August Temperature Spike in Seas Raises Alarm as El Niño Intensifies

The world's oceans have reached their highest recorded temperature, new data from the European Copernicus climate change service has revealed, a stark signal of the growing stress on the planet's marine systems. On Saturday, the average surface temperature of the seas outside the polar regions hit 21.1C (70F), edging past the previous record of 21.09C set in March 2024 and standing far above the average for this time of year. Scientists have expressed particular concern at the timing of the record, as global sea temperatures typically peak in March or April, following the southern hemisphere summer, not in August. The southern hemisphere contains more of the planet's ocean surface and thus exerts a greater influence on global averages.

The record heat is being driven by a combination of long-term, human-induced climate change and the intensifying natural El Niño weather phenomenon, which brings unusually warm waters to the surface of the eastern and central tropical Pacific. "This record is another clear signal of an ocean under growing stress," said Dr Samantha Burgess, deputy director of Copernicus. "El Niño is adding heat to the system, but it is doing so on top of decades of human-driven warming." The data, compiled from buoys, ships, and satellites measuring temperatures 10 metres below the surface, is particularly alarming because El Niño is still some way from its expected peak, which scientists now warn could be the strongest in centuries.

The implications of this oceanic heat are profound and wide-reaching. Warmer seas supercharge extreme weather, providing storms with additional moisture and energy, and can intensify heatwaves on land by reducing the cooling effect of sea breezes. The expanding volume of warmer water also raises sea levels, increasing the risk of coastal flooding, while intense marine heatwaves devastate fragile habitats such as coral reefs. The increasing frequency of these events is, as Dr Burgess noted, "putting increasing pressure on marine ecosystems and the communities that depend on them."

The extreme warmth is not confined to the Pacific. Waters around the UK and Europe are also experiencing unprecedented heat. The western English Channel has been in a state of almost continuous marine heatwave for more than three years, peaking at 7C above normal in July, according to Prof Tim Smyth, director of science at Plymouth Marine Laboratory. "This is unprecedented," he said. With El Niño expected to keep strengthening until the end of the year, scientists like Dr Jeremy Grist of the National Oceanography Centre warn that further records are likely. "All things being equal we might expect the ocean temperature record to be broken again in March [or] April 2027," he said. The data reinforces the troubling upward trend, a clear sign of the growing effect of humanity's greenhouse gas emissions on the world's seas.