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By Stephen Beech
Ocean temperatures reached a record high last year, reveals new research.
The seas stored more heat in 2025 than in any year since modern measurements began, according to the findings of an international research team.
Scientists say the heat increase over the last 12 months was 23 Zetta Joules – 23,000,000,000,000,000,000,000 Joules of energy – equivalent to around 37 years of global primary energy consumption at the 2023 level.
The findings, published in the journal Advances in Atmospheric Sciences, are the result of a major international collaboration, involving more than 50 scientists from 31 research institutions worldwide.
Earth’s oceans absorb more than 90% of the excess heat trapped by greenhouse gases, making it the main heat reservoir of the climate system.
Because ocean heat content (OHC) reflects the accumulation of heat stored in the ocean, scientists say it provides one of the best indicators of long-term climate change.
The latest assessment combines data from major international data centres and independent research groups from Europe, Asia and America.
The figures show that the 2025 OHC reached the highest level on record, confirming continued ocean heat gain.
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But ocean warming is not uniform with some areas heating up faster than others.
The analysis shows that, in 2025, around 16% of the global ocean area reached a record-high OHC, and about 33% ranked among the top three warmest values in their historical records.
The warmest areas included the tropical and South Atlantic and North Pacific Oceans, and the Southern Ocean.
The overall record shows stronger ocean warming trends since the 1990s than before.
Scientists say the increase in upper 2000m OHC is “fairly steady” in recent decades, although a small increase in rate can be detected.
Ocean heat content reached a record high in 2025 as it has in each of the past nine years.
Global annual mean sea-surface temperature (SST) in 2025 was the third warmest year in the instrumental record and remained about 0.5 °C above the 1981–2010 average baseline.
SST in 2025 is slightly lower than in 2023 and 2024, mainly due to the transition from El Niño to La Niña in the tropical Pacific.
(Photo by Josh Sorenson
Scientists say sea surface temperatures are particularly important because they affect weather patterns worldwide.
Warmer surface temperatures favor increased evaporation and heavier rains, causing more extreme tropical cyclones and weather events.
The research team say they played a major role in widespread flooding and disruption throughout much of South East Asia, drought in the Middle East, and flooding in Mexico and the Pacific Northwest, last year.
Rising ocean heat drives global sea-level rise, strengthens and prolongs heatwaves, and intensifies extreme weather by increasing heat and moisture in the atmosphere.
As long as the Earth’s heat continues to increase, scientists warn that ocean heat content will continue to rise and records will continue to fall.
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The final results will be included in a special collection on Ocean Heat Content Changes organized by the journal Advances in Atmospheric Sciences.
Cartoon characters of the sad shrimp and crab on the cover were suggested by Professor Lijing Cheng, the study’s corresponding author.
Cheng, from the Institute of Atmospheric Physics at the Chinese Academy of Sciences, said: “The idea comes from the ‘shrimp soldiers and crab generals’ guarding the underwater palace in Journey to the West.
“We reimagined them not as mighty guardians, but as vulnerable creatures whose armor – their shells and scales – is under attack by ocean warming, acidification and other ocean environmental changes.”
He added: “The biggest climate uncertainty is what humans decide to do.
“Together, we can reduce emissions, better prepare for upcoming changes, and help safeguard a future climate where humans can thrive.”