Spain is currently experiencing an unprecedented warming of its coastal waters, reaching record levels and having far-reaching effects on the weather, human health, and delicate marine ecosystems. Experts warn of “uncharted territory” and emphasize the urgency of combating global warming.
Alarming Records: Early Heatwaves in the Mediterranean and Cantabrian Sea
The data is clear: already by the end of June this year, the Dragonera buoy in the Balearic Islands reached a water temperature of 30.5 °C. This is the highest temperature ever recorded in this month in the western Mediterranean and is only slightly below the absolute record of 31.8 °C recorded last August at the same buoy. These “marine heatwaves,” comparable to the increasingly intense atmospheric heatwaves, have far-reaching consequences. They increase the number of tropical nights, create a more humid environment, and affect human health as the body has no opportunity to cool down and recover.
Samira Khodayar, an expert at the Centre for Environmental Studies of the Mediterranean (CEAM), puts it plainly: “It is another example of global warming. We have temperatures typical of August, we are in uncharted territory.” The first heatwave of summer 2025 was accompanied by a marine heatwave, showing anomalies of almost seven degrees above average in the Mediterranean and between 4.5 and 5 °C in the eastern Cantabrian Sea. The Dragonera buoy recorded 30.5 °C on June 30, an all-time high for this time of year, when temperatures are usually only 24 °C.
Northern Spain Also Affected: New Highs in the Cantabrian Sea
Not only the Mediterranean is affected by this development. Unusually high temperatures are also being recorded in the Cantabrian Sea. The buoys of Bilbao-Vizcaya reached record values of 23.93 °C and Villano-Sisargas 19.99 °C on the last day of June. The buoy of Cabo de Peñas also recorded a maximum of 20.97 °C on June 27. These temperatures have continued to rise, with 24.4 °C in Bilbao Vizcaya on July 5 and 20.1 °C in Gijón on July 2. This corresponds to anomalies of 4.5 to 5 °C above normal in the eastern Cantabrian Sea. José Miguel Viñas, a meteorologist at Meteored (tiempo.com), expresses concern: “It is very striking that at this time of summer, the water temperatures in La Concha, San Sebastián, are already 25-26 degrees. This can happen at the end of summer, but for it to happen now is very strange.”
The Mediterranean: A Climate Change Hotspot
Samira Khodayar warns that the Mediterranean, the area most affected by climate change, is warming 20 percent faster than the rest of the planet. “All regions of the world are suffering from global warming, but with different intensities. The Mediterranean region is particularly sensitive to the effects of climate change,” says the CEAM coordinator. The reasons for this lie in the Mediterranean’s low interaction with other bodies of water and the high solar radiation due to its latitude.
Mar Gómez, Doctor of Physics and head of meteorology at eltiempo.es, adds that this year’s record temperatures in the Cantabrian Sea surpass the marine heatwave of 2022, which was already considered an unprecedented event. The western Mediterranean has warmed by more than 1.5 °C since the 1980s, and the frequency of marine heatwaves has doubled within a decade.
Impacts on Health and Meteorology: A Double Threat
This ongoing warming has serious consequences for human health and the weather. Marine heatwaves bring more water vapor into the atmosphere, leading to an increase in tropical nights – nights with a minimum temperature above 20 °C. This reduces the cooling effect of the night breeze, which becomes warmer and more humid, affecting thermal comfort and rest. Particularly vulnerable groups such as children, the elderly, and the chronically ill are severely affected.
Samira Khodayar warns: “We have nights with higher temperatures, what we call tropical nights, over 20 degrees and even hot, over 25 °C. A few decades ago, we talked about 20 nights per summer, and last summer we exceeded 100 nights with more than 20 °C. We are seeing an extreme of the most extreme phenomena.” The continuous heat, day and night, for long periods poses an enormous strain on the human body. Sleep disturbances affect concentration, cognitive performance, and mood, as Mar Gómez adds.
The State Meteorological Agency (Aemet) emphasizes that heat is a “silent threat” that causes thousands of indirect victims each year, as it aggravates pre-existing pathologies. In coastal areas, this excess mortality is closely linked to anomalous sea temperatures. The confluence of atmospheric and marine heatwaves intensifies these effects.
Ecological and Meteorological Consequences: A Cascade of Problems
Such warm seas not only affect human health but also the environment. They lead to drier conditions on land, favor heatwaves, prolong droughts, and increase the risk of wildfires, which are becoming more intense and difficult to control. Furthermore, sensitive marine ecosystems such as corals, Posidonia meadows, and algae are being damaged, all of which are essential for biodiversity and CO₂ absorption. If the sea loses this capacity, climate change accelerates even further.
José Miguel Viñas points to another consequence: if atmospheric instability prevails, the increased water vapor can lead to heavier rain or hail showers. This means that in combination with an isolated low-pressure area at high altitudes (DANA), very intense storms can form with torrential rains, flash floods, and extreme wind or hail episodes.
Outlook for Summer: Uncharted Territory
Given these developments, the question arises as to what sea temperatures can be expected by the end of summer. For Samira Khodayar, the maximum sea surface temperatures are usually recorded in August. Since temperatures at this level have already been reached weeks before August, a further record break is likely if conditions remain as they are. The trend indicates a continuous and gradual increase.
Justino Martínez, a researcher at the Institute of Marine Sciences of the CSIC and ICATMAR, confirms that the atmosphere plays the leading role: “The sea warms up because the atmosphere is hot.” It is already becoming clear that summers are getting longer and spring and autumn are getting shorter. High temperatures will now accompany us for most of the summer and parts of spring and autumn.
