Cooling only the far northern latitudes of an idealized ocean model produced 74% of the carbon dioxide drawdown seen when researchers cooled the entire planet, a University of St Andrews-led team reports in the Journal of Climate. The result suggests a way growing ice sheets in the Northern Hemisphere could have pushed the Southern Ocean around Antarctica to store more carbon during past ice ages, according to a university summary.

The finding comes from a simplified computer model, not from ice cores or seafloor sediments. It shows that the mechanism can work in principle, but it does not show how much it contributed during real ice ages. That limitation is central to reading the study correctly.

The question still matters today. The Southern Ocean is one of the planet's main storehouses for carbon, and how its circulation responds to changes far away affects how much carbon dioxide stays in the atmosphere. Understanding that link in the past helps scientists judge how sensitive the ocean's carbon storage is to shifts in circulation.

A Puzzle Spanning Two Hemispheres

Ice ages are paced by slow changes in Earth's orbit, which first trigger ice sheet growth in the Northern Hemisphere. Yet much of the drop in atmospheric carbon dioxide during glacial periods is thought to come from changes in the Southern Ocean, half a world away. Ice-core records show carbon dioxide fell to roughly 180 to 190 parts per million in the coldest glacial periods, compared with about 280 before the industrial era.

Scientists have long struggled to explain how a northern trigger produced a southern response. The two regions are separated by thousands of miles of ocean, and the timing of changes recorded in ice cores from Greenland and Antarctica has not always lined up neatly, leaving room for competing explanations involving winds, sea ice and ocean mixing. The new study tests one pathway. Growing ice sheets cool the North Atlantic, that cooled water travels south in the deep ocean to the Southern Ocean, and, if the cooling is strong enough, it changes circulation around Antarctica in ways known to trap more carbon below the surface.

Cooling the North, Watching the South

Lead author Madison Shankle of St Andrews' School of Earth and Environmental Sciences and colleagues ran a set of simulations in an idealized ocean model. In some, they cooled only the far northern latitudes to mimic a large ice sheet. In a reference run, they cooled the whole planet.

The northern-only cooling produced nearly three-quarters of the carbon drawdown of the global cooling run, along with what the paper's title calls glacial-like ocean features. Shankle said the team has "evidence of a plausible mechanism" connecting the two hemispheres. In the St Andrews release, she added that "our study takes the step of actually demonstrating them working in model simulations."

Co-authors include Graeme MacGilchrist, Andrea Burke and James Rae of St Andrews, Alice Marzocchi of the U.K.'s National Oceanography Centre and Malte Jansen. The model output is publicly archived on Zenodo, allowing other researchers to check the results. The paper appears in volume 39, issue 22 of the Journal of Climate under DOI 10.1175/JCLI-D-25-0355.1.

A Simplified Ocean, Not the Real One

The model is idealized, meaning it strips the ocean down to its basic physics and geometry rather than reproducing real coastlines, winds, and seafloor. That makes cause and effect easier to trace, but it also means the 74% figure applies to the model, not to Earth's actual ice ages. The university's summary also did not say how quickly the southern response would develop, a key question for matching the mechanism to dated climate records. Real glacial oceans also responded to changes in winds, sea ice, dust, and biology that a simplified setup may not capture.

The study is peer-reviewed, and its conclusions are worded with care. The mechanism is described as plausible, and it depends on the northern cooling being strong enough. Testing it will require full climate models with realistic geography and comparisons with chemical records preserved in deep-sea sediments.

Echoes in Today's Changing Polar Oceans

The study looks backward, but the ocean connections it tests are the same ones scientists are watching now. The Atlantic overturning circulation links the North Atlantic to the Southern Ocean, and researchers continue to study how it may change as the planet warms. A separate study highlighted by the American Geophysical Union this month suggested that part of the cross-hemisphere circulation could persist for centuries even if deep-water formation in the North Atlantic shut down.

Neither study predicts what will happen next. The St Andrews work involves cooling, not warming, and it uses a simplified ocean — so it cannot be read as a forecast. What it adds is evidence that a change in one hemisphere can reach the carbon-storing waters around Antarctica, which is why scientists track both polar regions together.

Readers do not need to take any action because of this research. It does not forecast future ice ages, and it does not change projections of near-term warming. Its value is in explaining how distant parts of the ocean are linked — a connection that matters as scientists watch today's Atlantic overturning circulation and Antarctic sea ice for signs of change.

The next step is for other groups to test the mechanism in more complex climate models and against the sediment record. The bottom line is that a simplified model shows northern cooling alone can push the Southern Ocean to lock away most of the carbon seen in a fully cooled world, a strong clue to the ice-age puzzle that still needs confirmation in the real climate record.

What Readers Want to Know

What did the study find?

In an idealized ocean model, cooling only the far north produced 74% of the carbon dioxide drawdown seen when the whole planet was cooled.

How would northern ice sheets affect the Southern Ocean?

Cooled North Atlantic water travels south in the deep ocean and, if cold enough, can change circulation around Antarctica so more carbon stays trapped below the surface.

Is this proof of how ice ages worked?

No. It shows a plausible mechanism in a simplified model. It still needs testing in realistic models and against sediment records.

Who did the research?

A team led by Madison Shankle of the University of St Andrews, with colleagues including researchers at the National Oceanography Centre.

Does this predict a coming ice age?

No. The study is about past glacial cycles and does not forecast future climate.

Why does it matter today?

It shows how strongly the Southern Ocean's carbon storage can respond to distant changes in ocean circulation.