2023-07-30 09:15:27
©Luis Acosta / AFP
Atlantic Green
Currents in the Atlantic Ocean are now in a phase of modification, say some scientists, if not decline. Yet other experts argue that he has no evidence or certainty on the matter. What exactly do you need to understand?
Atlantico: More and more scientists are concerned about the evolution and health of the Atlantic meridian overturning circulation, also called AMOC. What is the problem exactly ?
François Sarano: To begin with, it is essential to understand that currents – whether marine or aerial – are all due to a phenomenon of different densities of water masses or water masses. Masses of different densities play with each other and this produces a current. In the air, the variations mainly concern temperatures: the warmer the air, the less dense it is (therefore the higher it rises), while in the opposite case it sinks. At sea, the situation is a little more complex because a second factor must be taken into account: salinity. Like temperature, it affects the density of water. Very salty water is very dense, even more when it is very cold. It therefore tends to sink. A less dense mass of water (warm and not very salty) tends to rise to the surface. They play with each other like rivers might flow inside the earth. Water masses flow through each other.
What makes the great marine currents (which, let’s remember, go around the planet), what is called the thermohaline circulation (for thermal and salinity), is the formation of polar pack ice. In the northern hemisphere, it is the Arctic Ocean. In the southern hemisphere, it is the sea ice around the Antarctic continent. This is the engine of the great ocean circulation. When seawater freezes, it is very cold and it expels salt. Under the pack ice, there is therefore very heavy, very dense water, because it is both very cold and very salty. Naturally, it sinks to the bottom and moves in the sea depths, to the South Atlantic (coming from the Arctic). But this water, which circulates, must obviously be replaced… and it is with hot water from tropical regions.
The warm water then flows north, since the earth’s rotation causes currents to be driven clockwise in the northern hemisphere and counterclockwise in the southern hemisphere.
From then on, the disappearance of the pack ice causes less freeze-up… and therefore a drop in engine speed. There is necessarily a modification of the motor and therefore of the importance of the currents (both in their intensity and in their quantity). Yes, we are therefore right to be concerned since the freeze-up is much less today and the phenomenon does not seem to want to stop. The risk is to see the major ocean currents change, which is not without impact on the entire climate of the planet. The hot water that came to warm our regions (like the gulf stream) may no longer pass in the long term. This is also true for cold water currents. There will no longer be the same cold-hot contrasts.
We are changing the distribution.
Some scientists also point out that the way we measure the health of the Atlantic meridian overturning circulation is not perfect: relying on sea surface temperatures alone might not be enough. What about?
They are absolutely right. The temperature of the surface water is, in fact, not sufficient to measure the good health of the meridian overturning circulation of the Atlantic. However, it remains a very important indicator and, above all, it is not the only one we use. Already today, we note the warming of deep water (particularly in the Mediterranean Sea). Attention ! We must not stop at crisis phenomena alone, such as the heat wave recently observed in the North Atlantic (we can also speak of a paroxysmal phenomenon, in this case). It is far from being the only event to remember: it is far too occasional… and it results from a specific context. All of the conditions necessary for the emergence of such a situation have been met. It is an outward sign of profound modification. The indicators are red on all sides. The presumption of disorderly conduct no longer suffers from any doubt; even though there will always be difficulties of measurement and precision, given the tremendous complexity of the phenomena.
It’s true that not all studies say exactly the same thing: some are more conservative than others…and scientists are right to be cautious. We cannot, because of this same complexity, know exactly what will happen. However, there is no doubt that there is indeed a change.
Others, among those responsible for observing the meridian overturning circulation of the Atlantic via the RAPID program, say they have no evidence of a lasting decline. How to explain the lack of consensus today?
Here again, the RAPID scientists are right: there is no evidence, as it stands, of a decline over time in the meridian overturning circulation of the Atlantic. There is no proof yet and there certainly won’t be until this has happened. What we have is the accumulation of warning signs, small events that speak volumes. Therefore, it is true to say that we cannot say with certainty that there will be a major change in this ocean circulation.
However, I have the impression that scientific rigor is pushing us into the same situation as forty years ago, when we were talking about global warming. Some very serious scientists have refused to alarm out of scientific rigour. The possible consequences were considered excessive, for lack of evidence. However, we must not lose sight of the range of clues available to us. It must be taken very seriously; without abandoning any scientific rigour.
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