The Mediterranean is overheating, if not literally boiling. According to the French Meteorological Service, on Monday, June 30, 2025, the average sea surface temperature along the French Mediterranean coast reached a record high for the month of June at 26.04°C. This temperature is nearly 2°C higher than the 1991-2020 average, and in marine areas around the French Riviera, Corsica, the Gulf of Lion, and the Balearic Islands, temperatures have risen by as much as 5°C.
According to a report by the French newspaper Le Monde, this exceptionally early heatwave affecting mainly the western part of the Mediterranean could seriously test marine ecosystems and fuel extreme weather events if it persists or intensifies.
Timothée Guinaldo, an oceanographer with France’s National Meteorological Research Center (Météo-France/CNRS), noted that marine heatwaves are now common in the Mediterranean. Whereas they occurred only once a year in isolated regions forty years ago, they now appear on average four times per year across the entire basin.
“The Mediterranean is becoming increasingly warmer both in summer and winter, and we never reset the clock,” said Guinaldo, warning that prolonged marine heatwaves could lead to mass species die-offs, including sea urchins, mollusks, corals, and mussels.
Species that are more fixed to one area are at risk because they cannot move to less exposed locations, explains Justiniano Martinez, a researcher at the Institute of Marine Sciences in Barcelona and the Catalan ICATMAR institute. He also highlights that rising temperatures lead to oxygen depletion in seawater, though researchers have not yet observed mass mortality related to heat. However, he adds, there is already an impact on reproduction processes in shellfish, causing premature spawning.
Jean-Pierre Gatuzou, another oceanographer, warns that the Mediterranean is warming at a rate of 0.4°C per decade, leading to its “tropicalization.” He notes that 1,200 species have arrived in the Mediterranean from the Red Sea over the past twenty years, including lionfish, jellyfish, and plankton species. Some have permanently established themselves, particularly along Turkish and Greek coasts, sometimes reaching as far as France.
A warmer Mediterranean means more climate-related extreme weather events. Warm, moist air from the basin prevents nighttime temperatures from dropping along coastal areas, contributing to an increase in tropical nights—when temperatures remain at or above 20°C. It also causes intense storms and exacerbates “Mediterranean episodes,” violent downpours that typically occur in autumn but require specific atmospheric conditions to trigger them. Global warming makes these severe phenomena even more intense and frequent, Gatuzou concludes.