From his oceanography laboratory in Villefranche-sur-Mer, Jean-Pierre Gattuso scrutinizes every day what scientists call the "marine heat wave phenomenon". “Since the beginning of June, we have been in the middle of it, reports the research director at the CNRS. We are living through a very unusual period. above the normals recorded during the last decade".
In Corsica, a peak of 30.7°C was recorded on Sunday July 24, off Alistro, a village located on the east coast of the Island of Beauty, according to the Keraunos observatory. While usually the temperature of the Mediterranean Sea oscillates between 21°C and 24°C. From the boot of Italy to Spain, the entire western Mediterranean has been experiencing a maritime heat wave since June.
An excess of energy in the Mediterranean
According to several researchers contacted by L'Express, the marine heatwave episode is directly linked to the strong continental heat that has hit southern and western Europe since the beginning of June. “A marine heat wave sets in when there is an excess of energy, and therefore an excess of heat”, observes Jean-Pierre Gattuso.
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"If the ocean is warming up, it is because it is the receptacle of too much energy released by human actions", adds Sabrina Speich, physicist oceanographer, specialist in climate change. We talk a lot about the warming of the atmosphere, but in truth, 90% of this heat that has accumulated since the pre-industrial era has escaped into the ocean.
The Mediterranean is far from the only place where the waters are warming up. In recent days, a sea heat wave has been observed on the Great Barrier Reef in eastern Australia, but also in Western Australia, according to the international working group on marine heatwaves. More surprisingly, the Arctic is also subject to an abnormal increase in these temperatures, also categorized by researchers as "a marine heat wave".
"The absence of wind favors these events"
Another phenomenon explains the warming of the oceans and seas: the absence of wind. "When the water is stable, horizontally and vertically, these heat phenomena tend to intensify, specifies Jean-Pierre Gattuso. Because in normal times, the wind allows hot water to mix in surface with the colder water of the depths, which generates a rapid cooling".
Problem: "There is no cooling weather signal in the coming days", indicates to L'Express Alix Roumanac, president of predict service, the risk subsidiary of Météo France. "In the Mediterranean, the freshness comes from the blows of the Mistral and the Tramuntana. But since June, we have been in an anticyclonic situation. The air is blocked, which holds back the winds", he adds.
Species appear, others disappear
These temperatures are not without consequences on the balance of marine life. "We already know that in the Mediterranean, nearly 50 species are threatened", explains Jean-Pierre Gattuso, co-author of an international study carried out by 70 researchers, about the harmful effects of sea heat waves on the fauna and underwater flora. For four years, between 2015 and 2019, the team of researchers found that 90% of the Mediterranean Sea had already been affected by an episode of abnormal water temperature increase. “Each year, there are one or more episodes of this type, the direct impact of which is the massive excess mortality of plants and animals”, announces the researcher from the Villefranche oceanography laboratory.
Scientists have noted the disappearance of sea urchins, sponges and even gorgonians (species of jellyfish). While the posidonia, emblematic plant of the Mediterranean and essential to its ecosystem, "supports these heats very badly". "So much so that Spanish researchers estimate that it could completely disappear from the Balearic Islands by 2050", worries Jean-Pierre Gattuso, whose team is currently trying to follow the slightest change. "We are working with a local diving club which has been asked to alert us if they notice any mortalities of organisms in the harbour, given the temperatures of the last few weeks".
When certain species disappear, others move on and, on the other hand, find good acclimatization conditions in the Mediterranean. "This is the case of barracudas but also rabbitfish (Siganus corallinus), reports Thierry Thibaut, professor of marine ecology at the Mediterranean Institute of Oceanography. This is not without impact on the functioning of the ecosystem. because the rabbitfish, for example, eat everything they find in terms of grasses and algae, which risks modifying the ecosystem".
Especially since nearly 50% of the world's population lives less than 50 kilometers from a coast, recalls Sabrina Speich: "If the ecosystem is impoverished, humans will suffer, from a food point of view but also economical.
Weather consequences for the fall
In addition, these marine heat waves are likely to have several meteorological effects by the fall. The specialists of Météo France fear significant rainy events from the start of the school year in September, even if it is too early to know if Italy, Spain or even France will be concerned. And in particular the appearance, as was the case last year in southern Italy, of a medicane (contraction between Mediterranean and hurricane in English). "It is an intense, sudden and rapid low pressure system that appears when the cold drops of autumn meet a mass of warm water", explains Alix Roumanac.
“The major floods that we had in the hinterland of Nice in 2019 followed an atmospheric and marine heatwave episode, adds Jean-Pierre Gattuso. This is what is more commonly called Cévennes episodes or Mediterranean episodes".
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