In response to climate change, the ocean has tended to stabilize more and more for 50 years, at a rate six times greater than past estimates, according to a study by researchers shows a study conducted by researchers from CNRS, Sorbonne University, and Ifremer.
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Patrick Smith / GETTY IMAGES NORTH AMERICA / Getty Images via AFP
Sinks of carbon and immense reserves of heat, the oceans are a major regulator of the global climate.
“A kind of buffer against climate change that makes it more bearable,” says oceanographer Jean-Baptiste Sallée.
Is climate change undermining this role?
Its consequence is to intensify the demarcation between the surface layer of the oceans and those of the depths, shows, this Wednesday, a study published in
Nature
and coordinated by Jean-Baptiste Sallée.
With the potential consequence, behind, of complicating the exchanges between these layers, however essential to allow the heat and the carbon captured on the surface to diffuse in all the column of water of the oceans.
There are surface waters and deep waters.
Between these different layers of the oceans, exchanges are numerous and permanent.
And fortunately.
They are the ones that allow the oceans to be at the heart of the global climate system.
"A sort of great regulator and a buffer against climate change, which makes it more bearable for us", compares the oceanographer Jean-Baptiste Sallée, researcher at the Locean * laboratory.
Heat ... and carbon sponges
How? 'Or' What ?
The oceans, which cover 71% of the earth's surface, already act as huge reservoirs of heat.
“90% of the excess heat on the Earth's surface generated by global warming is captured by the oceans and buried in the depths,” he explains.
This thanks, in particular, to the thermohaline circulation.
In detail, the ocean receives heat from solar radiation, mainly in the tropics.
These warmer waters “rise towards the poles, where they arrive cooled,” explains Ifremer.
By convection, these cold and salty waters, more dense and therefore heavier, will then flow under the warmer water and not very salty, to then set out again in the opposite direction, towards the equator.
"
Heat sponges, the oceans are also used for CO2, "always according to the same mechanism", indicates Jean-Baptiste Sallée.
CO2 is transmitted to the ocean by simply dissolving the gas in seawater. The colder the water, the greater the quantity of carbon absorbed, the CO2 being more soluble in them.
The cold currents then take care of carrying the CO2 to the bottom, where it is stored.
The IPCC (Intergovernmental Panel on Climate Change) insisted on the role of ocean carbon sinks in a special report published in September 2019. They thus absorb 20 to 30% of CO2 emissions due to the activities of man, scientists say.
A greater demarcation between surface and depth?
It remains to be seen whether the oceans will be able to continue to play this role of thermostat with the same efficiency under the effect of global warming.
This is the question posed by a team of researchers from CNRS, Sorbonne University, Ifremer as well as Germany, the United Kingdom and the United States, coordinated by Jean-Baptiste Sallée.
Their study was published this Wednesday in the scientific journal
Nature
.
"We show that the fundamental structure of the ocean is changing under the effect of this climate change," explains Jean-Baptiste Sallée.
More precisely, the surface of the oceans is separated from the deep ocean.
There is like a barrier between the two, more and more difficult to cross.
"
Idealized diagram of the vertical structure of the world ocean.
- © Jean-Baptiste Sallée, Locean (CNRS / MNHN / IRD / Sorbonne University)
How to explain it?
“On the one hand, the surface of the ocean is heating up considerably.
On the other hand, in some regions, especially the poles, the salinity of the water is falling due to the melting of glaciers and the increase in precipitation, explains Jean-Baptiste Sallée.
As a result, the surface water becomes lighter.
”So that they sink less easily in the depths, forcing the diffusion of heat and carbon throughout the water column of the ocean.
This demarcation between surface and depth intensifies by 9% every ten years, indicates the study.
“It's much faster than we thought,” continues Jean-Baptiste Sallée.
But we also realized, in this study, that the surface layer thickens.
It is getting deeper and deeper, especially under the effect of intensifying winds, another consequence of climate change.
"
Impacts on the climate and biodiversity?
This phenomenon could have consequences on biodiversity.
Starting with phytoplankton.
First link in the marine food chain, these microscopic plants live in suspension in surface waters and drift with the currents.
“This phytoplankton is mixed in this surface layer, sometimes being found sometimes on the surface, sometimes at the bottom of this layer,” continues Jean-Baptiste Sallée.
But the more this layer widens, the more this phytoplankton will be stirred deeply.
And the less time it will spend on the surface, where it is best lit by the sun, the light it feeds on.
"
Bad news in the fight against climate change and the erosion of biodiversity, presented as the two great challenges of the 21st century?
Jean-Baptiste Sallée is not so categorical.
"Our study focused on the change in the structure of the oceans linked to climate change, that is to say this demarcation which intensifies between the surface layer and the depths of the oceans," recalls the CNRS researcher.
Further work is needed to measure the precise consequences, both on marine biodiversity and on the capacity of the oceans to capture heat and CO2.
"
“In both cases, we are faced with complex phenomena, with possible feedbacks,” he continues.
We can imagine that the phytoplankton will manage to adapt to this less exposure to light.
Perhaps also that the part of carbon or heat that the oceans will not absorb more or less will be captured elsewhere, on the land surface.
"
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CNRS / MNHN / IRD / Sorbonne University
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