For decades, the frozen wilderness at the bottom of the world defied global warming trends, with ice levels actually growing – until 2015 when it suddenly reversed. A study led by the University of Southampton shows that a series of compounding events flipped the Southern Ocean – which surrounds Antarctica – out of balance, dragging unusually warm, salty water from the deep up to the surface.
It was so extreme that it wiped out vast areas of ice equivalent to the size of Greenland, leading to record-breaking lows in 2023. Oceanographer Aditya Narayanan says that Antarctic sea ice in the Southern Ocean helps drive the planet’s ocean overturning circulation.
“However, since 2015, the region has undergone a huge transformation, with extreme ice loss around the continent. What started as a slow build-up of deep-sea heat under the Antarctic sea ice was followed by a violent mixing of water, ending in a vicious cycle where it’s too warm to let ice recover. It’s concerning because massive loss of sea ice destabilises the world’s ocean current systems, warming our planet far quicker than expected.”
The study concludes that the sea ice decline happened in three stages, driven by shifting winds and warming oceans.
- Around 2013 – strengthening winds began pulling warm, salty water from the deep ocean, known as Circumpolar Deep Water, closer to the surface
- In 2015 – intense wind mixed the deeper heat directly into the surface layer, rapidly melting sea ice, particularly in East Antarctica
- Since 2018 – the ice-ocean system has become trapped in a cycle where, with less ice to melt, the surface remains salty and warm, inhibiting new ice from forming
The study also found a striking asymmetry in how the ice is retreating across the continent, with East Antarctic loss almost entirely ocean-driven, fuelled by an upward surge of warmer deep water. In West Antarctica, heat was trapped in the ocean by intense cloud cover, which was funnelled by warm air from the subtropics down to the pole, melting the sea ice during the summers of 2016 and 2019.
Co-author Alessandro Silvano said: “This isn’t just a regional problem, Antarctic sea ice acts as Earth’s mirror, reflecting solar radiation back into space. Its loss could destabilise the currents that store heat and carbon in the ocean, accelerating global warming, and also destabilize ice shelves that prevent glaciers from sliding into the sea, raising global sea levels.”
Climate change is fuelling stronger winds, exposing the Southern Ocean’s surface and pushing deep-sea heat to the surface. Co-author Alberto Naveira Garabato warns that if this continues into the 2030s, the Southern Ocean could be pushed into a “prolonged low sea-ice state. The ocean may transition from a stabiliser of the world’s climate to a powerful new driver of global warming.”
(Adapted from content provided by the University of Southhampton.)
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