Carbon pollution has driven a massive expansion of marine heatwaves across European waters in 2026, according to a rapid analysis by World Weather Attribution. Some areas recorded sea temperatures as much as 6C above normal, while the hottest two-week periods would have been virtually impossible in a preindustrial climate in three of the four regions studied.
The analysis found that climate breakdown accounted for about two-thirds of the observed excess heat. It estimated that human-caused warming raised temperatures by about 2C in the Mediterranean, 1.4C in the Bay of Biscay and Iberian Peninsula, and 1.3C around Ireland and the Channel.
Where the heat has expanded
Marine heatwaves covered 90% of the Bay of Biscay and Iberian coast region and 80% of the western Mediterranean during 2026, the analysis found. In a climate without human-caused warming, researchers estimated those areas would have been about half as large.
The eastern Mediterranean saw its expected heatwave area fall from 70% to 9% when the effect of climate breakdown was removed from the analysis. In the Celtic region, covering much of the waters around the British Isles, the heat reached levels that historically occurred about once a century; without climate breakdown, the expected affected area fell from 80% to 30%.
Related coverage: Sea temperatures reached a July record during Europe’s extreme heat.
What the analysis shows
Researchers compared observed sea surface temperatures with climate-model simulations to assess the role of global heating. In three of the four regions, they said the hottest fortnights of 2026 would have been virtually impossible in a preindustrial climate. The Celtic region was the exception to that finding, although its heat was still unusually frequent by historical standards.
The analysis has not yet been submitted for peer review. It is World Weather Attribution’s first study focused on the effect of fossil-fuel burning on marine heatwaves.
Why unusually hot seas matter
Hot seas can kill habitat-forming species such as corals and macroalgae, disrupt ecosystems and contribute to large-scale mortality among fish and seabirds. Warmer oceans can also raise dangerous overnight temperatures in coastal areas because seawater cools more slowly than land.
More context: Heat Levels in Nearly One-Fifth of World Cup Matches Raise Safety Concerns.
Claire Bergin, a Maynooth University scientist and co-author of the analysis, said the warming could reshape ecosystems on which coastal communities rely for food and income and could drive damaging extreme weather on land. Separate research published in January found that 5% to 25% of extreme rainfall over coastal land is accompanied by a nearby marine heatwave, while rainfall during extreme downpours influenced by a strong heatwave was 20% to 30% higher.
Also read: European Heat and Drought Put Olive Oil and Food Crops at Risk.
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