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In 2025, the ocean absorbed an extra 23 zettajoules of heat energy in 2025, breaking the ocean heat content record for the ninth consecutive year.
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The ocean soaked up more heat last year than in any year since modern measurements began around 1960, according to a new analysis published in Advances in Atmospheric Science.
The world's oceans absorb more than 90% of excess heat trapped in Earth's atmosphere by greenhouse gas emissions. As heat in the atmosphere accumulates, heat stored in the ocean increases, too, making ocean heat a reliable indicator of long-term climate change.
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Scientists measure the ocean's heat in different ways. One common metric is global annual mean sea surface temperature, the average temperature in the top few meters of ocean waters. Global sea surface temperature in 2025 was the third warmest ever recorded, at about 0.5°C (0.9°F) above the 1981-2010 average.
Another metric is ocean heat content, which measures the total heat energy stored in the world's oceans. It's measured in zettajoules: One zettajoule is equivalent to 1,000,000,000,000,000,000,000 joules. To measure heat content in 2025, the study's authors assessed ocean observational data from the upper 2,000 meters of the ocean, where most of the heat is absorbed, from NOAA's National Centers for Environmental Information, the European Union's Copernicus Climate Change Service, and the Chinese Academy of Sciences.
They found that in total, the ocean absorbed an additional 23 zettajoules of heat energy in 2025, breaking the ocean heat content record for the ninth consecutive year and marking the longest sequence of consecutive ocean heat content records ever recorded.
"Last year was a bonkers, crazy warming year," John Abraham, a mechanical engineer at the University of St. Thomas and a co-author of the new study, told Wired.
Sign up for the Live Science daily newsletter nowContact me with news and offers from other Future brandsReceive email from us on behalf of our trusted partners or sponsorsBy submitting your information you agree to the Terms & Conditions and Privacy Policy and are aged 16 or over.Twenty-three zettajoules in one year is equivalent to the energy of 12 Hiroshima bombs exploding in the ocean every second. It's also a large increase over the 16 zettajoules of heat the ocean absorbed in 2024. The hottest areas of the ocean observed in 2025 were the tropical and South Atlantic, Mediterranean Sea, North Indian Ocean, and Southern Ocean.
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The results provide "direct evidence that the climate system is out of thermal equilibrium and accumulating heat," the authors write.
A hotter ocean favors increased global precipitation and fuels more extreme tropical storms. In the past year, warmer global temperatures were likely partly responsible for the damaging effects of Hurricane Melissa in Jamaica and Cuba, heavy monsoon rains in Pakistan, severe flooding in the Central Mississippi Valley, and more.
"Ocean warming continues to exert profound impacts on the Earth system," the authors wrote.
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