With 30.8°C off Corsica and 29.4°C at Villefranche-sur-Mer, temperatures in the Mediterranean have reached record levels. – Technologist

Once again, the Mediterranean is overheating. The ongoing marine heatwave in the Ligurian Sea continued on Monday, August 12, reaching record levels. According to satellite data, over the past few days temperatures were 5 degrees above the reference period (the average of temperatures measured between 1991 and 2020) in the area between Nice, Corsica and the Gulf of Genoa in Italy. Extremely high temperatures were also recorded at local survey points.

On Sunday, August 11, a buoy at the entrance to the harbor at Villefranche-sur-Mer (department of Alpes-Maritimes), near Nice, registered 29.4°C, exceeding the 2022 record of 29.2°C. “This is unprecedented at this location,” said Jean-Pierre Gattuso, research director at the Oceanographic Laboratory in Villefranche. “This marine heatwave stands out from previous ones owing to its intensity.”

Another buoy, installed between Nice and Corsica, measured a temperature of 29.7°C late on Sunday, compared with 28.9°C on August 5. On Saturday, the temperature was also 30.8°C off Porto-Vecchio, Corsica. “We’re really seeing the effect of this weekend’s heat,” said Thibault Guinaldo, an oceanographer at the French National Center for Meteorological Research (Météo-France/CNRS). “We’re reaching orders of magnitude similar to those of the summer of 2022,” marked by extreme heatwaves.

This heatwave, which began in mid-July, has been classified as category two (out of four) for several weeks and is therefore considered severe. It is due to a combination of two phenomena: Firstly, very high temperatures in south-east France, where heatwaves have followed one after another, and secondly, the absence of wind, which prevents surface water from mixing with the cooler water further down. “At the beginning of July, we were right in the normal range for the season, but we’ve gained almost 6 degrees in one month!” said Guinaldo.

Growth of filamentous algae

The warming of seas and oceans, which absorb around 90% of greenhouse gas emissions, is directly linked to global warming caused by human activities, and in particular by the combustion of fossil fuels (oil, gas and coal). Since the 1980s, the Mediterranean has been warming by an average of 0.4 degrees per decade. “This latest heatwave comes as no surprise to scientists: The Intergovernmental Panel on Climate Change’s models predict that these phenomena will become more frequent, longer-lasting and more intense with global warming,” said Gattuso, “but it is very worrying.”

One of the main consequences of this type of extreme event is the mass mortality of certain species particularly vulnerable to excessive temperatures, such as gorgonians, oysters, mussels, sea urchins and corals. Warming also causes the growth of filamentous algae, which develop on gorgonians and suffocate them. “Every heatwave episode in the Mediterranean is followed by massive mortality,” said Gattuso. “At the end of September, divers generally start reporting the death of specimens.”

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