The Southern Ocean’s low-salinity water locked away CO2 for decades, but...

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 ability has seen no significant decline in recent decades. In a recent study, researchers from the Alfred Wegener Institute have discovered what may be causing this. Low-salinity water in the upper ocean has typically helped to trap carbon in the deep ocean, which in turn has slowed its release into the atmosphere – until now, that is, because climate change is increasingly altering the Southern Ocean and its function as a carbon sink. The study is published in the journal Nature Climate Change.

Oceans absorb around a quarter of all anthropogenic CO₂ emissions released into the atmosphere. Of this total, the Southern Ocean alone stores roughly 40 per cent, making it a key region for containing global warming. The Southern Ocean’s important role comes about due to the ocean circulation in the region, whereby water masses upwell from deeper levels, are renewed and then return to the depths. This process releases natural CO₂ from the deep ocean and absorbs and stores anthropogenic CO₂ from the atmosphere. How well the Southern Ocean is able to absorb anthropogenic CO₂ depends on how much natural CO₂ comes to the surface from the deep ocean: the more natural CO₂ that rises to the surface from the deeper levels, the less anthropogenic CO₂ the Southern Ocean is able to absorb. This process is controlled by ocean circulation and the stratification of different water masses.

The water that upwells from the depths in the Southern Ocean is extremely old, having not been at 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 cause more and more of this CO₂-rich deep water to rise to the surface. In the long term, this would reduce the Southern Ocean’s capacity to absorb human-made CO₂. However, contrary to climate model projections, observational data from recent decades has shown no reduction in its capacity as a CO₂ sink. A new study from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) now provides an explanation as to why, despite strengthening westerly winds, the Southern Ocean has continued to act as a CO₂ sink in recent decades and therefore been able to slow down climate change.

“Deep water in the Southern Ocean is normally found below 200 metres,” says Dr. Léa Olivier, AWI oceanographer and lead author of the study. “It is salty, nutrient-rich and relatively warm compared to water nearer the surface.” The deep water contains a large amount of dissolved CO₂ that entered the deep ocean from the surface a long time ago. Near-surface water, on the other hand, is less salty, colder and contains less CO₂. As long as the density stratification between deep and surface water remains intact, CO₂ from the deeper layers cannot easily rise to the surface.

Cold, low-salinity water keeps carbon-rich water contained – however, climate change brings CO₂ dangerously close to the surface

“Previous studies suggested that global climate change would strengthen the westerly winds over the Southern Ocean, and with that, the overturning circulation too,” says Léa Olivier. “However, that would transport more carbon-rich water from the deep ocean to the surface, which would consequently reduce the Southern Ocean's ability to store CO₂.” Although strengthening winds have already been observed and attributed to human-made change in recent modelling and observational studies, there is no evidence pointing to the Southern Ocean absorbing less CO₂ – at least at this point.

Long-term observations by the AWI and other international research institutes suggest that climate change may be affecting the properties of surface and deep water masses. “In our study, we used a dataset comprising 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 both circulation patterns and the 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 Léa Olivier. “We were able to determine that, since the 1990s, the two water masses have become more distinct from one another.” The Southern Ocean’s surface water salinity has reduced as a result of increased input of freshwater caused by precipitation and melting glaciers and sea ice. This “freshening” reinforces the density stratification between the two water masses, which in turn keeps the CO₂-rich deep water trapped in the lower layer and prevents it from breaking through the barrier between the two layers.

“Our study shows that this fresher surface water has temporarily offset the weakening of the carbon sink in the Southern Ocean, as model simulations predicted. However, this situation could reverse if the stratification were to weaken,” summarises Léa Olivier. There is a risk of this happening, as the strengthening westerly winds push the deep water ever closer to the surface. Since the 1990s, the upper boundary of the deep water mass has shifted roughly 40 metres closer to the surface, where CO₂-rich water is increasingly replacing the low-salinity winter surface water. As the transition layer between surface and deep water moves closer to the surface it becomes more susceptible to mixing, which could be primarily caused by the strengthening westerly winds. Such mixing would release the CO₂ that had accumulated beneath 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 Southern Ocean’s capacity to absorb anthropogenic CO₂ and therefore further drive climate change. “What surprised me most was that we actually found the answer to our question beneath the surface. “We need to look beyond just the ocean's surface, otherwise we run the risk of missing a key part of the story,” says Léa Olivier. “To confirm whether more CO₂ has been released from the deep ocean in recent years, we need additional data, particularly from the winter months, when the water masses tend to mix,” explains Prof. Alexander Haumann, co-author of the study. “In the coming years, the AWI is planning to carefully examine these exact processes as part of the international Antarctica InSync programme, and gain a better understanding of the effects of climate change on the Southern Ocean and potential interactions.“