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 thing happened in the western Mediterranean. A new analysis shows that the previously extraordinary heat is becoming part of the normal climate. Human-caused warming has added about 1.4°C to highs off the peninsula and about 1.9°C in the western Mediterranean.
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. He did not look for the fleeting maximum of a buoy, but for the 14 consecutive days with the highest average temperature, throughout the year and in July.
First they gathered three records of satellites, ships, buoys and floats. They then 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 the preindustrial one, and another without human warming. This is how they calculated the added degrees and the increase in probability.
The comparison reveals an enormous difference. In 2026, the 14-day mean high reached 21.4°C in the Bay of Biscay and Peninsula region and 27.9°C in the western Mediterranean. They are averages of large areas, not isolated measurements.
Now, such an episode can happen 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. It is not a calendar, but the probability of reaching or exceeding those temperatures.
That this year the maximum was already reached 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 registry. The range reflects the uncertainty, but the previous record was half a degree lower.
The work shows that the change is also seen on the map. Almost 90% of the Bay of Biscay region and the peninsula suffered a marine heat wave – at least five days among the usual warmest 10%; Without climate change it would have been less than 40%. In the western Mediterranean, the affected area goes from 41% to 79%; in the east, from 8% to 67%.
Observations and models point in the same direction, but thermometers are moving faster. Annual maximums have increased by 3.1 °C in the western Mediterranean and 2.6 °C in the eastern Mediterranean. Combining data and models, the part attributed to human warming remains at 1.9 and 1.8 °C. The authors admit that the figures may be understated.
“The uncertainty here is not so much in the direction of change as in its regional magnitude,” Carlos García-Soto, a researcher at the CSIC-IEO who is not involved in the work, explains to the Science Media Center Spain. More direct measurements, he adds, would help clarify the difference.
“Although warmer waters may seem attractive to bathers in colder countries like mine, 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 massive 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, an AZTI researcher not involved in the report, offers an example from the Basque Country. "This year we already had 25 °C in the water at the end of June, and during the summer it has practically remained 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 maximums. The figures for 2026 are, therefore, a still provisional photograph. In addition, to measure the background trend, the study uses a four-year average of global temperature, which reduces the temporary effect of El Niño. Borja considers that this is a limitation of the work. "They don't mention anything about El Niño and this year is going to be or is being one of the most intense in history." In his opinion, it could reinforce some extremes.
The sea heat also increases the humidity on the coast. For each degree, the air above the water can contain around 7% more vapor and favor intense rains if other factors coincide, such as those that occurred in the Valencia floods in 2024. If the planet reaches 2.8 °C above the pre-industrial level, a scenario compatible with current policies, episodes like the one in 2026 would be repeated practically every year in the four regions.
