layer 2,300 to 4,000 feet deep that encircles Antarctica.Science - Ars Technica: “researchers documented a sharp salinity increase in a vast layer of the Southern Ocean, extending from 2,300 to 4,000 feet deep and encircling Antarctica.”
- SupportedSimilar reversals appear in sediment records from past ice ages, but those shifts took centuries or longer rather than a single decade.The Conversation – Articles (AU): “Earlier shifts unfolded over centuries or longer, rather than in a single decade.”
