The Southern Ocean regained its strength to take up CO2 from the atmosphere

In September Science published an article called ‘The Reinvigoration of the Southern Ocean Carbon Sink‘. We asked one of the authors, Dr Peter Landschützer from ETH Zürich, to comment on this study.

The Study

Over the last decades scientists from all over the world have measured the amount of CO2 in the surface ocean, which is an important quantity to determine the uptake of CO2 from the atmosphere by the ocean and the rate of which the ocean slows down current climate change, but only recently these measurements have been combined to form the to-date largest database of these measurements, i.e. the Surface Ocean Carbon Atlas (SOCAT) version 2 (www.socat.info).

However, as these measurements are mainly following the main shipping routes, large parts of the global Ocean are still unobserved, which makes it difficult to estimate the exchange of CO2 with the atmosphere, particularly in the Southern Ocean, which is one of the ocean basins with the fewest observations, we had to come up with a method that is on the one hand able to represent the observed CO2 data, but also is able to estimate the surface ocean CO2 where we don’t have observations at all.

In our study, we used a statistical interpolation technique, which is based on neural networks. Essentially, the method establishes relationships between the observed CO2 and environmental data, such as sea surface temperature, sea surface salinity, sea surface chlorophyll, the depth of the upper well mixed layer of the ocean and the atmospheric CO2 content. Once we have these relationships, we can estimate the sea surface CO2 where we don’t have observations. However, we also test our results with an additional interpolation method and an estimate based on atmospheric CO2 observations, rather than oceanic observations.

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At first we did this for the global ocean, but it was the Southern Ocean that immediately caught our attention. South of 35°S we found found strong variability in the surface ocean CO2 content, so much that from 1992 through 2002, the Southern Ocean almost gradually lost its ability to take up carbon from the atmosphere, whereas from 2002 through 2011 we found this trend to reverse and the Southern Ocean becoming the strong sink for atmospheric CO2 that we would expect.

When we discovered this strong sink variability, we were puzzled: “How could this be possible?”. Previous studies have suggested that the westerlies wind belt in the Southern Ocean is gaining strength, which means that more CO2 is recovered from the deep ocean to the surface and the Southern Ocean is losing its ability to take up CO2 from the atmosphere. This was exactly what we found from 1992 through 2002, but why would the trend reverse after 2002 since overall the winds still got stronger in the Southern Ocean?

We found our answers when we had a closer look on how exactly the Southern Ocean winds developed. Over the last decade we discovered very asymmetric surface pressure trends in the Southern Ocean, which led to changes in the local wind fields. While in the Pacific sector of the Southern Ocean the stronger winds still led to an increased transport of CO2 from deeper ocean layers to the surface, the changing wind field further brought cold air and water directed from Antarctica which resulted in a cooling of the surface waters and an enhanced ability to take up CO2 from the atmosphere. In the Atlantic sector of the Southern Ocean we found the opposite. While local wind trends resulted in the transport of warm waters from the north, they also prevented the transport of CO2 from deeper layers of the ocean which overall resulted in strengthening in the surface ocean capacity to take up CO2 from the atmosphere.

Therefore, our study suggests that the Southern Ocean carbon sink might be more affected by natural climate variability than previously recognized.

Background

While the Southern Ocean south of 35°S only accounts for about 26% of the surface ocean area, it has been shown that it accounts for roughly 40% of the oceanic uptake of anthropogenic CO2 since the beginning of the industrial era. The carbon that is taken up by the ocean is then transported and stored in deeper layers of the ocean. It is therefor a very efficient sink of atmospheric CO2 and very important in modulating global warming.

However, in the past scientists have pointed out that the changing climate also effects the Southern Ocean carbon sink. The westerlies wind belt was predicted to get stronger and shift towards the south, which results in a transport of of carbon back from the deep ocean to the surface and thereby limiting the ability of the Southern Ocean to take up additional CO2 from the atmosphere. These results were mainly based on models and were predicted to continue in the future. Given the importance of the Southern Ocean, these results caused strong concern, as the ocean is an important sink for CO2 emitted by humans and thereby slowing down the rate of global warming.

Most of these studies have been presented roughly a decade ago and fortunately, since then, we can now use a total of 2.6 million CO2 measurements south of 35°S to test if this predicted weakening of the Southern Ocean sink strength is confirmed by observations. What we found was that within the investigated timeframe of previous studies (1980s through to the early 2000s) there is indeed a decrease in the Southern Ocean CO2 sink strength, but we also find a new and certainly unexpected result, namely that the sink strength is more variable on decadal timescales and has lately immensely gained strength, which we link to natural climate variability.

Future Directions

“Is this good news?”, you may ask. It certainly is good news that the Southern Ocean CO2 sink has recently regained its sink strength, but one has to be cautious. We cannot use our method to predict the future development, but given the variability of the Southern Ocean sink it is possible that the CO2 sink trend reverses again and the Southern Ocean loses its capacity to take up CO2 in the future.

Furthermore, many new methods to interpolate the available observations arose within recent years, all of them benefiting from the increasing amount of measurements. But also the complementary nature of these methods can help to gain a better insight in the uncertainty of these estimates. To achieve this goal, the Surface Ocean pCO2 Mapping inter-comparison project was founded to better understand long term mean, seasonal cycle, inter-annual variability and trends of the global ocean carbon sink based on observations.

It is therefor essential that the international effort, that led to the measurements and the data collection, will be continued, to monitor the future development of this important global C02 sink.