Antarctica’s sea ice started shrinking dramatically in 2015 after resisting global warming for decades, and researchers now know why.
A study published May 8 in the journal Science Advances reveals that Antarctic sea ice succumbed to strong winds that disturbed the Southern Ocean’s layers, replacing cold and relatively fresh surface water with warmer, saltier water that caused some initial melting. As sea ice declined over the years and reflected less sunlight back to space, the ocean absorbed more heat, thus accelerating the loss way beyond what scientists were expecting.
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In February 2023, sea ice in Antarctica hit its lowest extent since records began.
(Image credit: European Union, Copernicus Climate Change Service data)
To pinpoint what caused such sudden and rapid sea ice loss, Narayanan and his colleagues used a model and observations from satellites and sensors in the Southern Ocean. The researchers fed the real-life data into the model to constrain its output and bring the results closer to what scientists have watched unfold in Antarctica since 2015.
“The model we used is sort of a hybrid,” Narayanan said. “It digests all of the observational products that we feed into it, and it also runs a numerical model, much like a climate model.”
Phase 1: Westerly winds push surface waters north
As in real life, sea ice in the model expanded between 2013 and 2015. The Southern Ocean’s surface was cold and relatively fresh during this period, but the simulation showed that a warm, salty layer deep beneath the surface was rising and eroding the winter water layer — a thick band of frigid water that, up until recently, served as a barrier to protect surface waters from warmer waters below.
Study co-author Theo Spira, a researcher in the Alfred Wegener Institute at the Helmholtz Center for Polar and Marine Research in Germany, reported in a March paper that the winter water layer has been thinning since 2005. That’s because the Southern Hemisphere westerlies, which are strong winds that blow eastward around Antarctica, picked up due to the ozone hole above the continent, Narayanan said. The ozone hole strengthened the Antarctic polar vortex, which, in turn, intensified the westerlies.
Strong westerly winds around Antarctica displace surface waters northward, causing the water below to rise to replace them. This process unfolds very slowly, and the immediate response of the Southern Ocean to stronger winds in the 2000s and early 2010s was to grow more sea ice, because cold fresh water reached farther out along the margins of the frozen continent, Narayanan said.
“The hypothesis already existed in the literature that when you strengthen the winds, you get a response from the ocean on two different timescales,” he said. “The immediate response is to see a growth in the sea ice coverage. But then, if you keep this going for a few years or up to a few decades, you start to get this slower response. It takes a while, but what happens is, the heat that’s deeper in the ocean starts to rise up, simply because you’re moving waters away.”
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Phase 2: Warm water rises and initiates melting
In 2015, the westerlies became even stronger, accelerating the movement of surface waters away from Antarctica and the rise of warmer, saltier layers to replace them. By this point, the ozone hole was recovering, but the atmosphere was warming due to human greenhouse gas emissions, which have the same effect of intensifying the westerlies, Narayanan said.
The model showed that warm, salty water penetrated the winter water layer and reached the surface, where the water underwent turbulent mixing due to the powerful winds. “After 2015, you clearly see enhanced mixing from below of heat and salt,” Narayanan said. “Our study bears it out that the initiator of sea ice loss was this heat from below.”
The salt weakened the layers that naturally occur in the Southern Ocean, meaning more heat and salt could migrate upward after the initial breach in 2015. This feedback mechanism sped up sea ice melt, particularly in East Antarctica, the study found.
Phase 3: Heat and salt trigger feedback loops
By 2018, so much sea ice had melted in Antarctica that the decline became a self-reinforcing process.
Sea ice loss reduced the amount of sunlight that was reflected into space by this white surface and increased the amount of heat absorbed by the Southern Ocean, especially in the summer. This delayed the growth of sea ice every subsequent fall, as the ocean had to transfer its excess heat to the atmosphere before it could produce sea ice. The later in the year that sea ice forms, the smaller sea ice extent becomes and the more heat the ocean absorbs, Narayanan said.
Between 2013 and 2015 (blue), sea ice extent grew compared with the 1979-to-2012 average (gray). Then, in 2016, sea ice extent dropped below the long-term average (orange). And in 2023, sea ice extent reached a record low (red).
(Image credit: National Snow and Ice Data Center)
Sea ice is a source of fresh water when it melts in the summer, and previously, this helped to keep the Southern Ocean’s surface cold and relatively fresh. But the less sea ice grows in fall and winter, the less fresh water is available to maintain the Southern Ocean’s natural layers. “A saltier upper ocean means you can keep the vertical layering weak and you keep the vertical mixing going,” Narayanan said.
This is what led to the record-low sea ice extent observed in 2023. And if humans keep pumping greenhouse gases into the atmosphere, there is little hope that Antarctica will recover, because our emissions are strengthening the westerlies and warming the atmosphere, Narayanan said.
“If we keep emissions going, we will see sea ice receding farther and farther out to the continent, but I’m not sure how quick that change is going to be,” he said.
An uncertain future
Climate change is expected to boost precipitation over the Southern Ocean, which could counteract the westerlies’ impact on sea ice. More melting of Antarctic glaciers and ice sheets could also restore the ocean’s layers. Therefore, it remains unclear if Antarctica has reached a tipping point, Narayanan said.
“Is it a collapse? Not yet,” he said; but currently, the frozen continent is completely out of whack and behaving like a “new system.”
The Southern Ocean has absorbed roughly 75% of the excess heat in the atmosphere over the past 50 years, and sea ice plays a major role in this storage. When sea ice forms, it releases salt that creates dense, northward-flowing currents, which carry heat and carbon from the atmosphere to the depths of the ocean.
As sea ice shrinks, salt becomes less concentrated in the Southern Ocean, thus preventing the water from sinking and storing heat and carbon at depth. “That’s something that’s concerning, if it [sea ice loss] changes the balance, and if it reduces the ability of the Southern Ocean to store heat and carbon at depth,” Narayanan said.
Plenty of organisms also rely on sea ice to survive, including krill, dolphins, whales and penguins. Sea ice loss has already impacted the ecosystem in Antarctica through mass die-offs in penguin colonies.
The sudden switch from high sea ice extent in the 2000s and early 2010s to record-low extent in the mid 2020s “is one of the largest present-day climatic shifts in the Earth system,” Narayanan and his colleagues wrote in the study. A cascade of undesirable events could come from this, including less carbon and heat storage, more global warming, ecosystem degradation, and exposure of Antarctic ice shelves to warmer water as sea ice disappears.
Narayanan, A., Ayres, H., England, M. H., Haumann, F. A., Mazloff, M. R., Silvano, A., Spira, T., Zhou, S., & Garabato, A. C. N. (2026). Compound drivers of Antarctic sea ice loss and Southern Ocean destratification. Science Advances, 12(19), eaeb0166. https://doi.org/10.1126/sciadv.aeb0166
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