Photo by Joshua Humpfer
The global average sea surface temperature has broken its all-time record. The Copernicus Climate Change Service (C3S) confirmed the figure reached 21.1°C on 22 August, surpassing the previous record of 21.09°C set in March 2024. The timing is notable in itself: global ocean temperatures typically peak in March and April, yet this record was set in August.
“The record is another clear signal of an ocean under growing stress,” said Samantha Burgess, Strategic Lead for Climate at ECMWF, which implements C3S. “El Niño is adding heat to the system, but it is doing so on top of decades of human-driven warming. As ocean temperatures continue to rise, the risks to marine ecosystems and coastal communities also increase. The consequences are already visible: the number of marine heatwave days globally has more than tripled since the early 1990s, putting increasing pressure on marine ecosystems and the communities that depend on them,” she said.
Already visible in UK waters
The record comes as El Niño continues to strengthen, with the Met Office warning it is on course to be the strongest in centuries and expected to keep intensifying until Christmas. Dr Jeremy Grist, senior research fellow at the National Oceanography Centre in Southampton, said the timing of the record was itself telling. “The fact that we are already breaking records is an early indicator of how strong the El Niño is becoming,” he said. “All things being equal we might expect the ocean temperature record to be broken again in March or April 2027.”
Waters around the UK are already experiencing the consequences directly. The western English Channel has seen almost continuous marine heatwave conditions for more than three years, peaking at 7°C above normal in July, according to Professor Tim Smyth, director of science at Plymouth Marine Laboratory. “This is unprecedented,” he said, adding that “the latest Copernicus data reinforce the troubling upward trend in ocean temperatures.”
New findings on frequency further west
Separate analysis published this month by the World Weather Attribution (WWA) network puts a number on how much more often these events are now striking Irish and wider European waters. Researchers studied four marine regions over two weeks in July, including the Celtic region covering Ireland’s Atlantic and southern waters and the Irish Sea, alongside the Bay of Biscay, the Western Mediterranean and the Eastern Mediterranean. The most severe warming was in the Western Mediterranean, where the average July temperature reached 27°C and one buoy recorded a record 33°C; waters around Ireland fared comparatively better, at 1.3°C above normal for July.
Claire Bergin, a researcher at Maynooth University’s Icarus Climate Research Centre and part of the WWA network, said the change in frequency was stark. A marine heatwave in Ireland would have been expected once a century in the pre-industrial era, she said, but is now expected roughly every three years. “It will happen every year as climate change intensifies,” she said, warning the shift “has the potential to completely reshape ecosystems that coastal communities rely on for food and income, and drive damaging extreme weather on land too.”
Thomas Frolicher, an environmental physicist at the University of Bern, drew the same conclusion in separate research published the previous week. “From the Atlantic waters off Ireland to the Mediterranean Sea, Europe’s seas are experiencing an alarming surge in temperatures,” he said. “Our research shows clearly that this is being driven by climate change, in particular by the continued burning of fossil fuels.”
Marine ecologist Professor John Bruno explained why small rises matter so much biologically: because most marine species are cold-blooded, their metabolism tracks water temperature directly, so warming waters mean animals need to eat more and many starve or their enzymes stop functioning correctly, with effects rippling up through the food chain. “We are rapidly approaching the absolute biological limits of adaptation for many species, threatening biodiversity, fisheries and coastal food systems,” he said.
Brian O’Donnell, director of Campaign for Nature, said the record underlined two priorities. “Fossil fuel emissions need to be rapidly reduced,” he said. “Meanwhile, it is essential that countries protect marine and coastal ecosystems… More than 190 governments have committed to protect and conserve at least 30% of the planet by 2030, and now is the time to turn that commitment into effective, durable conservation on the ground.”
C3S scientists say they will continue monitoring sea surface temperatures to determine whether the August peak marks a temporary spike or the start of a more sustained period of extreme ocean warmth.