- Human-caused climate change added about 2°C to the Mediterranean event and 1.3°C around Britain and Ireland.
- Marine heatwaves covered 80% of the Celtic region, compared with an estimated 30% without present-day warming.
- Researchers warn that further warming will push some ecosystems beyond their ability to adapt.
Human-induced climate change has transformed exceptional marine temperatures around Europe into increasingly common events, with significant consequences for coastal economies, infrastructure and power systems.
World Weather Attribution analysed the warmest 14-day periods recorded this year across the Celtic seas, the Bay of Biscay and Iberian coast, and the eastern and western Mediterranean.
July produced the highest global sea surface temperature recorded for that month, while parts of the Mediterranean were as much as 6°C above normal earlier in the summer. The average Mediterranean temperature reached a July record of approximately 27°C, with one buoy near Mallorca registering 33°C in early August.
Researchers found that human-caused warming added about 2°C to temperatures in both Mediterranean regions, 1.4°C around the Bay of Biscay and Iberia and 1.3°C in the Celtic region.
The Celtic area includes the Irish Sea, English Channel, Bristol Channel, waters west of Scotland and much of the sea surrounding Ireland.
In a climate 1.4°C cooler, comparable Celtic temperatures would have occurred roughly once in 100 years. Human influence has made the event about 30 times more likely.
In the other three regions, the observed warmest annual 14-day periods would have been “virtually impossible” in a pre-industrial climate, the researchers concluded.
Heat spreads across entire marine regions
The study also assessed the geographical reach of marine heatwaves, defined as temperatures above the local 90th percentile for at least five days.
Approximately 80% of the Celtic region experienced marine heatwave conditions, compared with 30% in the counterfactual cooler climate. Coverage increased from 40% to 90% around the Bay of Biscay, from 40% to 80% in the western Mediterranean and from 9% to almost 70% in the eastern Mediterranean.
Marine heat can cause mass mortality among corals, sponges and macroalgae that provide habitats for other species. Fish populations can move north or into deeper water, but seabed species and fixed organisms have fewer opportunities to escape.
Claire Bergin of Maynooth University, one of the study’s authors, told The Guardian that the changes could “completely reshape ecosystems that coastal communities rely on for food and income”.
Warmer seas also increase coastal humidity and reduce overnight cooling, worsening the health effects of terrestrial heatwaves. Higher evaporation can add moisture to weather systems, potentially contributing to more intense rainfall later in the season.
The analysis followed World Weather Attribution’s established rapid-attribution methodology, combining observations with climate models. However, it has not yet been through peer review as a standalone study.
There are also uncertainties. Observed Mediterranean warming was stronger than the changes reproduced by climate models, leading the researchers to use a conservative attribution estimate.
Under a world 2.8°C warmer than pre-industrial conditions, the study estimates that an event equivalent to 2026 would become another 1.3°C to 1.9°C hotter.
For the energy industry, the findings widen the physical risk discussion beyond air temperature. Offshore installations, subsea equipment, coastal substations and thermal power stations were generally designed using historical environmental ranges. Marine ecosystems can also affect operations directly through changing biofouling, fisheries interactions and jellyfish concentrations near cooling water systems.
The immediate requirement is better monitoring and adaptation. The longer-term conclusion is less comfortable: the authors found that adaptation could reduce vulnerability but would not protect all species or economic activity if ocean warming continues.
