Waters between Spain's Balearic Islands and the Italian coast were up to 5°C warmer than at the same time last year, Spain's AMET weather agency said on Friday, while also warning that temperatures around the Spanish coast would be 3°C to 4°C higher until at least mid-August.
Spain's ports' authority said in a statement the water in Cabo de Gata, in the country's southeastern corner, registered a ten-year temperature record of nearly 28°C on Monday.
Marine heatwaves, which are far less researched than heatwaves on land, are becoming more frequent due to climate change, adding pressure to ecosystems already struggling from over-fishing and plastic pollution.
Ocean scientist Jean-Pierre Gattuso told Reuters that water near the French coastal city of Nice was measured at 29.2°C on July 25, around 3.5°C higher compared to the same day last year.
"This is an absolute record since at least 1994 and very likely earlier," he said. "The ocean and sea are kind of a sponge for the heat," Gattuso explained.
Marine heatwaves also struck the Mediterranean in 2015 to 2019, leading to mass die-offs of marine life, according to a study this week from Spain's Institute of Marine Sciences.
This year's heatwave is worse. "It is [lasting] longer, and also the magnitude is larger," Gattuso said. The die-offs "will probably come later in August."