The Southern Ocean, the body of water encircling Antarctica, is the largest regional ocean sink for anthropogenic carbon. It absorbs roughly 550 million metric tons of carbon each year, equivalent to about two billion metric tons of carbon dioxide, and accounts for approximately forty percent of global oceanic anthropogenic carbon uptake. This sequestration function is a critical component of the global carbon cycle, and changes in its capacity, whether from warming, acidification, or circulation shifts, have direct implications for the pace of atmospheric CO2 accumulation and the rate of global warming.
Carbon uptake in the Southern Ocean operates through two interrelated mechanisms: the physical, or solubility, pump and the biological carbon pump.
The solubility pump works because cold water dissolves CO2 more readily than warm water. Dense, cold surface water in the Southern Ocean absorbs atmospheric CO2 and then sinks into the deep ocean as part of the thermohaline circulation, carrying dissolved carbon with it. This carbon can remain sequestered in the deep ocean for centuries to millennia before returning to the surface. The formation of Antarctic Bottom Water, one of the densest and coldest water masses in the global ocean, is a major driver of this process. As the Southern Ocean warms and freshens from glacial melt, the density gradients that drive this sinking are weakened, potentially reducing the efficiency of the physical pump.
The biological carbon pump operates through photosynthesis. Phytoplankton in the Southern Ocean fix atmospheric CO2 into organic carbon during the austral summer bloom. When these organisms die, much of their carbon sinks to the ocean floor rather than being respired back into the atmosphere. The efficiency of this pump depends on nutrient availability, particularly iron, which is limiting in large parts of the Southern Ocean. Upwelling zones that bring iron-rich deep water to the surface support some of the most productive biological carbon uptake in the ocean. Changes in circulation that alter upwelling patterns therefore affect the biological pump directly.
The Southern Ocean’s carbon uptake capacity is not unlimited, and evidence suggests that warming and acidification are already affecting it. As ocean temperatures rise, the solubility of CO2 in seawater decreases. As the chemistry shifts and pH falls, calcifying organisms that form an important part of the biological pump are stressed. The efficiency of the solubility pump declines as the temperature differential between surface and deep water narrows and density-driven overturning weakens.
Research published over the last decade has found that the growth of the Southern Ocean carbon sink has not kept pace with the growth of atmospheric CO2 concentrations. In effect, the ocean is absorbing more carbon in absolute terms because there is more CO2 available to absorb, but its uptake efficiency, relative to what a larger sink could theoretically absorb, appears to be declining. If this trend continues, a larger fraction of anthropogenic CO2 emissions will remain in the atmosphere rather than being absorbed by the ocean, accelerating warming.
Sea ice plays a complex and still not fully understood role in Southern Ocean carbon dynamics. Ice-covered areas limit direct air-sea CO2 exchange, while the formation and melt of sea ice drives changes in salinity and density that influence deep water formation. The algae that grow on and within sea ice contribute to the biological pump: when ice melts in spring, seeding the water column with algae and nutrients, it helps trigger the phytoplankton bloom that drives seasonal carbon uptake.
The sharp declines in Southern Ocean sea ice extent observed since 2016 therefore affect carbon cycling in ways that are not yet fully quantified. More open water may allow greater air-sea CO2 exchange in some conditions, but it also disrupts the seasonal patterns of productivity on which the biological pump depends. Disentangling these effects is a current priority in Southern Ocean research.
The Southern Ocean’s role as a carbon sink is one reason why changes in Antarctic systems matter globally, even to people living far from any coast. The ocean’s ability to absorb and sequester CO2 is one of the largest moderating factors in the relationship between human emissions and atmospheric CO2 concentrations. Every ton of carbon absorbed by the Southern Ocean is a ton that does not accumulate in the atmosphere. If that sequestration capacity declines, the effective CO2 budget consistent with any given warming target shrinks.
Understanding how the Southern Ocean carbon sink works, how it is changing, and what drives those changes is not a specialized scientific problem. It is one of the most consequential questions in climate science, with direct bearing on projections, policy, and the timeline of physical change across the entire planet.