Global ocean temperatures have reached record highs, with the average sea surface temperature this summer reaching 21.1°C, according to the Copernicus program, a European Earth observation initiative. This marks the hottest year on record since measurements began, with ocean warming now reaching 1.1 to 1.2°C above pre-industrial levels. The temperature rise has far-reaching effects, influencing marine ecosystems and even the climate in land areas far from the coast, such as Switzerland. A buoy near Majorca in the Balearic Islands recorded a sea temperature of 33°C, which is nearly as warm as a hot bath. This extreme temperature is not confined to the Mediterranean; in some regions, water temperatures have exceeded seasonal averages by as much as 6°C. Thomas Frölicher, a professor of climate and environmental physics at the University of Bern, explains that the most vulnerable marine ecosystems are nearing the limits of their ability to adapt, which could lead to significant impacts on biodiversity, fisheries, food security, and coastal communities. The primary cause of ocean warming is greenhouse gas emissions, including carbon dioxide and methane, which trap heat in the atmosphere. Oceans absorb over 90% of the excess heat generated by human activities. Marine heatwaves, similar to extreme heat events on land, are becoming more frequent, intense, and prolonged. In Europe, the proportion of ocean waters affected by these heatwaves has risen from about 40% in the 1980s to nearly 98% during 2023–2025. A particularly severe marine heatwave in the northeast Pacific, lasting almost a year, occurred about a decade ago, according to Frölicher. Warming is not limited to the ocean's surface; heat accumulates throughout the water column, especially in the first 2,000 meters. Recent measurements in areas like Antarctica and the North Atlantic show that even deep waters are warming. Climate change, driven largely by the burning of fossil fuels such as coal and oil, is the main factor behind rising temperatures in European seas, according to a study by the World Weather Attribution (WWA), a group of experts analyzing climate change's role in extreme weather events. El Niño, a natural and cyclical climate phenomenon, causes unusual warming in the equatorial Pacific. This year's El Niño is particularly intense, possibly the strongest ever recorded. It redistributes heat between the tropical Pacific and the atmosphere, temporarily increasing global sea surface temperatures beyond those caused by climate change alone. A recent study indicates that El Niño events are now more extreme than at any point in the last 1,000 years. Ocean warming influences weather patterns in Europe by increasing the energy and moisture content of storms, leading to more intense rainfall and evaporation. It can also amplify the strength of tropical cyclones and affect monsoon systems. In Europe, exceptionally warm Mediterranean waters have been linked to extreme precipitation events, such as the floods in the Spanish city of Valencia in 2024. High temperatures in the Mediterranean increase evaporation, enriching the atmosphere with heat and moisture. When these air masses reach southern Switzerland, they can trigger heavy rainfall, as explained by Frölicher. Similar processes in the North Atlantic can lead to intense precipitation in Switzerland when warm, humid air meets the Alps. Exceptionally high sea surface temperatures and marine heatwaves are having "devastating consequences for marine ecosystems," warns WWA. They are causing mass mortality in key species like corals, sponges, and macroalgae. In the Mediterranean, invasive species such as the lionfish are spreading, threatening local ecological balance, according to Guido Gnone, scientific coordinator of the Aquarium of Genoa. While some species may benefit from new habitats, such as the Mediterranean sea turtle Caretta caretta, climate change is likely to lead to significant biodiversity loss. Warmer seas also promote the spread of waterborne diseases like vibriosis. The proliferation of jellyfish, encouraged by these new environmental conditions, can harm beach tourism and lead to temporary beach closures. Adaptation measures are essential. According to the United Nations University's Institute for Environment and Human Security (UNU-EHS), creating marine protected areas, managing fisheries sustainably, and restoring coastal ecosystems like mangroves can help build ocean resilience.