Declining ice has led to saltier and denser water that could influence atmospheric circulation and weather in the Southern Hemisphere – and even globally
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Melting ice in Antarctica is changing seawater and could drive major shifts in the world’s oceans, a new study published in Nature has revealed.
Normally, ice forms in Antarctica between May and July and then melts when temperatures rise, releasing freshwater into the ocean. However, researchers found that this layer of water, known as Antarctic Intermediate Water (AAIW), became saltier and denser between 2016 and 2024, coinciding with record-low Antarctic sea ice. This weaker layer could mean heat can travel farther into the ocean, suggesting scientists may have underestimated the extent of deep-ocean warming and rising sea levels.
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The Antarctic Intermediate Water spans much of the Southern Hemisphere’s four oceans and helps store carbon, supply oxygen, and drive global ocean currents. Researchers stress that melting Antarctic sea ice influences ocean circulation thousands of kilometres away.
‘It was quite surprising. This alteration is happening very quickly,’ said study lead and University of Melbourne associate professor Bishakhdatta Gayen. ‘Since we have records in oceanography, we have never seen this water alter.’
‘If the altered AAIW changes the way surface water is carried down into deeper layers, it could affect how much carbon the ocean locks away – although the magnitude and timing are uncertain,’ Gayen continued.
The team analysed data from a network of around 4,000 robotic floats that sample the upper ocean, along with satellite and atmospheric datasets. The Antarctic Intermediate Water last changed around 130,000 years ago, when little ice covered Antarctica.
‘That transition took thousands of years, not two decades,’ said Gayen.
Antarctica contains 90 per cent of the planet’s freshwater ice, with more than 99.5 per cent of the continent covered in ice.
The Antarctic Ice Sheet measures nearly 4.9 kilometres (3 miles) at its thickest point and contains about 30 million cubic kilometres (7.2 million cubic miles) of ice. If the entire ice sheet melted, sea levels would rise about 58 metres (190 feet).
Between 2002 and 2025, Antarctica lost about 135 gigatons of ice per year, raising global sea levels by 0.4 millimetres annually.
Over the last 50 years, the west coast of the Antarctic Peninsula has become one of the planet’s fastest-warming regions, affecting everything from the distribution of penguin colonies to the abundance of Antarctic krill, which are vital to marine food chains. Glaciers and ice shelves in Antarctica have also retreated in recent years due to global warming, with some collapsing completely.
