Southern Ocean’s low-salinity Antarctic waters continue absorbing CO₂ despite climate model predictions

Climate models suggest that climate change could reduce the Southern Ocean’s ability to absorb carbon dioxide (CO₂). However, observational data actually shows that this capacity has not seen a significant decline in recent decades.

In a recent study, researchers from the Alfred Wegener Institute discovered the possible cause of this phenomenon. Low-salinity waters in the upper layers of the ocean have generally helped trap carbon in the ocean depths, slowing its release into the atmosphere – until now, as climate change increasingly alters the Southern Ocean and its function as a carbon sink. The study is published in the journal Climate change.

Oceans absorb about a quarter of all anthropogenic CO₂ emissions released into the atmosphere. Of this total, the Southern Ocean alone stores around 40%, making it a key region for containing global warming. The important role of the Southern Ocean is due to ocean circulation in the region, whereby water masses rising from deeper levels are renewed and then return to the depths. This process releases natural CO₂ from the depths of the ocean and absorbs and stores anthropogenic CO₂ of the atmosphere.

To what extent is the Southern Ocean capable of absorbing anthropogenic CO₂ depends on the amount of natural CO₂ comes to the surface from the depths of the ocean: the more natural the CO₂ that rises to the surface from the deepest levels, the less anthropogenic CO there is₂ the Southern Ocean is capable of absorbing. This process is controlled by ocean circulation and the stratification of different water masses.

The water rising from the depths of the Southern Ocean is extremely old, having not been on the surface for hundreds or thousands of years. Over time it has accumulated large amounts of CO₂ which naturally return to the surface through the upwelling process. Model studies show that strengthening westerly winds, caused by climate change, will generate more and more of this CO₂-rich deep waters to rise to the surface. In the long term, this would reduce the Southern Ocean’s capacity to absorb human-caused CO2.₂.

However, contrary to climate model projections, observational data from recent decades have shown no reduction in its capacity as CO.₂ to flow. A new study from the Alfred Wegener Institute, Helmholtz Center for Polar and Marine Research (AWI), now explains why, despite strengthening westerly winds, the Southern Ocean continued to act as a CO.₂ sink in recent decades and was thus able to slow climate change.

“The deep waters of the Southern Ocean are normally found below 200 meters,” explains Dr Léa Olivier, AWI oceanographer and lead author of the study. “It is salty, nutrient-rich and relatively warm compared to water closer to the surface.”

Deep waters contain a large amount of dissolved CO₂ that entered the ocean depths from the surface long ago. On the other hand, water close to the surface is less salty, colder and contains less CO₂. As long as the density stratification between deep water and surface water remains intact, CO₂ from deeper layers cannot easily rise to the surface.

Cold, low-salinity water keeps the water carbon-rich. However, climate change brings CO₂ dangerously closer to the surface.

“Previous studies suggested that global climate change would strengthen westerlies over the Southern Ocean and, therefore, the reversal of the circulation,” explains Olivier. “However, this would transport more carbon-rich water from the depths of the ocean to the surface, which would consequently reduce the Southern Ocean’s capacity to store CO₂.”

Although stronger winds have already been observed and attributed to human-caused changes in recent modeling and observational studies, there is no evidence that the Southern Ocean is absorbing less CO₂, at least at this stage.

Long-term observations by AWI and other international research institutes suggest that climate change could affect the properties of surface and deep water bodies.

“In our study, we used a dataset including biogeochemical data from a large number of marine expeditions in the Southern Ocean between 1972 and 2021. We looked for long-term anomalies, as well as changes in circulation patterns and properties of water masses. In doing so, we only considered processes related to the exchange between the two water masses, namely circulation and mixing, and not biological processes for example,” explains Olivier. “We have seen that since the 1990s, the two water masses have become more distinct from each other.”

The salinity of surface waters in the Southern Ocean has declined due to increased freshwater input from precipitation and melting glaciers and sea ice. This “refreshing” reinforces the density stratification between the two water masses, which keeps the CO₂-rich deep water trapped in the lower layer and prevents it from breaching the barrier between the two layers.

“Our study shows that these cooler surface waters temporarily compensated for the weakening of the carbon sink in the Southern Ocean, as predicted by model simulations. However, this situation could be reversed if stratification were to weaken,” summarizes Olivier.

There is a risk of this happening as strengthening westerly winds push deep water ever closer to the surface. Since the 1990s, the upper limit of the deep water mass has moved about 40 meters closer to the surface, where CO₂-rich waters increasingly replace low-salinity winter surface waters. As the transition layer between surface and deep waters moves closer to the surface, it becomes more susceptible to mixing, which could be primarily caused by strengthening westerly winds. Such a mixture would release the CO₂ accumulated under the surface water layer.

A recently published study suggests that this process may have already begun. The result would be that more CO₂-rich deep water could reach the surface, which would in turn reduce the ability of the Southern Ocean to absorb anthropogenic CO₂ and therefore contribute further to climate change.

“What surprised me the most was that we found the answer to our question beneath the surface. “We have to look beyond just the surface of the ocean, otherwise we risk missing a key element of the story,” Olivier explains.

“To confirm whether more CO₂ has been released from the ocean depths in recent years, we need additional data, especially over the winter months, when water masses tend to mix,” explains Professor Alexander Haumann, co-author of the study. “In the coming years, AWI plans to closely examine these precise processes as part of the international Antarctica InSync program and better understand the effects of climate change on the Southern Ocean and potential interactions.”