The Mediterranean Sea is undergoing an extremely intense marine heatwave, which scientists say could be a one-in-a-billion climatological event. This means that the observed temperatures, which are eight degrees Celsius above the normal in some places for this time of the year, would be virtually impossible without accelerated global warming, a consequence of human-caused climate change.
During a marine heatwave, the temperatures of the surface and sub-surface of a sea are much higher than the normal for extended periods of the time.
While the marine life of the Mediterranean would be getting impacted by the much higher-than-normal temperatures currently, they also do not bode well for severe weather in the future months. The increased moisture in the atmosphere, resulting from the heat, could lead to more intense storms with catastrophic rainfall.
“What are the odds this July’s Mediterranean heat is natural? About 1 in a billion! It's the most extreme Mediterranean heatwave on record. Peak anomalies +8°C (+14F) now off France / Italy with absolute peak temps near 31°C (88F),” wrote Jeff Berardelli, chief meteorologist and climate specialist for WFLA TV news channel, on the social networking platform X (formerly Twitter).
The overall deviation of the sea surface temperatures (SST) in the Mediterranean is six standard deviations from the normal for this time of the year, which comes to around a one in a billion chance of the event occurring, according to Berardelli.
“Does it mean the Mediterranean (Med) is hotter than it has ever been? No. It means it is the hottest relative to where it should be in early July,” as per Berardelli.
The near surface temperatures in Europe have been higher than normal from the beginning of 2025, but a heat dome that developed in June has increased the temperatures drastically over Mediterranean Sea, becoming more adverse in July. This is only the beginning of the summer season in Europe.
“SST in the Mediterranean are now ‘in their own league’, already way above normal for the late summer peak,” explained Berardelli.
The worst-affected are parts of the western Mediterranean Sea, with the central region also severely impacted, according to Severe Weather Europe, a weather and climate analysis blog. In the western Mediterranean, the higher increases in temperatures were observed in Balearic Sea and the Tyrrhenian Sea.
The immediate reasons for a marine heatwave over a part of a sea are lower wind activity and less mixing of water among the layers of the sea, mainly due to the formation of a heat dome.
Within a heat dome, which is a large region of high pressure area in the atmosphere, the air sinks downward heating up the air. Unless an external weather system breaks the continuity of the dome, the extreme heat builds up within increasing temperatures, causing both atmospheric and marine heatwaves.
“Heat domes also strongly impact the seas, not only the land areas. The long-lasting and persistent excessive heat events gradually warm the water temperatures, which, without a significant frontal system, do not mix enough to remain cool. So, the sea waters accumulate heat and develop marine heatwaves,” according to the Severe Weather Europe blog.
“The Mediterranean heat dome is a continuation of a large high-pressure ridge across the Atlantic basin. Of course the Med dome is much more intense. So it begs the question, are these stronger heat domes over Europe driven by climate change? The answer is probably yes, due to circulation changes, and some excess heat also from reduced aerosols,” wrote Berardelli on X.
A research paper published in the journal Nature Communications Earth and Environment in December 2024 highlighted that when marine heatwaves in the Mediterranean Sea in recent years occurred concurrently with atmospheric heatwaves, they were much more severe.
Another research paper published in the journal Aquaculture in July 2024 showed that marine heatwaves have tripled in frequency in the last 40 years and last 50 per cent longer. The immediate impacts of the marine heatwave would be on the ecological systems of the Mediterranean and the livelihood of people dependent on it. The paper said that fishing areas in the Spanish part of the Mediterranean show a steady temperature increase of 0.75 C per decade.
Between 2015 and 2019, there were five consecutive mass mortality events in the Mediterranean due to marine heatwaves, that included thousands of kilometres of coastline and a range of marine habitats, according to a research paper published in the journal Global Change Biology in May 2022.
The high sea temperatures, especially if they continue into late summer, would be further cause of concern in terms of extremely heavy rainfall in the form of storms in the future months, according to the Severe Weather Europe blog. Such torrential rainfall can lead to flash floods and landslides causing loss of lives and damage to infrastructure.
“High moisture translates into higher temperature dew points, thus more convective energy to fuel thunderstorms in the coming weeks and months,” said Severe Weather Europe. A dew point is the temperature to which ambient air needs to be cooled for it to be 100 per cent saturated. “Also note that higher sea temperatures keep the air temperature dew points higher, resulting in more moisture for excessive and intense rainfall events,” the blog added.
“Close monitoring of the extremely warm Mediterranean Sea during summer is needed, as warm seas play an essential role in autumn rainfall events,” said Severe Weather Europe in its blogpost.
“Numerous cases have occurred where several hundred millimetres of rainfall led to destructive flooding across coastal Italy, Spain, the Alps and the western Balkans,” it added.
A year’s worth of rainfall fell on the intervening night of October 29 and October 30 in 2024 in many regions of eastern and south eastern Spain that led to the deaths of more than 200 people, in one of the worst weather-related disasters in continental Europe.