The world’s oceans have reached their hottest recorded temperatures in history, and experts warn additional warming is on the horizon in coming months — a shift that poses cascading threats to marine ecosystems, human food security, and global weather patterns, the European Union’s Copernicus Climate Change Service announced Monday.
According to Samantha Burgess, Copernicus’ strategic climate lead, the combined forces of a rapidly expanding, already massive natural El Niño event and human-caused climate change pushed average global sea surface temperatures to 21.1 degrees Celsius (70 degrees Fahrenheit) on Friday and Saturday of last weekend. This new mark narrowly surpasses the previous daily average record set in March 2024, marking an unprecedented deviation from long-term climate patterns.
“This is yet another sign of our climate system under strain and the ocean under immense pressure,” Burgess emphasized in the announcement.
Leading ocean scientists and former senior environmental officials have echoed deep alarm over the milestone. Jane Lubchenco, a renowned oceanographer and former administrator of the U.S. National Oceanic and Atmospheric Administration (NOAA), described the record as “very troubling.” In an emailed statement, she warned, “An ocean this warm destabilizes weather patterns and threatens food security, economic prosperity and marine life. A warmer ocean is not our friend.”
The impacts of elevated ocean temperatures extend far beyond coastal waters. Warmer oceans become more acidic and hold less dissolved oxygen, directly harming the commercial and subsistence fishing industries that billions of people depend on for nutrition and livelihoods, Burgess explained. Warming also reshapes major ocean current systems and amplifies the intensity of tropical and extratropical storms, boosting rainfall totals and increasing the height of destructive storm surges.
For coastal communities already grappling with summer heat waves, ocean warming creates an additional secondary threat: when seawater temperatures are abnormally high, overnight cooling from onshore sea breezes is eliminated, leaving communities to swelter through unrelenting hot nights. Burgess pointed to this summer’s heat events across Europe as a real-world example of this dangerous amplification effect.
In a typical climate cycle, global average sea surface temperatures peak in March or April, as the Southern Hemisphere — which is dominated by ocean area — finishes its summer season. This year, however, the all-time record was broken in August, signaling that temperatures will continue to climb until the current El Niño reaches its peak. El Niño, a natural periodic warming of central and eastern Pacific waters that alters global weather patterns and raises worldwide average temperatures, is forecast to hit its maximum strength between November 2026 and January 2027.
Burgess noted that the current El Niño is already comparable in size to the strongest events in recent history, and it continues to grow, with peak strength expected to reach unprecedented levels. It accounts for a large share of the current ocean warming, while long-term warming driven by fossil fuel burning has already raised global ocean temperatures by 0.8 degrees Celsius (1.4 degrees Fahrenheit) overall.
“The thing that makes my heart sink is that this starts so early and it’s so out-of-normal that you know when it hits its peak, it’s going to be unprecedented,” said Boris Worm, a marine biology professor at Canada’s Dalhousie University. “The ocean is not supposed to do this. It’s just that there is no normal anymore.”
Marine ecologists warn that the timing of this record sets up devastating impacts for coral reefs and global fisheries in the coming months. “We’re not even in October yet. Usually the Caribbean peaks at like late September, early October. And I think next year in the Southern Ocean, the Great Barrier Reef, yeah I’m really worried about what’s going to happen in the southern hemisphere in February, March,” said John Bruno, a marine ecologist at the University of North Carolina.
Bruno added that only 20 percent of the heat-sensitive coral that existed in the 1990s remains alive today. While a small number of fast-growing “weedy” coral species have shown limited adaptation to warmer waters, long-lived iconic species such as staghorn coral continue to face catastrophic die-offs, he explained.
Both Worm and Bruno warn that widespread fisheries collapse and resulting food shortages are a major looming risk. The world’s largest single fishery, the Peruvian anchovy fishery, typically suffers major collapses during strong El Niño events, and this year’s extreme heat makes a collapse particularly likely, Worm said. The loss of anchovies ripples through the entire marine food web, as the small fish are a primary food source for larger fish, marine mammals, and seabirds. Unlike larger mobile marine life that can swim long distances to find cooler water, anchovies cannot relocate to more habitable areas, leading to failed reproduction and mass starvation when waters overheat — a shift that also harms human communities that rely on the fishery.
“Not everybody is hit equally, obviously, but the whole system collapses,” Worm explained. “It’s like when you pull out the rug from underneath an economy. It’s, like, the whole thing collapses. Some sectors may do a little bit better, but overall, everybody suffers.”
One of the more visible changes for coastal visitors is a surge in jellyfish populations, which thrive in warmer waters. Burgess noted increased jellyfish sightings off European coasts this summer, and Bruno said the same pattern is likely emerging along the U.S. East Coast.
Rick Spinrad, another former NOAA administrator, emphasized the ocean’s foundational role in regulating the planet’s climate. “The ocean is the planet’s thermostat, so as long as we depend on a healthy ocean for our safety, prosperity and wellbeing, these record-breaking sea surface temperatures portend even greater challenges for us all,” he said.