Parts of the Mediterranean are more than 6C warmer than normal for the time of year, scientists have said, sparking fears that the sea’s fragile ecosystems are suffering the equivalent of a “marine wildfire” and being permanently altered by global heating.
Water temperatures have been well above average since May and hit a peak of 30.7C off the eastern coast of Corsica last weekend, meaning the summer of 2022 is likely to set new records for both the intensity and duration of the marine heatwave.
Several areas of southern France this month experienced record air temperatures, which combined with low winds has produced a layer of surface water that is significantly hotter and much deeper than usual, marine ecologists say.
“A water temperature of 28C or 29C may feel pleasant to bathers, but it is worrying for the Mediterranean’s ecosystems,” Frédéric Denhez told BFMTV. “The Mediterranean is starting to resemble the Red Sea, and its species are not adapted to that.”
Rubén del Campo of the Spanish national meteorological service told Le Monde that with cooler deep water no longer rising to the surface, the Mediterranean’s native populations of “corals, of shellfish and of fish are suffering enormously”.
Scientists consider the Mediterranean a biodiversity hotspot, accounting for less than 1% of the world’s ocean surface but home to about 10% of all marine species. The sea hosts up to 20,000 marine species of fauna and flora, 25% of which are endemic.
“The most adaptable organisms will resist – although they may become weaker – by adjusting their physiology or migrating,” said Emilie Villar, a Marseille-based marine ecologist. “But weaker ones are likely to perish,” she told La Provence newspaper.
In all, 700 Mediterranean species are threatened with extinction, Villar said. “If the shock lasts too long, or if the species is fixed and cannot migrate, others will fill the void – or, if conditions become too harsh, the Mediterranean risks dying out.”
One recent study found maritime heatwaves had already destroyed up to 90% of coral populations in parts of the Mediterranean, with red corals and gorgonians or sea fans particularly hard hit. Sea urchins and sea sponges have also been badly affected.
David Diaz of the Spanish oceanographic institute told Le Monde such ocean heatwaves were “the equivalent of underwater wildfires, with fauna and flora dying just as if they had been burned”.
Posidonia or Neptune grass, which is endemic to the Mediterranean and plays a key role for ecosystems by storing CO2, has also been severely affected, scientists say. The scale of destruction is “not surprising”, said the oceanographer Jean-Pierre Gattuso.
“We are seeing both a gradual overall warming, which leads to a steady migration of marine species, and sudden spells of intense warming, which is causing significant mortality,” Gattuso told BFMTV.
A WWF report last year found that water temperatures in the Mediterranean were rising 20% faster than the global average, making it the world’s fastest-warming sea.
Nearly 1,000 exotic species – including 126 species of fish, several of them highly invasive and destructive of the Mediterranean’s marine habitat – had already migrated into the sea, the report said, some from the Red Sea via the Suez Canal.
The previous temperature record for the Mediterranean was set in August 2018, when the water off Marseille was measured at 6.6C higher than the seasonal average, while the sea’s longest marine heatwave so far observed was in 2003, lasting from 3 August to 2 September.