Scientists warn that climate change is causing record marine heatwaves, with Portuguese man o' war and jellyfish increasingly appearing on European coasts, including the Croatian Adriatic.
Climate change has raised the temperature of the Mediterranean Sea by about 2°C, and the consequences are increasingly visible on European beaches: jellyfish and Portuguese man o' war, once rare visitors, now appear almost every year. The scientific network World Weather Attribution (WWA) has published a study showing that this year's record marine heatwaves from Ireland to the Mediterranean are largely a result of human influence on the climate.
According to data reported by the news agency Servimedia, some regions recorded sea surface temperatures as much as 6 degrees above normal. In the Bay of Biscay and along the Iberian coast, 90% of the area experienced marine heatwaves, as did 80% of the western Mediterranean. Without current global warming of 1.4°C, this would be expected to be around 40%. In the eastern Mediterranean, 70% of the area was affected by heatwaves, whereas without climate change, that percentage would be only 9%.
Researcher Claire Bergin from the Icarus Climate Research Centre at Maynooth University in Ireland highlights: "This summer has been exceptionally hot across Europe, but the impacts on our coastal waters have not been documented with the same depth. Our attribution analysis shows that anthropogenic climate change has directly added about 2°C to the Mediterranean Sea to date and between 1.3 and 1.4°C to the seas of western Europe."
Marine ecologist John Bruno from the University of North Carolina at Chapel Hill warns that extreme sea temperatures can lead to mass die-offs of fish, birds, and marine mammals. Warmer seas also increase humidity in coastal areas, further amplifying the risk of extreme heat on land.
Particularly concerning is the spread of Portuguese man o' war, organisms whose tentacles can reach 30 meters and whose sting can be extremely painful. Elvire Antajan from the French oceanographic institute Ifremer says that along the French Atlantic coast, they used to appear roughly every seven or eight years, but now they are recorded almost every year. "We see them more and more. We are a bit puzzled," she said.
On the French Basque coast, more than 1,000 stings from Portuguese man o' war were recorded in just two weeks, and about ten percent of those stung require hospital treatment. Authorities in Arcachon temporarily banned swimming at the main city beach, and warnings were also issued in San Sebastián and the Pyrénées-Atlantiques region. Portuguese man o' war have even been recorded as far north as the United Kingdom, up to Cumbria.
The scale of the problem also became evident in France, where an EDF nuclear reactor in the north of the country remained offline after a "massive influx" of jellyfish clogged the cooling system pumps. The Financial Times reports that this year's large jellyfish occurrence coincides with a historic marine heatwave in Europe.
Albert Calbet from the Institute of Marine Sciences in Barcelona cited the example of the mauve stinger jellyfish: its mass occurrences in the Mediterranean used to be recorded about once every 12 years, but now they appear almost every year. However, scientists caution that the link between sea warming and jellyfish is not straightforward. Professor Delphine Thibault from Aix-Marseille University explains that some species thrive better in cooler waters, and an increase in jellyfish near the coast can appear months after a heatwave.
The Croatian Adriatic has not been without unusual sights in recent summers either. Swimmers increasingly report large aggregations of gelatinous organisms, although they are not always jellyfish. Dr. Olja Vidjak from the Institute of Oceanography and Fisheries in Split explained that in some cases, these are planktonic tunicates, not jellyfish: "These are not jellyfish, but planktonic tunicates (Tunicata) from the order Salpida (salps). Together with doliolids (Doliolida) and pyrosomes (Pyrosomatida), salps belong to the class Thaliacea; they are not dangerous and have no stinging cells."
Recently, photos on social media have shown the sea near Rovinj teeming with jellyfish just below the surface, further confirming that changes in the marine ecosystem are increasingly felt in the Croatian part of the Adriatic.
