Past climate shifts altered Southern Ocean currents and carbon exchange: Study warns it may be happening again

Human-induced climate change is causing shifts in the world’s largest ocean current and westerly wind systems also seen during periods of ice age and warmer intervals in Earth’s history, researchers claim.

Their study highlights the role of the Antarctic Circumpolar Current (ACC) in regulating dynamics in the Southern Ocean and global climate patterns over the past 1.5 million years.

The international team, led by Cardiff University researchers, show how southern migration of the westerly winds and the ACC towards the pole during periods of past global warming increased the amount of natural carbon released to the atmosphere by the Southern Ocean.

The team warns that human-induced climate change has brought about a similar process, which is underway today and likely to continue under global warming without appropriate climate action.

Their findings, published in Science Advances, offer vital insights into how heat, salt, and carbon-rich waters flow, filling a critical gap in the understanding of ocean circulation and its relationship with past and future global climate changes.

“Our study highlights the complex interplay between ocean currents and climate patterns,” said lead author Dr. Aidan Starr, who carried out the research at Cardiff University during his doctoral study and as a research associate in the School of Earth and Environmental Sciences.

The Southern Ocean plays a central role in the global uptake of heat and carbon, with approximately 40% of annual global CO₂ emissions absorbed by the world’s oceans entering through this region.

This phenomenon is largely attributed to its unique upwelling and circulation characteristics.

The team reconstructed the speed of ocean currents near the bottom of the Southern Ocean, south of Africa, by measuring marine sediment core material retrieved on Expedition 361 of the International Ocean Drilling Program.

They uncovered systematic variations in the strength and position of the ACC during periods with extensive ice sheets known as glacial periods or ice ages, and in the warmer times without them, known as interglacial periods.

Their findings suggest that during particularly warm periods, known as super-interglacials, the mid-latitude ACC slows down, while the flow in the high-latitude Drake Passage, where the Atlantic and Pacific Oceans meet, accelerates.

This indicates a poleward shift in the Southern Hemisphere’s westerly winds, which coincides with a strengthening and similar southward shift of the ACC during warmer climatic conditions.

The team says this southward shift in the ACC and westerly wind systems has massive implications for the way the Southern Ocean absorbs heat and carbon.

Now at the University of Cambridge, Dr. Starr added, “The urgency for comprehensive climate action has never been clearer, given the delicate balance that exists within these oceanic systems. By linking ACC flow patterns with the flow of water from the deep ocean to the surface, we gain a clearer understanding of how these dynamics have varied over millennia and what this means for our current climate trajectory.”

The researchers also examined the relationship between ACC circulation and the upwelling of carbon-rich deep waters.

By measuring the carbon isotopes in organisms that live in the upper ocean water column and on the seafloor, the team demonstrated that changes in the ACC’s flow align with significant upwelling events around Antarctica.

Co-author Professor Ian Hall of Cardiff University’s School of Earth and Environmental Sciences, added, “The long-term perspective we gain from paleoclimate data reveals a previously unrecognized connection between the retreat or collapse of the Antarctic Ice Sheet and the reorganization of the ACC.

“This relationship points to important feedback mechanisms that could be activated by the projected retreat of the modern Antarctic Ice Sheet due to human-induced climate change. If the ice sheets continue to recede, we may see further disruptions in ocean circulation, with cascading effects that could impact global climate patterns.”