Earth’s history shows how Antarctic ice-sheet melting altered ocean circulation

What is it about?

Over the past three million years, Earth’s climate has alternated between long glacial periods and warmer deglaciations, marked by the retreat of ice sheets. While the effects of Northern Hemisphere ice-sheet melting on North Atlantic circulation are well documented, the role of Antarctic ice-sheet dynamics in shaping the Southern Ocean remains much less understood. Yet the Southern Ocean is central to the global climate system: it connects the Atlantic, Indian, and Pacific Oceans and represents the main interface of exchange between the atmosphere and the deep ocean, a carbon reservoir nearly one hundred times larger than the atmosphere. These exchanges are largely governed by ocean stratification—the vertical layering of water masses that regulates mixing. To reconstruct past changes in these processes, scientists analyzed sediment cores from the Southern Ocean seafloor and measured the isotopic composition of organic matter preserved in diatom shells, tiny marine algae that are highly abundant in these regions. The results reveal that during deglaciations, stratification intensified near Antarctica as large volumes of freshwater from melting ice sheets entered the ocean. Farther north, near the polar front, the combined influence of freshwater input and strong westerly winds enhanced the upwelling of deep waters, sustaining partial ocean ventilation at the global scale, releasing CO₂ into the atmosphere, and contributing to global warming.

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Photo by Jay Ruzesky on Unsplash

Photo by Jay Ruzesky on Unsplash

Why is it important?

In the natural context of glacial cycles, this stratification of polar waters was temporary. It was eventually offset by intensified deep-water upwelling linked to the strengthening and poleward shift of the westerly winds. But the current situation is different: human-induced climate warming could lead to sustained melting of the Antarctic ice sheet and, consequently, prolonged stratification of the Southern Ocean. This evolution raises a crucial question: will the ocean continue to play its role as a buffer against climate change by absorbing a significant share of the heat and CO₂ emitted by human activities? Climate archives from the past nevertheless provide part of the answer. They suggest that wind-driven dynamics in the Southern Ocean, combined with freshwater input, could help maintain sufficient deep-water ventilation even in the presence of strong stratification in polar regions. The results indicate that this “window” to the deep ocean could remain open in the near future, allowing the ocean to continue absorbing a significant fraction of anthropogenic CO₂ and excess heat. This confirms the central role of the Southern Ocean in regulating the global climate.

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