The European sea usually reserves its highest temperatures for August or September. But this year, at the end of July, it already reached its highest value since daily satellite records began in 1982. The same happened in the western Mediterranean. A new analysis shows that that previously extraordinary heat is becoming part of the usual climate. Human-caused warming has added around 1.4 °C to the maxima off the peninsula and nearly 1.9 °C in the western Mediterranean.
The study reconstructs a sea without human warming
To find out how this change has occurred, an international team of researchers from World Weather Attribution (WWA) analyzed four regions, from the Celtic seas to the two halves of the Mediterranean. They did not look for the fleeting maximum of a buoy, but the 14 consecutive days with the highest average temperature, throughout the year and in July.
First, they gathered three records from satellites, ships, buoys, and floats. Then they compared them with 12 models from the international climate project CMIP6 and discarded those that did not reproduce the observations well. Finally, they faced two worlds: the current one, 1.4 °C warmer than preindustrial, and another without human warming. This is how they calculated the added degrees and the increase in probability.
Heat once millennial now repeats every five years
The comparison reveals a huge difference. In 2026, the 14-day maximum average reached 21.4 °C in the Bay of Biscay and peninsula region and 27.9 °C in the western Mediterranean. These are averages over large areas, not isolated measurements.
Now, such an episode can repeat once every five years in both regions. Without human warming, it would have been expected once every 1,200 years off the peninsula and every 2,000 in the western Mediterranean. This is not a calendar but the probability of reaching or exceeding those temperatures.
That this year the maximum was already broken in July is especially unusual, since the sea usually reaches its maximum in August or September. Off the peninsula, such a month would be expected once every 20 to 90 years, according to the record. The range reflects uncertainty, but the previous record was half a degree lower.
The work shows that the change is also visible on the map. Almost 90% of the Bay of Biscay and peninsula region experienced a marine heatwave — at least five days among the warmest 10% usual —; without climate change it would have been less than 40%. In the western Mediterranean, the affected surface goes from 41% to 79%; in the eastern, from 8% to 67%.
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The Mediterranean advances faster than the models
Observations and models point in the same direction, but the thermometers advance faster. Annual maxima have increased 3.1 °C in the western Mediterranean and 2.6 °C in the eastern. By combining data and models, the part attributed to human warming is 1.9 and 1.8 °C. The authors admit that the figures may be underestimated.
“The uncertainty here is not so much in the direction of change as in its regional magnitude,” explains Carlos García-Soto, a CSIC-IEO researcher not involved in the work, to the Science Media Center Spain. More direct measurements, he adds, would help clarify the difference.
The heat that starts underwater reaches the coast
“Although for bathers from colder countries, like mine, warmer waters may seem attractive, the consequences for marine life below the surface are more serious,” warns Claire Bergin, a researcher at Maynooth University and lead author of the report, in a WWA statement. Warming can cause mass mortalities of corals, sponges, and underwater meadows. It also favors invasive species, bacteria, and some toxic algae, with effects on fishing, tourism, and health.
Ángel Borja, a researcher at AZTI not involved in the report, offers an example about the Basque Country. “This year we already had 25 °C in the water at the end of June, and during the summer it has remained practically at 24 °C. Before, this temperature was only reached occasionally in August,” he explains to the Science Media Center Spain. And he adds: Mass arrivals of jellyfish, salps, and other species that cause damage to fishing, aquaculture, or tourism are becoming more frequent.
The analysis was closed at the beginning of August, when the sea was still abnormally warm and had not yet reached the time of year when it usually records its maxima. The 2026 figures are, therefore, still a provisional snapshot. In addition, to measure the underlying trend, the study uses a four-year global temperature average, which reduces the transient effect of El Niño. Borja considers this a limitation of the work. “They do not mention anything about El Niño and this year is going to be or is being one of the most intense in history.” In his view, it could reinforce some extremes.
Marine heat also raises humidity on the coast. For each degree, the air over the water can contain about 7% more vapor and favor intense rains if other factors coincide, such as those that occurred in the 2024 Valencia floods. If the planet reaches 2.8 °C above preindustrial levels, a scenario compatible with current policies, episodes like that of 2026 would repeat practically every year in the four regions.
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