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.

In a paper published Wednesday, 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. The shift reverses decades of freshening and echoes ocean changes usually associated with major global climate shifts over millennia but is now happening in just a few years.

The switch from a decades-long trend of freshening water to increasing salinity has implications that could extend beyond Antarctica. The layer, known as Antarctic Intermediate Water, spreads northward into the Indian, Pacific, and Atlantic oceans, transporting heat, carbon, oxygen, and nutrients toward the tropics, and even across the equator, said co-author Bishakhdatta Gayen, a researcher and professor at the University of Melbourne and at the Centre for Atmospheric and Oceanic Sciences in Bengaluru, India.

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