MONACO — The surface of the Mediterranean reached 30.4 degrees Celsius off the coast of Corsica in August, a record, but surface temperatures are a fickle measure, pushed up and down by a few days of wind. What alarmed oceanographers who gathered here this week was the layer beneath: the top 200 meters of the sea held more heat this summer than in any year since systematic measurements began, by a margin they described as outside anything in the record.
The finding, presented at a meeting of Mediterranean marine scientists at the principality’s oceanographic institute, draws on 240 robotic floats that drift through the basin, sinking and rising every 10 days to profile temperature and salinity. The heat content of the upper layer was 2.1 standard deviations above the 2000 to 2020 average, and a marine heat wave, defined as temperatures above the 90th percentile for the season, persisted for 71 consecutive days in the western basin.
“The surface warms every summer and cools every winter. Below 50 meters, the heat does not leave,” said Isabelle Marchetti, an oceanographer at the institute who presented the data. “What we are watching is the sea filling up with warmth from the top down, year after year, and the floats are telling us the filling has accelerated.”
The Mediterranean is nearly enclosed, shallow by oceanic standards, and surrounded by some of the fastest-warming land on earth, which makes it a kind of early-warning system for the global ocean. Its deep water, formed when cold winter winds chill the surface off the French coast and in the Adriatic, has in recent winters formed less often and less densely, meaning the heat accumulated in summer is not flushed downward as it once was.
The biological toll was visible before the physics was tallied. Nikos Papadakis, a marine ecologist at the Hellenic Centre for Marine Research, reported mass mortality of gorgonian corals and sponges at depths of up to 40 meters from the Balearic Islands to the Aegean, the fourth such event in six years and the deepest. “These are animals that live for centuries,” he said. “They do not recover between heat waves that come every other year.”
Fishermen have noticed, too. Salvatore Greco, who works out of Sciacca on the south coast of Sicily, said his nets this summer came up with species he had seen only in photographs. “Lionfish. Rabbitfish. The ones from the Red Sea,” he said. “And the sardines have gone somewhere colder. I would too.”
Dr. Marchetti said the floats would continue to report through the winter, when scientists will learn whether the deep-water formation that normally resets the sea occurs at all. “If it does not, the heat stays,” she said. “And then next summer starts from here.”
A version of this article appears in print on Oct. 12, 2026, Section A, Page 6 of the European edition with the headline: The Mediterranean Just Had Its Warmest Summer on Record Below the Surface, Too.
