What hides the cooling of the last decades on the surface of the Southern Ocean

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In recent decades, the surface of the Southern Ocean has cooled slightly. An evolution that may seem paradoxical in the context of global warming. However, a study published on January 21 in the journal Nature communications reminds us that it is, so to speak, the tree that hides the forest.

The complexity of warming around Antarctica

If 90% of the additional energy trapped by the greenhouse gases that we release is stored in the ocean, it is distributed unevenly between the different ocean basins of the world. Indeed, between 1970 and 2017, 35% to 40% of this heat was absorbed by the Southern Ocean. Over the more recent period from 2005 to 2017, this proportion reaches 45% to 60%. In other words, by its imposing mass and its very distinct dynamics, the Southern Ocean plays a major role in the regulation of the global climate.

Credits: CC0 Public Domain.

However, the local variation of warming in this area of ​​the globe is complex. Also, there is a differentiated distribution of trends at the ocean surface. Maximum warming is observed south of the Amundsen and Bellingshausen Seas. Elsewhere, there is rather a cooling. A fact that may seem paradoxical given what has been mentioned above. However, the paradox disappears when we look at what is happening a few hundred meters deep.

Southern Ocean: rapid and marked warming at depth

Thanks to previous work, we already knew that warming was still present below the surface. But the data remained limited. However, in a study published on January 21, French and Australian researchers published the most accurate dataset ever produced regarding the evolution of temperatures in the Southern Ocean. The latter covers the last 25 years and specifies the changes over the first 800 meters of water according to a north-south section.

southern ocean
A. Trend in summer temperatures (NDJF) between 1993 and 2017, in ° C per decade. The black rectangle indicates the measured section. B. Trend of temperatures as a function of depth (vertical axis) and longitude (horizontal axis) over the same period. In addition, the hatches correspond to areas where the trends are not significant. Credits: Matthis Auger et al. 2021.

It can be seen that the slight surface cooling masks a much faster and more marked rise in temperature at depth. More precisely, at a rate of 0.04 ° C per decade – which is ample when one knows the quantity of water involved and the heat capacity of this one. In addition, scientists found that these warmed waters rose to the surface at a rate of around 40 meters per decade. Three to ten times more than previous estimates!

Unsurprisingly, such a burial of heat has strong implications for the gigantic Antarctic ice sheet. In particular, it weakens the bases of certain sectors of the shell. In fact, the probability of seeing sudden ice losses – as was the case with Feedback A, B and C – is increasing. Unfortunately, significant uncertainties still persist about the actual vulnerability of the southern ice sheet. Continuing research and strengthening measurement campaigns will be the keys to a better understanding of the processes involved.


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