BRUSSELS — Europe’s seas are overheating and climate change is to blame, researchers said Wednesday, warning that marine heat waves are threatening ecosystems and coastal communities. 

The Atlantic and the Mediterranean are far hotter than normal, with some parts of the oceans around Europe reaching 6 degrees Celsius above average this week.

The culprit is climate change, scientists from the World Weather Attribution consortium said in a study released Wednesday. They found global warming is the driver behind this year’s high ocean temperatures.

The report lands during a summer marked by record-breaking heat waves on land, droughts and fires — all worsened by climate change, according to the same scientists. In the sea, the soaring temperatures harm marine species, boost jellyfish populations and increase the risk of destructive storms later in the year.

“This summer, we’ve seen some really exceptional ocean temperatures around Europe,” said Catherine Gregory, a climate scientist at the University of Bern who contributed to the study.

“By July, the average temperature of the entire Mediterranean Sea was just over 27C, which was the highest July value on record,” she added. “We’ve had a buoy reading to the west of Mallorca measuring more than 33C in open water … and we’ve seen similarly extreme conditions in the North Atlantic.” 

The scientists used peer-reviewed methods to analyze four regions: The Eastern and Western Mediterranean; the Atlantic around the Bay of Biscay and Iberian Peninsula; and the seas around Ireland and western Britain. 

“We find it’s the most extreme July marine heat wave conditions in the record for the Western Mediterranean, the Celtic region, and for the Bay of Biscay-Iberian Peninsula region,” said Gregory. 

The scientists concluded climate change made both Mediterranean regions 2C warmer while heating the Iberian waters by 1.4C and the Irish seas by 1.3C. 

Global warming has also vastly expanded marine heat waves, the researchers found when comparing this year’s conditions to a simulated world without climate change. This year, 90 percent of the Bay of Biscay experienced hot temperatures and 80 percent of the Western Mediterranean; without global warming, only 40 percent of each region would have been affected.

The difference is even more dramatic in the Eastern Mediterranean, where 70 percent of the seas experienced marine heat waves this year, a figure plunging to 9 percent in a world without climate change. Around Ireland, the extent drops from 80 percent to 30 percent. 

Oceans absorb the vast majority of the excess heat produced by burning fossil fuels. The warming waters push some species to relocate in search of cooler seas, while those that cannot move — such as corals or seagrasses — are at risk of mass die-offs. 

“We have what we call winners and losers. So a lot of species wane in abundance or completely disappear while others bloom and proliferate,” said John Bruno, an ecologist from the University of North Carolina at Chapel Hill and one of the study’s co-authors. 

In the United Kingdom, for example, fishermen have found octopuses spreading north in the warming waters, decimating the shellfish catch with their voracious appetite for crabs. In France, jellyfish — which thrive in warmer waters — are clogging up nuclear power stations. 

Beyond the impact on ecosystems and the fishing industry, marine heat waves increase the extreme weather threat to coastal communities. The scientists noted that warm oceans supercharge the dangerous combination of heat and humidity along the shores, while also building up moisture that can fuel storms, such as the deadly 2024 floods in Valencia. 

“We find that when we have very warm sea surface temperatures, they provide added energy to storms,” said Gregory. “And then we also have the added impact of additional stress during land heat waves as well.”