Marine heatwaves have engulfed much of Europe's coastal waters this summer, with human-caused global warming responsible for roughly two-thirds of the excess heat observed, according to new analysis. Some areas recorded sea-surface temperatures more than six degrees Celsius above normal.
The Bay of Biscay and Iberian Peninsula coast saw 90 percent of their waters hit by marine heatwaves in 2026, while 80 percent of the western Mediterranean experienced the same. Climate change has warmed Mediterranean waters by about two degrees and the Bay of Biscay region by 1.4 degrees. Without human-driven warming, those heatwave figures would be roughly half their current levels. The hottest fortnights of the year would have been virtually impossible in a preindustrial climate in three of four sea regions studied.
The ecological shifts are already visible: fishermen in southwest England have reported a sharp increase in common octopus sightings since 2025, a species native to warmer North African waters. Rising sea temperatures also increase coastal humidity and heighten extreme rainfall risks, with downpours influenced by strong marine heatwaves producing 20 to 30 percent more rain.