Tag: sheet growth

New research from the University of St Andrews, published in the Journal of Climate (22 October 2026), proposes a mechanism linking Northern Hemisphere ice sheets to Southern Ocean changes that helped drive past ice ages via CO₂ drawdown.

Ice ages are largely driven by more atmospheric CO₂ being stored in the ocean, a process mainly controlled by changes in the Southern Ocean around Antarctica (involving ocean circulation and sea ice).

Orbital cycles (Milankovitch) initiate Northern Hemisphere ice- sheet growth, but the link from those distant northern ice sheets to the Southern Ocean CO₂ sink has been unclear.⁠

The study proposes that growing northern ice sheets cool nearby North Atlantic waters. These cooler waters circulate southward to the Southern Ocean. If the cooling is strong enough, it can trigger circulation and sea- ice changes there that enhance oceanic CO₂ storage.⁠

Researchers tested this with ocean model simulations that cooled only the far northern latitudes (mimicking large ice- sheet effects) versus cooling the entire planet.

Northern- only cooling produced ~74% of the CO₂ drawdown seen in the global- cooling case, supporting the idea that northern cooling can propagate southward and contribute substantially to glacial CO₂ reduction and related ocean features.⁠

Lead author Dr. Madison Shankle (School of Earth and Environmental Science) noted that the work provides evidence of a plausible mechanism connecting Northern Hemisphere ice sheets with Southern Hemisphere Ocean circulation changes leading to CO₂ drawdown, long a challenge in ice- age theory, and demonstrates it in model simulations.⁠

The paper is titled “Isolated Northern Hemisphere Cooling in an Idealized Model Promotes Ocean Carbon Storage and Glacial- like Ocean Features” (DOI: 10.1175/JCLI-D-25-0355.1).

Understanding these natural carbon- cycle processes are relevant for assessing how the Earth system responds to modern anthropogenic CO₂ increases.

Milankovitch cycles (also called Milanković cycles) are periodic variations in Earth’s orbital geometry and axial orientation that change the amount and distribution of solar radiation (insolation) reaching different parts of the planet over tens to hundreds of thousands of years.

These variations are widely accepted as the primary pacemaker of the Pleistocene ice ages (the glacial- interglacial cycles of the last ~2.6 million years).

They are named after the Serbian mathematician and astronomer Milutin Milanković, who calculated them in the early 20th century.

There are three main cycles:

1. Eccentricity (shape of Earth’s orbit)

• Period: Dominant cycles of ~100,000 years and a longer ~400,000-year cycle.
• Earth’s orbit around the Sun varies from nearly circular (low eccentricity) to more elliptical (higher eccentricity).
• Higher eccentricity slightly increases the total annual solar energy Earth receives and, more importantly, amplifies the contrast between perihelion (closest approach to the Sun) and aphelion (farthest point).
• Effect on climate is relatively modest on its own, but it modulates the strength of the other two cycles.

2. Obliquity (axial tilt)

• Period: ~41,000 years.
• Earth’s axial tilt varies between about 22.1° and 24.5° (currently ~23.4°).
• Greater tilt increases the seasonal contrast (hotter summers and colder winters), especially at high latitudes.
• Reduced tilt decreases seasonal contrast, making high- latitude summers cooler, conditions favorable for ice -sheet growth because snow and ice are less likely to melt completely in summer.

3. Precession (axial wobble and precession of the equinoxes)

• Period: Climatic precession has periods of roughly 19,000 and 23,000 years (the axial precession itself is ~26,000 years).
• Earth “wobbles” on its axis like a spinning top, and the orientation of the orbit also slowly rotates. Together these change the season in which Earth is closest to the Sun (perihelion).
• Currently, Northern Hemisphere winters occur near perihelion. When perihelion coincides with Northern Hemisphere summer, summers become warmer (and winters milder) in the north, and vice versa.
• This strongly affects the intensity of seasons in each hemisphere.

Combined effect and link to ice ages

The three cycles interact, producing complex, quasi- periodic changes in the seasonal and latitudinal distribution of sunlight. The most climatically important quantity is usually Northern Hemisphere high- latitude summer insolation. Cool Northern summers allow snow to persist year- round, enabling ice sheets to grow; warm Northern summers promote melting.

These orbital forcings are relatively small in energy terms. Their effects are strongly amplified by feedback in the Earth system, especially:

• Ice- albedo feedback (more ice reflects more sunlight, further cooling the planet).
• Changes in atmospheric CO₂ (largely controlled by ocean processes, particularly in the Southern Ocean).
• Changes in ocean circulation, vegetation, and dust.

This is why the previous research you asked about is relevant: orbital changes initiate Northern Hemisphere ice- sheet growth, and the new work explores one mechanism by which those ice sheets can influence Southern Ocean circulation and CO₂ drawdown, helping drive the planet deeper into a glacial state.

In summary, Milankovitch cycles do not act alone, but they set the timing and pacing of the ice ages by modulating the distribution of sunlight over long timescales.

Milankovitch Cycles, the Last Glacial Period, and Why Earth Is Still in an Ice Age

Isolated Northern Hemisphere Cooling in an Idealized Model Promotes Ocean Carbon Storage and Glacial-like Ocean Features

Using idealized ocean- model simulations, the authors show that cooling only the Northern Hemisphere high latitudes (mimicking the effect of growing Northern Hemisphere ice sheets) is sufficient to drive a large fraction of the ocean carbon storage and Southern Ocean changes characteristic of glacial periods.

In the key experiments:

• They cooled air temperatures only north of 40°N (roughly 13% of the model surface) to glacial- like values while holding the rest of the planet at pre-industrial conditions.
• This isolated northern cooling produced ~74% of the CO₂ drawdown (ocean carbon uptake) that occurred in a parallel simulation in which the entire planet was cooled.
• The northern- only cooling led to an additional ~18.8 billion tonnes of carbon stored in the ocean.

Growing Northern Hemisphere ice sheets cool nearby North Atlantic surface waters. These cooler, denser waters are transported southward via the global overturning circulation.

When the cooling signal reaches the Southern Ocean, it alters circulation and promotes expanded sea ice. These Southern Ocean changes enhance the ocean’s ability to store carbon (through a combination of physical and biological carbon pumps), lowering atmospheric CO₂ and helping push the climate deeper into a glacial state.

This provides a plausible “teleconnection” linking the two classic features of ice ages that are geographically far apart:

• Orbital (Milankovitch) forcing initiates Northern Hemisphere ice- sheet growth.
• The Southern Ocean is the main region responsible for glacial- interglacial CO₂ drawdown.

The paper addresses a long- standing gap in ice- age theory: how northern ice sheets, once formed, can influence the distant Southern Ocean processes that store CO₂.

The idealized modeling approach isolates the pure effect of northern cooling and demonstrates that it can generate glacial- like ocean features even without global cooling or other forcings.

The work is consistent with the broader view that orbital cycles set the pacing of ice ages, while feedback involving ice sheets, ocean circulation, sea ice, and the carbon cycle amplify the response.

Note: The full text on the AMS journals site is behind a paywall and login. The details above are drawn from the official press release, the paper’s metadata, and secondary reporting.

For the complete methods, figures, and discussion (including limitations and sensitivity tests), institutional access or the published PDF is required.

Title: Isolated Northern Hemisphere Cooling in an Idealized Model Promotes Ocean Carbon Storage and Glacial-like Ocean Features

Journal: Journal of Climate, Volume 39, Issue 22 (2026)

DOI: 10.1175/JCLI-D-25-0355.1

Provided: University of St Andrews

Authors: Madison G. Shankle (University of St Andrews), with co- authors including Graeme A. MacGilchrist, Andrea Burke, Alice Marzocchi, Malte F. Jansen, and James W. B. Rae.

Abstract

Earth’s past glacial cycles are paced by orbital insolation changes, but internal climate feedbacks modulating atmospheric carbon dioxide (CO₂) are necessary to explain the full magnitude of glacial–interglacial climate variability. Identifying such feedbacks and linking them to orbitally paced forcing remains a challenge, impeding a comprehensive understanding of the glacial cycles. Orbital variations, determining the latitudinal distribution and seasonality of insolation, are primarily important for allowing the buildup of large Northern Hemisphere ice sheets. Meanwhile, the processes canonically understood to drive CO₂ drawdown and mediate ocean–atmosphere carbon partitioning are largely hosted in the far-flung Southern Ocean. This study explores the role of remote Northern Hemisphere temperature forcing in influencing Southern Ocean conditions that impact global ocean circulation and carbon content. In a set of idealized ocean–sea ice model simulations, we find that cooling air temperatures to glacial-like levels at the high northern latitudes (emulating the thermal effect of a large Northern Hemisphere ice sheet) drives a substantial CO₂ drawdown of 32 ppm—or 74% the magnitude of a simulation forced by globally glacial-like temperatures everywhere (43 ppm). Northern cooling is found to cool not only northern-sourced middepth waters but ultimately also global deep waters, resulting in a strong solubility-driven CO₂ drawdown component. It also drives several characteristic features of glacial periods, including expanded southern sea ice and a colder, saltier, and better stratified deep ocean, which contributes to air–sea CO₂ disequilibrium. These results demonstrate an interhemispheric connection between Northern Hemisphere conditions paced by orbital variations and Southern Hemisphere processes that modulate deep-ocean carbon content.