More greenhouse gases from the sea

The ocean surrounding Antarctica, which oceanographers often refer to as the Southern Ocean, plays an important role in regulating the Earth’s climate evolution. It absorbs carbon dioxide (CO2) from the atmosphere on one hand, but releases natural carbon as well. How will this buffer work in the future? Will the Southern Ocean continue to help reduce greenhouse gases, or will it increasingly serve as a source of them? At the Oeschger Centre, several climate research teams are searching for answers.

If it gets warmer in the Southern Ocean, more icebergs will be produced and more CO2 will be released from the ocean. (Image: E. Galbraith)

The oceans absorb huge quantities of CO2 – in their depths they store about 50 times the amount found in the atmosphere. And so far, the sea has absorbed 30% of the anthropogenic CO2 emitted since the Industrial Revolution. The Southern Ocean thus plays a crucial role in the exchange between the voluminous deep sea and the atmosphere. It acts like a valve, or an open window, permitting the exchange of CO2 between water and air. For some time, climatologists have assumed that changes in the marine carbon cycle play a key role in the rise and fall of CO2 in the atmosphere – and also in the natural variations in the climate over the past million years.

In a recent edition of the scientific journal Nature, Bernese geologist Samuel Jaccard was the first to report that during the climatic cold phases, a greater quantity of CO2 was in fact stored in the Southern Ocean – and that during the warm phases, it was released again contributing to warm the global climate. Using deep-sea sediment cores, Jaccard and his colleagues have been able to reconstruct the efficiency at which the deep ocean was storing CO2 from over the past 80,000 years. “We can show how fast and through which processes the exchange of CO2 between the deep sea and the atmosphere took place,” says Jaccard. A second important result of his study: The communication between the depths of the Southern Ocean and the atmosphere is related to the production of deep water in the North Atlantic. If less deep water flows out of the North Atlantic, then the temperature rises in the Southern Ocean. That increases the exchange through the “polar window”, and more CO2 is released into the atmosphere. “Based on our results, we can assume that in the future, the Southern Ocean CO2 sink could decrease,” says Samuel Jaccard. “Not because it absorbs less anthropogenic CO2, but because it will release more previously sequestered, natural carbon.”

The French research vessel Marion Dufresnde in the Crozet Islands in the Southern Ocean (Image: A. Mazaud/CEA)

Distribution of CO2 in the ocean and atmosphere 

The reconstruction of this gas emission from the ocean’s abyss is consistent with the past changes in atmospheric CO2 concentrations, as the Oeschger Centre has been able to demonstrate thanks to air bubbles trapped in Antarctic ice. The analysis of ice cores has led to the conclusion that the amount of carbon in the entire system remained constant over long periods of time. What changes is its partitioning between the ocean and the atmosphere.

Lake sediments are another natural climate archive that researchers at the Oeschger Centre use to better understand climatic processes. Geographer Krystyna Saunders examined sediments from lakes on the remote Macquarie Island in the South Pacific. Thanks to the changing levels of seasalt concentration, she was able to demonstrate how the direction and strength of the prevailing westerly winds had changed over the past 20,000 years. This makes it possible to draw conclusions about changes in the wind regime when the last ice age ended and the southern hemisphere warmed up. The prevailing westerly winds shifted southwards thereby impacting ocean circulation.

Simulations with a Bernese 3D model

Reconstructions of various parameters of past climates help determine whether the ocean will continue to act as a greenhouse gas sink, or whether it will increasingly become a source of CO2. A decisive factor here is also the amount of greenhouse gas emissions that humanity is allowed to generate within the framework of the climate targets set in the Paris Agreement. A recent Oeschger Centre study shows the urgency of measures to be taken to abate carbon emissions. The remaining policy options are quickly dwindling; every decade of delay will shave 0.5°C off the climate target.

Retrieving marine sediments from the Southern Ocean (Image: A. Mazaud/CEA)

Climate physicists Patrik Pfister and Thomas Stocker performed this study on the CO2-budget using a climate model developed in Bern. Not content to work with climate archives alone, the various research groups at the Oeschger Centre also use computer models to form a better understanding of the CO2 cycle. The Bernese 3D model makes it possible to simulate the past 100,000 years of climate history. “If a model can understand the past,” says carbon cycle specialist Fortunat Joos, “then that increases the confidence in the future projections that we can project with it.”

Model simulations always contribute to a better understanding of the processes in the climate system. For example, they can explain what drives the mechanism that partitions? CO2 between the ocean and the atmosphere. Yet the factors contributing to this evolution are still hotly debated within the research community – just like the question of whether the Southern Ocean will continue to absorb fossil CO2 from the atmosphere. “There are indications that the buffering capacity of the Southern Ocean will deteriorate in the future,” says Fortunat Joos, “but for the moment, this is only a hypothesis; we can’t prove it yet.”