Will the Mediterranean Sea soon reach 30°C? Five questions about the current historic marine heatwave

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The heatwave currently affecting France is spreading to the Mediterranean Sea and could cause temperatures to rise to 30°C or more in the coming days, particularly off the French Riviera. This is a first since 2003.

• Did Mediterranean sea surface temperatures vary significantly this summer?

Since the beginning of summer, the surface temperature of the Mediterranean Sea has been yo-yoing. Already in June, during the first heatwave of the summer, the water was 5°C above normal, according to data from the European Copernicus Marine program. The average surface temperature reached 23.8°C.

By the end of July, the opposite phenomenon had occurred: the thermometer was only showing 17.5°C at the surface of the Mediterranean, under the influence of a mechanism called "upwelling" , which consists of deeper, generally cooler, waters rising to the surface under the effect of the wind. The surface temperature had dropped to 15°C, the water being colder than that of the North Sea or the Baltic.

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Decryption Marine heatwaves: what consequences for biodiversity?

In mid-August, the heatwave affecting France is once again heating up the Mediterranean. According to meteorologist Paul Marquis, interviewed by " Le Parisien ", "we can expect it to be 29°C or 30°C next weekend [of August 15] and perhaps even up to 31°C at the beginning of next week." A temperature never before reached.

The phenomenon of marine heatwave refers to a surface temperature exceeding 25°C, 26°C or 27°C for several days, over a wide area. "These phenomena have unfortunately not been exceptional for the last four or five years ," oceanographer François Sarano told "Le Parisien ." "What is worrying is that these episodes are occurring earlier and earlier and are becoming longer and longer," Jean-Baptiste Sallee, research director at the CNRS, explained to " Ouest-France ."

Due to global warming linked to human activities, the frequency of marine heatwaves has doubled since 1982, and their intensity is increasing, according to a 2019 report by the Intergovernmental Panel on Climate Change (IPCC). On average, the Mediterranean gains 0.4°C per decade and now experiences four marine heatwaves per year, compared to one forty years ago, reports " Le Monde ."

Such episodes lead to visible changes in marine ecosystems, endangering some emblematic species of the Mediterranean. Locally, some populations are simply threatened with extinction. In the summer of 2022, when the surface water temperature of the French Mediterranean had jumped, species of sea sponges were eradicated from certain depths. "It was the first time I used the term 'extinction'," Thierry Perez, a CNRS researcher at the Mediterranean Institute of Biodiversity and Marine and Continental Ecology, pointed out to "Nouvel Obs" during a previous episode of marine heatwave. "In August, two months after the heatwave, nothing had survived. And even today, above 20 meters deep, you can count them on the fingers of one hand on the Marseille coast," he explained.

Encouraged by increasingly warmer waters, hundreds of invasive species adapted to high temperatures, originating from the Red Sea, have instead invaded the Eastern Mediterranean via the Suez Canal, disrupting ecosystems, according to scientists. In September 2022, the Food and Agriculture Organization of the United Nations (FAO) warned of the presence of at least "900 non-native species" that have taken up residence there, "more than half of them permanently." This is the case, for example, of the lionfish (Pterois miles), a scorpionfish averaging 26 cm in length with long, spotted fins.

This "tropicalization" of the Mediterranean could also occur in the coming years via the Strait of Gibraltar, far from the Suez Canal, estimate the authors of a study published in April 2024 in the American scientific journal PNAS. According to an intermediate climate scenario, the warming of the Atlantic Ocean could lead certain species to move up from the southern coasts of West Africa to the western Mediterranean by 2050, the authors of the study predict. In a more pessimistic scenario, they warn, the Mediterranean will even be "entirely tropicalized" by 2100.

Marine heatwaves also promote the proliferation of "heat-dependent bacteria" such as Vibrio. The lack of oxygen in the water, caused by the increase in heat, can also allow toxic species to develop. This is the case, for example, of ostreopsis, a microscopic algae that can cause coughs, skin irritation or gastric disorders. It is developing so much that the French Agency for Food, Environmental and Occupational Health Safety (ANSES) issued a warning in June 2023. These tropical algae arrived in the Mediterranean around twenty years ago and have been spreading since 2018 in France on the Atlantic coast.

In addition to the threats to marine ecosystems, rising sea mercury leads to increased evaporation, which is conducive to the formation of storms, thunderstorms, and extreme rainfall. In September 2023, a "medicane"—a contraction of "Mediterranean" and "hurricane" —hit Libya: Storm Daniel left 4,333 dead and approximately 8,540 missing.

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