The seas and oceans cover a little more than 70% of the terrestrial globe and play a key role, still unknown in many respects, in mitigating the impact of human activities on the climate.
To perfect this knowledge, the international Argo program, in which thirty countries including France participate, has deployed for twenty years a network of floats in the four corners of the globe, capable of diving independently to a depth of 2,000 m.
The standard model measures water temperature and salinity.
But a new generation is coming, capable of descending to 6,000 m and/or measuring new parameters.
In short, Argo will move up a gear, Ifremer announced this Thursday.
The past seven years have been the hottest on record since the pre-industrial era, the World Meteorological Organization (WMO) pointed out on Wednesday.
It confirmed the observation made ten days earlier by Copernicus, the European Earth monitoring network.
A warming trend that is far from new.
“Since 1980, each decade has been warmer than the previous one,” recalls Virginie Thierry, researcher in physical oceanography at the French Research Institute for the Exploitation of the Sea (Ifremer).
The next question is where does this heat go?
We know that part of it is absorbed by the oceans, but many points remain to be clarified.
“How does this heat enter the ocean?
Where exactly is it stored?
What is its impact on marine biodiversity?
And on the climate?
», list Virginie Thierry.
4,000 autonomous floats from the Arctic to the Antarctic
To answer this, you need
data taken over the long term. This is the challenge of the international Argo program, the first global ocean observation network, which was created in the early 2000s. About thirty countries contribute to it, including France with 10% of instruments deployed. Because, that is what it is, above all, the “Argo” program: a network of 4,000 autonomous floats scattered over all the seas and oceans. From the Arctic to the Antarctic, and even in marginal seas.
“These Argo floats are tubes 20 cm in diameter and 1.5 m high with an antenna, describes Virginie Thierry. They are totally autonomous and drift with the currents. For five years – their average lifespan – they tirelessly repeat the same cycle. First dive to 1,000 m depth and, once there, drift for nine days. Then descend an additional 1,000 m, before reaching the surface. “During this ascent, throughout the water column, these floats measure the temperature and salinity, sometimes also a multitude of other parameters depending on the sensors they carry,” continues the Ifremer researcher. These data are then transmitted by satellite to data centers on land, which will decode them and control their quality before making them available online, accessible to all.And the Argo floats, for their part, plunge again.
Already a precious help for Science…
An existence dedicated to Science therefore.
But not in vain, far from it.
“The Argo program has thus made it possible to measure that more than 90% of the excess heat due to human activities was absorbed by the oceans, begins Virginie Thierry.
This absorption is not without consequence on the ocean itself since, as it warms up, it expands, which contributes to the rise in sea level. Still from the Argo floats, we have been able to show that 40% of the rise in sea level is due to this thermal expansion of the oceans.
So much for the temperature.
On the salinity side, the Argo program has made it possible to highlight an acceleration of the hydrological cycle.
Clearly, “the salinity is increasing where it was already high, which means that where there was little rain, there is even less today.
And it continues to drop where it was already low, which means that there is more rain and melting ice in these places, ”explains Virginie Thierry.
A new generation to probe the deep ocean
Now is the time to step up a gear. The Argo program thus becomes “OneArgo” and plans to reach 4,700 floats in operation by 2030. France will invest 21 million euros to participate in this reinforcement, with the aim of deploying 80 new floats per year. But the challenge is not only quantitative. Over the past ten years, a new generation of floats, capable of descending much deeper, has emerged. Up to 4,000 or even 6,000 m deep. In the ocean depths, therefore, where the pressure is 600 times greater than that which we experience on Earth.
These deep floats should be deployed from 2024 and OneArgo aims to have 1,200 in its network by 2030. With the hope that they will allow a better understanding of the role of the oceans in mitigating the impact of human activities on the climate.
“For example, it will be a question of understanding how this heat is distributed once absorbed,” continues the Ifremer researcher.
Until recently, it was thought that this heat was stored in the surface layers.
But oceanographic campaigns carried out by ships have been able to observe, very clearly, a signal of warming up to 6,000 m depth, particularly in the Southern Ocean.
Also measure life in the oceans
It remains to precisely quantify this excess heat which penetrates to the deep ocean, to know how it propagates at this depth and to determine its contribution to the rise in sea level. This will be the primary mission of these new floats. generation. But not the only one. Technological progress has also gradually made it possible to embed new sensors on these floats, to analyze other parameters. Acidity, quantity of oxygen, light, nitrate and chlorophyll, list Ifremer. These are all indicators that will make it possible to assess life in the oceans.
"This category of new floats, called biogeochemicals (BGC), began to be deployed from 2015 and will accelerate within the framework of OneArgo", specifies Virginie Thierry.
One thousand BGC floats are expected in 2030. “And a new generation is already being prepared, warns the researcher.
It will allow us to go a little further up the food chain, with measurements on high plankton and small carnivores.
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