Oceans hit their highest temperature on record: 21.1C as El Niño keeps growing

The world's oceans are hotter than ever recorded. On Saturday, the average surface temperature of the planet's seas outside the polar regions hit 21.1C, according to figures from Europe's Copernicus climate change service.

The record in context

That reading edges past the 21.09C recorded in March 2024 and sits far above average for the time of year. The data is based on sea temperatures 10m below the surface, using measurements from buoys, ships and satellites combined into a global estimate.

Why the timing matters

Average worldwide sea temperatures normally peak in March or April, at the end of southern-hemisphere summer, not in August. The southern hemisphere contains more of the planet's ocean surface than the north and therefore exerts a bigger influence on the global average. What concerns scientists most is that the oceans are already this warm while the natural El Niño phenomenon is still well short of its expected peak.

"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," she added.

What warmer seas mean

El Niño brings unusually warm water to the surface of the eastern and central tropical Pacific, and it is expected to keep strengthening until Christmas — on course, scientists warn, to be the strongest in centuries.

Warm seas fuel more extreme weather: they give storms extra moisture and energy, and in some coastal regions they intensify heatwaves on land. Warmer water also takes up more space, raising sea levels and the risk of coastal flooding, while intense ocean heat damages habitats such as coral reefs. The oceans absorb more than 90% of the excess heat trapped by humanity's greenhouse gas emissions.

Dr Jeremy Grist, a senior research fellow at the National Oceanography Centre in Southampton, said the fact that records are already falling is an early indicator of how strong El Niño is becoming, and that the record may be broken again in March or April 2027. In the western English Channel, marine heatwave conditions have been almost continuous for more than three years, peaking at 7C above normal in July.