The Southern Ocean dominates the oceanic uptake of human-made carbon dioxide (CO2). How much CO2 it can store depends on the salinity content of its surface waters. Climate scientists from the University of Bern have now shown this with computer simulations from an ensemble of Earth system model simulations. Using this new relationship, they have substantially reduced the large uncertainties in the estimates of the future CO2 sink in the Southern Ocean.

May 05, 2021 – Press Release University of Bern/ SU

Anyone researching the global carbon cycle has to deal with unimaginably large numbers. The Southern Ocean – the world’s largest ocean sink region for human-made CO₂ – is projected to absorb a total of about 244 billion tons of human-made carbon from the atmosphere over the period from 1850 to 2100 under a high CO₂ emissions scenario. But the uptake could possibly be only 204 or up to 309 billion tons. That’s how much the projections of the current generation of climate models vary. The reason for this large uncertainty is the complex circulation of the Southern Ocean, which is difficult to correctly represent in climate models.

Salinity of the surface waters is key

“Research has been trying to solve this problem for a long time. Now we have succeeded in reducing the great uncertainty by about 50 percent,” says Jens Terhaar, Postdoc at Climate and Environmental Physics, Physics Institute and the Oeschger Centre for Climate Change Research at the University of Bern. Together with the professors Thomas Frölicher and Fortunat Joos from the University of Bern, Terhaar has just presented in the scientific journal “Sciences Advances” a new method for constraining the Southern Ocean’s CO₂ sink. The link between the uptake of human-made CO₂ and the salinity of the surface waters is key to this. “Our discovery that these two factors are closely related helped us to better constrain the future Southern Ocean CO₂ sink”, explains Thomas Frölicher. A better constraint Southern Ocean carbon sink is a prerequisite to understand future climate change. The ocean absorbs at least one fifth of human made CO₂ emissions, and as such slows down global warming. By far the largest part of this uptake, about 40 percent, occurs in the Southern Ocean.

In their study, the three climate scientists show why the salinity content of the ocean surface waters is a good indicator of how much human-made CO₂ is transported into the ocean interior. Models that simulate low salinity in the Southern Ocean surface waters have too light waters and therefore transport less water and CO₂ into the ocean interior. As a result, they also absorb less CO₂ from the atmosphere. Models with higher salinity, on the other hand, show higher absorption of CO₂ from the atmosphere. The salinity of the Southern Ocean surface waters, determined through observations, allowed the researchers from Bern to narrow down the uncertainty in the various model projections.

Towards achieving the Paris climate target

The new calculations from Bern, some of which were carried out on “Piz Daint”, not only reduce uncertainties in CO₂ uptake and thus allow more accurate projections, but also show that by the end of the 21st century the Southern Ocean will absorb around 15 percent more CO₂ than previously thought. [Emphasis added.] This is only a tiny bit of help on the extremely challenging path to achieving the Paris temperature goal of 1.5 degree. “The reduction of human-made CO₂ emissions resulting from the combustion of fossil fuels remains extremely urgent if we are to achieve the goals of the Paris climate agreement,” clarifies Fortunat Joos.

This article appeared on the Swiss National Superconducting Centre (CSCS) website at https://www.cscs.ch/science/earth-env-science/2021/strength-of-carbon-sink-in-the-southern-ocean-depends-on-salinity/

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