Anyone studying the global carbon cycle faces incredibly high numbers. The Southern Ocean – the world’s largest ocean sink region for man-made CO2 – is expected to absorb a total of around 244 billion tonnes of human-made carbon from the atmosphere over the period 1850-2100 under high CO2 emissions scenario. But the absorption could be only 204 or up to 309 billion tonnes. This is how the projections of the current generation of climate models vary. The reason for this great uncertainty is the complex circulation of the Southern Ocean, which is difficult to represent correctly in climate models.
“Research has been trying to solve this problem for a long time. We have now succeeded in reducing the large uncertainty by around 50 percent,” says Jens Terhaar of the Oeschger Center for Climate Change Research at the University of Bern.
With Thomas Frölicher and Fortunat Joos, also researchers at the Oeschger Center, Terhaar has just presented in the scientific journal “Sciences Advances” a new method for constraining the CO of the Southern Ocean2 sink. The link between human CO absorption2 and the salinity of surface water is key. “The discovery that these two factors are closely related has helped us to better constrain future CO from the Southern Ocean2 sink ”explains Thomas Frölicher.
Towards the achievement of the Paris climate objective
A better constraint on the carbon sink of the Southern Ocean is a prerequisite for understanding future climate change. The ocean absorbs at least a fifth of human-made CO2 emissions, and as such slows global warming. By far, most of this uptake, about 40 percent, occurs in the Southern Ocean.
Berne’s new calculations don’t just reduce CO uncertainties2 absorption and thus allow more precise projections, but also show that at the end of the 21st century, the Southern Ocean will absorb about 15% more CO2 than previously thought. This is just a little help on the extremely difficult path to reach the Paris temperature target of 1.5 degrees. “The reduction of CO of human origin2 emissions resulting from the combustion of fossil fuels remain extremely urgent if we are to achieve the objectives of the Paris climate agreement, ”explains Fortunat Joos.
Best possible model predictions
In their study, the three climatologists show why the salinity content of ocean surface water is a good indicator of the amount of CO of human origin.2 is transported inside the ocean. Models that simulate low salinity in the surface waters of the Southern Ocean have waters that are too clear and therefore carry less water and CO2 in the interior of the ocean. As a result, they also absorb less CO2 of the atmosphere. Models with higher salinity, on the other hand, show higher CO absorption.2 of the atmosphere. The salinity of the surface water of the Southern Ocean, determined by observations, allowed the Bernese researchers to reduce the uncertainty of the various projections of the model.
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