Advances in Atmospheric Science reports the oceans absorbed a record 23 zettajoules of heat in 2025 — the ninth straight annual record. That increase (up from ~16 ZJ in 2024) is roughly equivalent, in energy terms, to 12 Hiroshima bombs detonating every second. Data from the upper 2,000 meters and multiple international datasets show strong warming in the tropical and South Atlantic, Mediterranean, North Indian and Southern Oceans. Researchers warn this heat accumulation drives heavier precipitation, stronger storms, and long-term shifts in the climate system.
World’s Oceans Absorbed Record Heat in 2025 — Equivalent to 12 Hiroshima Bombs Every Second
Credit: Thomas Vimare/Unsplash
A new analysis published in Advances in Atmospheric Science found that the world’s oceans absorbed more heat in 2025 than in any year since modern measurements began around 1960. The study reports a total increase of 23 zettajoules of heat absorbed by the oceans — the ninth consecutive annual record.
How Scientists Measured Ocean Warming
Researchers calculated ocean heat content using observational records from the upper 2,000 meters of the ocean — the layer that captures most excess heat — and combined datasets from NOAA’s National Centers for Environmental Information, the European Union’s Copernicus Climate Change Service, and the Chinese Academy of Sciences. Ocean heat content is expressed in zettajoules (ZJ); 1 ZJ equals 1,000,000,000,000,000,000,000 joules.
Key Findings
The analysis found oceans absorbed an additional 23 ZJ of heat in 2025, up from roughly 16 ZJ in 2024. To give a visceral sense of the energy involved, the authors note this increase is equivalent to the energy of about 12 Hiroshima bombs detonating every second throughout the year — an analogy that highlights the scale of heat accumulation (it is an energy equivalence, not a literal comparison of impacts).
The regions warming most strongly in 2025 included the tropical and South Atlantic, the Mediterranean Sea, the North Indian Ocean, and the Southern Ocean. Global annual mean sea surface temperature for 2025 was the third warmest on record, about 0.5°C (0.9°F) above the 1981–2010 average.
Why This Matters
Oceans absorb more than 90% of the excess heat trapped by greenhouse gases in Earth’s atmosphere, so rising ocean heat content is a robust indicator of long-term climate change. A warmer ocean alters atmospheric circulation, increases the frequency and intensity of marine heatwaves, and shifts global precipitation patterns.
Warmer ocean and global temperatures contribute to more intense rainfall events and can fuel stronger tropical storms. The authors of the study and other researchers point to recent extreme weather — including impacts from Hurricane Melissa in Jamaica and Cuba, heavy monsoon rains in Pakistan, and severe flooding in parts of the United States — as examples of events likely influenced in part by elevated ocean heat.
“Last year was a bonkers, crazy warming year,” said John Abraham, a mechanical engineer at the University of St. Thomas and a co-author of the study, in an interview with Wired.
Conclusions
The authors state the results provide direct evidence that the climate system is out of thermal equilibrium and accumulating heat. Continued monitoring of ocean heat content is critical for understanding long-term climate trends and for anticipating changes to weather patterns, marine ecosystems, and coastal impacts.
Originally published on Eos.org; datasets and methods summarized from the published paper in Advances in Atmospheric Science.
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