The European Union’s Copernicus Climate Change Service said on Monday that the average surface temperature of the world’s ocean, measured between 60 degrees south and 60 degrees north, reached 21.1 degrees Celsius on 22 August — the highest daily figure in a record that begins in 1979. The previous mark, 21.09 degrees, was set in March 2024.
The margin is a hundredth of a degree, which on its own would be a footnote. What makes the reading unusual is the calendar. The global ocean has a seasonal shape of its own: because so much more of the southern hemisphere is water, the planetary sea surface warms towards the southern summer and normally peaks in March or April. Late August sits near the bottom of that annual curve. A record set now is a record set against the season rather than with it, which means the underlying warmth is large enough to overcome the ordinary rhythm of the year.
Two things are pressing in the same direction. The World Meteorological Organization, in its seasonal update published on 31 July, said a strong El Niño was developing rapidly in the tropical Pacific, with the seasonal average sea surface temperature anomaly in the monitoring region forecast to reach about 2.9 degrees over August to October and to peak around November. The same update noted a positive Indian Ocean Dipole developing, at a seasonal mean of 0.6 degrees. El Niño events redistribute heat from the ocean towards the atmosphere and reliably lift global temperatures for a year or more afterwards; they do so, however, on a baseline that has been climbing for decades.
“This record is another clear signal of an ocean under growing stress. El Niño is adding heat to the system, but it is doing so on top of decades of human-driven warming,” said Samantha Burgess, strategic lead for climate at the European Centre for Medium-Range Weather Forecasts, which runs the Copernicus service.
For readers in this part of the world the practical question is what a strong El Niño does to the coming months. The WMO’s outlook describes above-normal temperatures across much of the globe and substantial shifts in rainfall patterns, without putting figures on any one region. Warmer seas also carry more energy for tropical storms and more stress for coral. None of that is settled by a single day’s reading — but the day is worth marking, because the ocean is the part of the climate system that remembers longest.