Rising emissions weaken carbon storage in the Southern Ocean

The Southern Ocean, vast and cold, swallows more carbon than its size suggests. It’s a carbon sink, a crucial one, pulling atmospheric carbon down into its icy depths. But what happens when the rules change?

New research from the University of East Anglia (UEA) takes a hard look at this question. The study warns that the ocean’s role in carbon absorption might weaken as greenhouse gas (GHG) emissions climb.

The old driver, the ozone hole, might be stepping back. In its place? Rising emissions, warming waters, and unpredictable winds.

Ozone depletion’s reversible impact

Back in the 20th century, the ozone hole ruled the Southern Ocean’s winds. It churned up the waters, dragging carbon-heavy currents to the surface. The result? The ocean became less capable of soaking up more carbon.

But now, the ozone hole is healing. And as it recovers, its grip on the winds is slipping. That might sound like good news, but there’s a catch.

Dr. Tereza Jarníková of the Tyndall Cente for Climate Change Research at UEA is the lead author of the study.

“An interesting, and hopeful, highlight of this work is that the effects of human-caused ozone hole damage on the winds, circulation, and carbon uptake of the Southern Ocean are reversible, but only under a lower scenario of greenhouse gas emissions,” noted Dr. Jarníková.

Wind patterns in the Southern Ocean

In the 20th century, ozone depletion drove wind patterns in the Southern Ocean. The stronger winds pulled carbon-laden waters to the surface, reducing the ocean’s carbon sink potential.

Now, as the ozone hole mends, those winds are calming down. But will that last? The study suggests it won’t.

Under a high-emission scenario, wind speeds could surge by 0.75 meters per second (2.46 feet per second) by the end of the century. In a low-emission world, the increase might hold at 0.35 meters per second (1.15 feet per second). In both cases, GHGs, not ozone, would be the dominant force behind the winds.

Carbon storage in the Southern Ocean

The ocean’s mixed-layer depth (MLD) tells a story of shifting currents. When the MLD deepens, carbon-rich waters rise to the surface, limiting how much more carbon the ocean can absorb.

Ozone depletion deepened the MLD in the past, slashing the Southern Ocean’s carbon sink capabilities. But as the ozone hole heals, the MLD could stabilize.

Still, there’s a new threat. Rising GHGs could keep pushing those waters up. In a high-emission scenario, MLD deepening could intensify, making it harder for the ocean to absorb atmospheric carbon.

Ocean currents and carbon uptake

Meridional overturning circulation (MOC) acts like a conveyor belt, carrying nutrient-rich waters to the surface. This process fuels biological productivity, pulling carbon down into the deep ocean.

But the study finds that the MOC’s influence on carbon uptake is waning. Why? Rising surface temperatures are flattening the carbon gradient between surface and deep waters. The result is that less carbon gets absorbed.

Temperature: The new carbon thief

The Southern Ocean is warming. By 2100, surface temperatures could rise by 1.6°C under a high-emission scenario. Warm water holds less carbon, making it harder for the ocean to trap atmospheric CO2.

Even if winds intensify and the MLD deepens, the warming waters might still win. The ocean’s capacity to absorb carbon is shrinking, driven by temperature increases that are now outpacing other factors.

Future scenarios: A tug of war

The UEA team used the UK Earth System Model (UKESM1) to simulate three possible ozone futures.

In the first scenario, the ozone hole never forms, maintaining stable winds and circulation. In the second, the hole forms but gradually recovers after the 1987 Montréal Protocol. The third scenario envisions the ozone hole persisting at its 1987 size, continuing to influence wind patterns and circulation.

Layered over these scenarios are two GHG pathways: one with emissions peaking and then falling, and another where emissions keep climbing. The results reveal a tug of war between ozone recovery and GHG-driven warming.

Ocean loses its grip on carbon

The Southern Ocean’s days as a reliable carbon sink may be numbered. The study suggests that by mid-century, GHG emissions will dominate wind patterns, mixed-layer depth, and overturning circulation.

In the past, ozone depletion drove the winds. It pushed carbon-rich waters to the surface, reducing the ocean’s capacity to absorb more carbon.

But as the 21st century unfolds, GHGs will take the lead. Temperature changes, not wind shifts, will determine how much carbon the ocean can hold.

The research delivers a clear message and the findings serve as a stark reminder. The Southern Ocean’s ability to absorb carbon depends on emissions, temperature, and circulation.

The study was funded by the UK Natural Environment Research Council (NERC) and the Royal Society. The findings are published in the journal Science Advances.

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