The world’s oceans are edging dangerously close to record-breaking temperatures, with sea surface readings in some regions now 4°C above historical averages an ominous sign of accelerating global warming.
“This is global warming playing out before our eyes,” warned Professor Matthew England of the University of New South Wales, co-author of a new study on the subject.
According to new data released by the EU’s Copernicus Climate Change Service, May 2025 was the second warmest globally on record. While it was the first time since July 2023 that average global surface temperatures dipped just under the crucial 1.5°C threshold above pre-industrial levels (recorded at 1.4°C), the larger trend remains grim.
Extreme weather events are intensifying in tandem. North America, northern and central Europe, and parts of Asia were significantly drier than usual. In contrast, regions like southern Europe, the eastern United States, Australia, and parts of Africa saw excess rainfall.
Amid this growing climate emergency, momentum is building at the UN Ocean Conference in France, where 50 countries—including the European Union ratified the High Seas Treaty. With over 80 nations now on board, the treaty aims to protect 30% of the world’s oceans by 2030.
However, experts say time is running out. The ocean, long seen as a critical buffer against climate change, may no longer be able to shield humanity from its worst effects.
Warning Signs Multiply
The global average sea surface temperature hit 20.79°C in May 2025, just 0.14°C below the all-time record set in May 2024. Copernicus described this as part of an “alarming” two-year streak of marine warming.
Simultaneously, the U.S. National Oceanic and Atmospheric Administration (NOAA) raised red flags about atmospheric CO₂ levels, which reached 426 ppm in March, a sharp rise from 300 ppm just six decades ago. At Hawaii’s Mauna Loa Observatory, CO₂ even exceeded 430 ppm, a record high.
Historically, oceans have absorbed 25–30% of atmospheric carbon dioxide and 90% of the heat generated by greenhouse gases. But that safety net is now under threat.
“We’re seeing stronger and stronger bursts of ocean warming marine heatwaves that are pushing us past records at an alarming rate,” said Dr. Michael Meredith of the British Antarctic Survey.
Julien Nicolas, senior scientist at Copernicus, explained that while El Niño contributed to the spike in temperatures, the deeper concern lies in persistent, long-term warming. “As sea surface temperatures rise, the ocean’s ability to absorb carbon diminishes,” he said. “This could speed up the greenhouse effect and intensify climate change.”
Hardest-Hit Regions
Some areas are experiencing the brunt of the crisis. In May, a marine heatwave off the coasts of the UK and Ireland shattered previous records. The Mediterranean Sea also saw above-average warmth. A Nature study linked the record North Atlantic warming in summer 2023 to heightened heat exchange with the atmosphere rather than just internal ocean dynamics indicating that more frequent and severe marine heatwaves are likely in the future.
With oceans heating faster than ever, scientists and policymakers alike are calling for urgent, unified global action to protect marine ecosystems, enhance climate monitoring, and curb emissions before Earth’s natural buffers collapse beyond repair.
