Abstract:
Scientists have proposed a new mechanism to explain how the Earth's past ice ages were formed: Ocean cooling caused by the expansion of the northern hemisphere ice sheets may be transmitted to the southern hemisphere through global ocean circulation, changing the circulation and sea ice conditions of the ocean around Antarctica, thereby allowing more carbon dioxide to be stored in the deep sea, promoting further cooling of the global climate. The study sheds new light on long-elusive climate connections between the northern and southern hemispheres.
This study was led by researchers from the University of St. Andrews in the UK, and the results were published in the Journal of Climate on October 22, 2026. The research focuses on a long-standing scientific question: Scientists already know that the Southern Ocean plays an important role in regulating atmospheric carbon dioxide concentrations, but there has been a lack of clear physical explanations of how the huge ice sheets formed in the Northern Hemisphere affect the oceans surrounding Antarctica thousands of kilometers away.
The earth has experienced many ice ages in its long history. During these periods, global temperatures drop and large areas of land in the Northern Hemisphere are covered by ice sheets. Changes in the Earth's orbit will change the solar radiation received at different latitudes, creating conditions for the formation and expansion of ice sheets; at the same time, atmospheric carbon dioxide concentration, ocean circulation, and the ability of ice and snow to reflect sunlight will also affect the development and duration of ice ages. Scientists believe that these factors do not operate independently, but may influence each other through a series of feedback mechanisms.
The mechanism proposed by the research team begins in the high latitudes of the Northern Hemisphere. When orbital changes cool these regions in the summer, ice sheets may gradually form and expand. Large ice sheets will cool nearby North Atlantic waters and change the temperature structure and density distribution of the water. As seawater participates in the global ocean circulation system, this cooling signal may gradually propagate southward, eventually affecting the Southern Ocean around Antarctica.
The Southern Ocean is an important part of the Earth's carbon cycle. The ocean absorbs carbon dioxide from the atmosphere and stores some of it in the deep sea through seawater sinking, deep water circulation, and other physical and biological processes. If circulation and sea ice conditions in the Southern Ocean change, the carbon exchange between the ocean and the atmosphere may also change, leaving more carbon dioxide in the ocean instead of being released back into the atmosphere.
Atmospheric carbon dioxide is an important greenhouse gas, and reduced concentrations reduce the atmosphere's ability to retain heat. Therefore, if the expansion of the northern hemisphere ice sheet can promote the storage of more carbon in the Southern Ocean through ocean circulation, it may form a gradually increasing cooling feedback: northern cooling promotes the expansion of the ice sheet, the ice sheet affects the ocean circulation, the Southern Ocean further absorbs and stores carbon dioxide, and the weakening of the atmospheric greenhouse effect is conducive to continued global cooling.
To test this hypothesis, the researchers conducted ocean climate model simulations. Rather than directly cooling the entire planet, they lowered temperatures only in the northernmost regions of the ocean to simulate the effects of large ice sheet expansion on surrounding water. The research team then looked to see whether this localized cooling could be transmitted to the Southern Hemisphere through ocean circulation and cause changes that would be beneficial to ocean carbon storage.
Simulation results show that cooling the high-latitude waters of the Northern Hemisphere alone can produce a significant global response. Cooling just the northern oceans would result in a 74% reduction in atmospheric carbon dioxide compared to a scenario in which the researchers cooled the entire planet simultaneously. This result suggests that local cooling in the Northern Hemisphere could have considerable impacts on the global carbon cycle through long-distance ocean connections.
The work provides a possible explanation for two climate phenomena that have previously been difficult to link: the expansion of ice sheets in the Northern Hemisphere, and a decline in atmospheric carbon dioxide triggered by changes in ocean circulation in the Southern Hemisphere, said lead author Madison Shanker of the School of Earth and Environmental Sciences at the University of St Andrews. The research team proved through model simulation that this connection is feasible in terms of physical mechanism.
However, the 74% figure comes from specific model experiments and does not mean that every expansion of the Northern Hemisphere ice sheet in real history will necessarily cause the same proportion of carbon dioxide decline. What the model results demonstrate is that this mechanism may play an important role, rather than that its actual contribution to the formation of each glacial period has been precisely determined. The real Earth's climate system is also affected by factors such as wind fields, sea ice extent, seawater salinity, marine life activity and the location of continents.
The significance of this study lies in linking climate processes in the northern and southern hemispheres that have been studied relatively separately in the past. Previously, when scientists explained the decline in atmospheric carbon dioxide during ice ages, they often focused on changes in the ocean around Antarctica. New research suggests that the Southern Ocean is not an independent climate system and its state may be affected by changes in ice sheets and oceans as far away as the Northern Hemisphere.
Understanding this connection will not only help reconstruct Earth's past climate history, but may also help scientists improve climate models. The ocean can absorb, transport and store large amounts of carbon over long time scales, and changes in its circulation will affect atmospheric carbon dioxide concentrations and further change global temperatures. If models could simulate these processes more accurately, they could better explain the transitions between warm and glacial periods in Earth's history.
Of course, the Earth's naturally formed ice age mechanism in the past cannot be directly equated to the evolution path of today's global warming. Modern atmospheric carbon dioxide concentrations are significantly affected by human fossil fuel burning and land use changes, and the speed and background conditions of current climate change are also different from many paleoclimate scenarios. Researchers still need to further confirm the strength of this connection between the northern and southern hemisphere oceans at different historical times and the role it may play in future climate change.
Overall, this study reveals a cross-hemispheric feedback mechanism that may drive the earth into an ice age: ocean cooling caused by the expansion of the northern hemisphere ice sheet may affect the carbon storage capacity of the Southern Ocean through global circulation, thereby amplifying the cooling process of the entire Earth system. It reminds scientists that Earth's climate is not determined by any one region alone, but by a complex interconnected system of atmosphere, oceans, ice sheets and carbon cycles. To understand how the ice ages began, it is necessary to trace how these changes were transmitted and mutually reinforced across half the world.
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