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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While oil is known to float, new research finds that over time it can permeate even extremely deep marine habitats, with animal populations declining by as much as 80 percent.

Coral-fringed tropical atolls are among the landforms most threatened by global warming, but many have a potential superpower: Healthy ecosystems may help them keep pace with rising seas.

Atoll islands with native vegetation and thriving seabird populations are “naturally active systems that can change and adapt to their environmental conditions if they are allowed to do so,” said Nick Graham, a marine ecologist at Lancaster University in the United Kingdom, who led an international team of scientists that studied island resilience across 28 atoll islands in the Pacific and Indian oceans.

Their paper, published Wednesday in Science Advances, shows that restoring atoll ecosystems reinforces their growth. Seabirds ferry nutrients from the open ocean and deposit them on the atolls as guano that nurtures land plants, marine life and coral growth. This provides more material for reef building, as parrotfish and other reef feeders turn dead coral into sand to maintain beaches, while native plants with extensive root networks stabilize sediments.

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