Every southern spring and summer, sea ice around Antarctica melts with the change of season. The melt injects fresh water into the sea, where it mixes and sinks, then slowly spreads northward into the Atlantic, Pacific and Indian oceans.

The water forms a layer called the Antarctic intermediate water, which sits around 700 to 1,200 metres beneath the surface. It forms a barrier that limits how much surface waters can mix with the deeper body of the ocean – and how much heat absorbed at the surface can move into the depths.

For as long as we’ve had reliable measurements, the Antarctic intermediate water has been growing steadily lighter and fresher. But in 2016, sea ice levels around Antarctica began to plummet.

Our new research shows that over the past decade, the Antarctic intermediate water has rapidly become saltier and more dense, and sitting deeper in the ocean. As a result, more heat may reach the deeper waters – with unknown consequences for the world’s climate.

Antarctic intermediate water travels from near Antarctica out of........

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