Ocean absorbs record heat, fuelling extreme weather

More heat accumulated in the ocean in 2025 than in any year since modern measurements began, says Dr Kevin Trenberth, an honorary academic at the University of Auckland.

Heat in the upper 2kms of the ocean increased by an estimated 23 zettajoules, according to a study co-authored by Trenberth, who is affiliated to the National Center for Atmospheric Research in the US.

That’s the equivalent of detonating hundreds of millions of Hiroshima atomic bombs – or roughly 200 times global electrical energy consumption in 2023.

“The ocean is the hottest on record,” says Trenberth. “We’re looking at creating a very different planet – do we really want to do that?”
 

The research by an international team of scientists, led out of the Chinese Academy of Sciences, was published in the journal Advances in Atmospheric Science.

The ocean absorbs most of the extra heat trapped by greenhouse gases. Rising ocean heat drives global sea-level rise because water expands and intensifies extreme weather by increasing heat and moisture in the atmosphere.

The global sea-surface temperature was the third warmest on record, slightly lower than in 2023 and 2024, mainly due to the transition from El Niño to La Niña in the tropical Pacific.

The decline in mean sea-surface temperature was small compared to an unexpectedly large increase in 2023. Warmer surface temperatures lead to more evaporation, heavier rains, and more extreme tropical cyclones and weather events.

“Sea surface temperatures were exceptionally high near and around New Zealand during December 2025 into January 2026 from La Nina and global warming,” says Trenberth. “There’s a link with our unstable showery recent weather.”

The research paper referred to uncertainty over space-based observations for monitoring climate change, an apparent reference to US capabilities under the Trump administration.