The average temperature of the ocean surface reached 21.1°C on Saturday (22), the hottest day ever recorded in the world's seas, according to the European Union's Copernicus climate change service. The mark tops the previous record of 21.09°C, set in March 2024, Folha de S.Paulo reported on Monday (24).
According to Copernicus, it is the highest value since 1979, when its ERA5 dataset, which goes back to 1940, became more reliable for measuring sea surface temperature with the help of satellites. The service's technicians say the new record is in line with the long-term warming of the oceans and with the emergence of El Niño, the cyclical natural phenomenon that warms parts of the Pacific and is expected to intensify climate anomalies in the second half of this year and in 2027.
"The oceans are under increasing stress. El Niño is adding heat to the system, and this comes in a context of decades of human-caused warming," said Samantha Burgess of the European Centre for Medium-Range Weather Forecasts (ECMWF), which runs Copernicus.
According to Burgess, the consequences are already visible: the number of marine heatwave days worldwide has more than tripled since the early 1990s, putting growing pressure on marine ecosystems and the communities that depend on them. Warmer waters transfer heat and moisture to the atmosphere, which raises the chance and the strength of storms. The oceans also absorb about 25% of the carbon dioxide emissions generated by human activity, mainly from burning oil, coal and natural gas, and warming reduces that absorption capacity.
A streak of records
Saturday's record comes after months of exceptional heat at sea. According to Brazilian meteorology site MetSul, by late July the average temperature of the ocean surface, excluding polar regions, had spent more than 50 consecutive days at record levels for the time of year, around 21°C. Copernicus itself had already registered, also in July, the highest sea surface temperatures ever measured for that month. MetSul notes that the strengthening of El Niño in the Equatorial Pacific contributes to the picture, although it does not by itself explain the observed heat, with extensive marine heatwaves also in the North Atlantic, the Indian Ocean and the Mediterranean, where the water approached 30°C in some areas.
The timing of Saturday's mark also draws attention, the Folha report notes: the previous record dated from March 2024, at the end of the Southern Hemisphere summer. Oceans cover about 71% of the Earth's surface and absorb more than 90% of the excess heat caused by greenhouse gases, which makes them the planet's main thermal regulator, according to MetSul.