The Mediterranean Sea has experienced an unprecedented situation this summer. Its water temperatures have broken all known records for this time of year, and this is no coincidence: behind this extreme heat lies the hand of humankind. An international team of scientists has confirmed that climate change has added around two degrees to the surface temperature of the Mediterranean, a figure that doubles the average global warming rate. The study, coordinated by the World Weather Attribution (WWA) network, makes it clear that the sea that washes our shores is no longer what it was decades ago.
Researchers from Switzerland, Sweden, the United States, Ireland, and the United Kingdom analyzed four European coastal regions, from the Celtic Sea to the eastern Mediterranean. In all of them, they found an upward trend exceeding global averages. But the phenomenon is most pronounced in the Mediterranean. While in the Bay of Biscay and the Iberian Peninsula, warming attributable to human activity is 1,4 degrees Celsius, in the western Mediterranean it reaches 1,9 degrees Celsius. And observational data point to even greater increases: up to 3,1 degrees Celsius in the western Mediterranean and 2,6 degrees Celsius in the eastern Mediterranean since the pre-industrial era.
A heat wave that used to last for thousands of years now repeats itself every five years
To reach these conclusions, the WWA team didn't just look at the thermometer. They combined three satellite records with data from ships, buoys, and floats, and compared them with twelve climate models from the international CMIP6 project. Then, they compared the current world, 1,4 degrees warmer than in pre-industrial times, with a hypothetical scenario without human-caused warming. The result is devastating: a marine heatwave like the one in 2026, previously expected once every 1.200 years off the Iberian Peninsula and every 2.000 in the western Mediterranean, could now occur every five years.
The average maximum temperature over 14 consecutive days reached 21,4 degrees Celsius this year in the Bay of Biscay and the Iberian Peninsula, and 27,9 degrees Celsius in the western Mediterranean. These are averages across large areas, not isolated measurements. And most strikingly, this record has already been broken in July, when the sea typically reaches its warmest point in August or September. A month as extreme as July 2026 would not be expected more than once every 20 to 90 years, according to historical records.
The Mediterranean is advancing faster than the models
One of the aspects that most worries the scientific community is that observations are outpacing predictions. Thermometers are registering greater warming than that projected by climate models. Annual maximum temperatures have increased by 3,1 degrees Celsius in the western Mediterranean and 2,6 degrees Celsius in the eastern Mediterranean, while the portion attributable to human activity remains at 1,9 and 1,8 degrees Celsius, respectively. The authors of the study admit that these figures could be underestimates.
Carlos García-Soto, a researcher at the CSIC-IEO not involved in the study, explains that the uncertainty lies not so much in the direction of the change as in its regional magnitude. More direct measurements would help clarify this difference. The European sea is entering uncharted climatic territory, and models are not yet able to capture the full speed of the phenomenon.
The heat that starts underwater reaches the coast
The consequences of this warming go far beyond what is visible on the surface. Claire Bergin, a researcher at Maynooth University and lead author of the report, warns that warmer waters may seem appealing to swimmers, but the consequences for marine life are serious. Extreme heat causes mass die-offs of corals, sponges, and seagrass meadows, and encourages the spread of invasive species, bacteria, and toxic algae. All of this has direct effects on fishing, tourism, and human health.
Ángel Borja, an AZTI researcher not involved in the study, offers a very relevant example: in the Basque Country, the water temperature had already reached 25 degrees Celsius by the end of June, and throughout the summer it has remained around 24 degrees. Previously, that temperature was only recorded sporadically in August. Massive blooms of jellyfish, salps, and other species that cause damage to fishing, aquaculture, and tourism are becoming increasingly frequent.
A warmer sea, more humidity, and a greater risk of torrential rains
The heat from the sea doesn't stay in the water. For every degree Celsius increase, the air above the sea can hold about 7% more water vapor. This extra moisture, when combined with other atmospheric factors, can fuel torrential rains like those that caused the floods in Valencia in October 2024. The Balearic Sea, off the Valencian coast, reached about 23 degrees Celsius, 1,5 degrees above average, and the event resulted in extraordinary rainfall totals such as 771 liters per square meter in Turís.
Experts clarify that a warm sea does not, in itself, guarantee a destructive storm. However, it does act as a risk multiplier: it provides more energy and moisture to convective systems. If the planet reaches 2,8 degrees Celsius above pre-industrial levels, a scenario compatible with current policies, episodes like the one in 2026 would be repeated practically every year in the four regions studied.
The expansion of marine heatwaves is another of the study's most striking findings. In 2026, 90% of the Bay of Biscay and the Iberian coast experienced a marine heatwave, and in the western Mediterranean, the affected area reached 80%. Without climate change, these figures would drop to 40% in both cases, and in the eastern Mediterranean, they would fall from the current 70% to just 9%. The Celtic region, meanwhile, would decrease from 80% to 30%.
Friederike Otto, Professor of Climate Science at Imperial College London, is emphatic: the seas are warming at a rate exceeding global warming averages, with direct consequences for life on land and in the sea. Delaying the transition to alternative energy sources is pushing many ocean ecosystems closer to extinction. She adds that we cannot simply try to adapt: there are insurmountable limits to adaptation in the oceans, and the longer we delay abandoning fossil fuels, the higher the price will be.
The analysis was completed in early August, when the sea was still abnormally warm and had not yet reached its peak temperature. The 2026 figures are therefore still a provisional snapshot of a relentless phenomenon. The Mediterranean is unable to escape this heatwave, and experts warn that the consequences will be irreversible if urgent action is not taken.
