Southern Ocean carbon ‘anomaly’ reveals what models can still miss

The chilly waters of the Southern Ocean that surround Antarctica like a moat are among the least explored and least understood of all the world’s oceans. This is stark because we also know that this ocean plays an outsized role in regulating the earth’s climate with its moody currents and its tremendous ability to absorb heat and carbon dioxide from the atmosphere.

The Southern Ocean covers roughly 25-30% of the global ocean area and absorbs about 40% of all the human-emitted carbon dioxide the oceans absorb.

This ability of the ocean comes in large part from its cold and relatively fresh surface layers, which sit like a lid on top of the warmer, saltier, carbon-rich reservoirs. This arrangement allows the ocean to trap a lot more carbon dioxide than it emits. Even a small shift in this layering — which could be due to freshwater influx, changing wind patterns, changes in circulation, etc. — can change whether the ocean continues to act as a buffer or if it becomes a terrible new source of atmospheric carbon dioxide.

For nearly two decades, scientists have used computer models to understand the ocean’s role in climate change — and the models have been saying that the Southern Ocean could become less of a sink as the world warms. Specifically, the models said stronger westerly winds and more greenhouse gases in the atmosphere would pump more carbon-rich deep water up towards the surface, which would release carbon dioxide into the air and reduce the ocean’s ability to buffer global warming.

Yet new data has found the exact opposite has happened.

Since the early 2000s, scientists have found the Southern Ocean to be soaking up more carbon, not less. A new long-term analysis of ocean chemistry by researchers — from the Helmholtz Centre for Polar and Marine Research and the Ludwig Maximilian University of Munich, both in Germany — has offered the clearest explanation yet for this unexpected resilience. While climate models did get important parts of the physics right, the researchers said the models missed a powerful surface process that temporarily masked the weakening they predicted.

The team’s findings were published in Nature Climate Change in October.

The carbon sink

The models’ reasoning was physically sound, so scientists didn’t have reason to doubt them. As the concentration of greenhouse gases rose and the ozone layer thinned, westerly winds in the southern hemisphere were to strengthen and shift poleward. This shift would mean stronger upwelling in the Southern Ocean, i.e. more deep, carbon-dioxide-laden waters rising towards the surface.

“The key assumption in earlier climate models is an intensification of the meridional overturning circulation of the Southern Ocean,” Léa Olivier, the new study’s coauthor and an oceanographer at the aforementioned institutions, said. “That would lead to more waters from the deep of the ocean … being in contact with the atmosphere, and therefore weakening the Southern Ocean carbon sink.”

So models predicted that as the winds got stronger, the Southern Ocean would start emitting more carbon, perhaps even accelerating climate change.

Using decades of hydrographic measurements across the Southern Ocean, the new analysis has reported that the deep waters are indeed rising. Specifically, the circumpolar deep waters, which are naturally rich in dissolved inorganic carbon and warmer than the layers above it, have moved up by around 40 metres since the 1990s.

This in turn has increased carbon dioxide pressure in the subsurface by about 10 microatmospheres, a shift consistent with model projections.

“The most surprising part of this study was seeing that the signal we expected was there — just in the subsurface layer … the deep waters getting closer and closer to the surface, slowly but surely replacing the waters that were there before,” Dr. Olivier said.

What the models missed

Even then the ocean wasn’t emitting more carbon dioxide, and the Germany team found why in a thin layer of freshwater at the surface.

Over the last few decades, the Southern Ocean has been becoming fresher (or less salty) thanks to more rainfall, transport of sea ice, and more meltwater from Antarctica’s glaciers. Fresher water is lighter. When it accumulates at the surface, it strengthens stratification, i.e. the amount of layering that separates the cooler and more buoyant surface from the warmer and saltier waters below.

This stratification prevented the carbon-rich water from the deep from being exposed to the atmosphere. Instead it seems to be trapped 100-200 m below the surface.

According to Dr. Olivier, this competing interplay between forces is exactly what models struggled to capture: “We have two competing mechanisms: the upwelling that brings the deep water up and the stratification that blocks the vertical exchanges. My guess would be that the stratification of the Southern Ocean is sometimes misrepresented.”

Indeed capturing the presence of this layer is quite challenging. Stratification is governed by many processes that are happening at wildly different scales. Dr. Olivier said this is mainly due to the complex physics of eddies and ice-shelf cavities. Eddies are only a few kilometres wide while ice cavities are much larger.

“The lack of data also plays a role,” she added.

Fleeting reprieve

The new study emphasised that the current situation may not last. Roughly in the first half of the 2010s, the stratified layer began to grow thinner. Recent observations have shown surface salinity rising again in parts of the Southern Ocean, suggesting the lid may be fading.

“We see a strong stratification, but it is getting more shallow,” Dr. Olivier explained. “Strong winds could more easily reach below the stratified layer and into the deep waters that are warm, saline and rich in carbon dioxide. Once these waters mix, it will be harder to re-stratify the upper layer.”

In other words, the predicted weakening of the carbon sink could re-emerge, and perhaps sooner than (now) expected. The deep carbon dioxide reservoir is already closer to the surface than it used to be. So if the stratification erodes further, the carbon that models expected to see at the surface decades ago could suddenly appear.

But rather than discredit models, the researchers said their findings reinforce how essential they are. Their projections helped direct policy attention to the processes that scientists needed to observe, and which ultimately helped explain why the Southern Ocean behaved unexpectedly.

The lesson is simple: models reveal vulnerabilities; observations reveal exceptions. And the earth’s climate system lies somewhere in between.

To know what comes next, scientists also need continuous, year-round observations in one of the world’s harshest environments. Whether it absorbs or releases carbon in the coming decades could profoundly alter the planet’s future and tell us just how well our models can keep up with the evolution of a deceptively small ocean.

Ashmita Gupta is a science writer.