A new international analysis published in Advances in Atmospheric Sciences on 9 January finds that the Earth's ocean stored more heat in 2025 than in any year since modern measurements began. The 2025 heat increase was 23 Zetta Joules (23,000,000,000,000,000,000,000 Joules of energy), which is equivalent to ~37 years of global primary energy consumption at the 2023 level (~620 Exa Joules per year). The finding is the result of a major international collaboration, involving more than 50 scientists from 31 research institutions worldwide.
Why are the oceans so important?
The ocean absorbs 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, it provides one of the best indicators of long-term climate change.
Study Results
The assessment combines data from major international data centers and independent research groups, including three observational products (Institute of Atmospheric Physics at the Chinese Academy of Sciences; Copernicus Marine; and NOAA/NCEI) and an ocean reanalysis (CIGAR-RT) from three continents: Asia, Europe, and America. These groups confirm that the 2025 OHC reached the highest level on record, confirming continued ocean heat gain.
The ocean warming is not uniform; some areas are warming faster than others. In 2025, about 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 re cord is characterized by stronger ocean warming trends since the 1990s than before. The increase in upper 2000m OHC is fairly steady in recent decades, though 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.
Ocean surface temperatures
Global annual mean sea-surface temperature (SST) in 2025 was the third warmest year in the instrumental record and remained about 0.5 °C (approximately 1°F) 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. Sea surface temperatures are particularly important because they affect weather patterns worldwide. Warmer surface temperatures favor increased evaporation and heavier rains, thus causing more extreme tropical cyclones and weather events. These played a major role in widespread flooding and disruption throughout much of Southeast Asia, drought in the Middle East, and flooding in Mexico and the Pacific Northwest, in 2025.
Why does this matter?
Rising ocean heat drives global sea-level rise via thermal expansion, 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, ocean heat content will continue to rise and records will continue to fall.
The final results will be included in a special collection on Ocean Heat Content Changes organized by the journal Advances in Atmospheric Sciences. Notably, the cartoon characters of the sad shrimp and crab on the cover were suggested by the study's corresponding author, Lijing Cheng from the Institute of Atmospheric Physics at the Chinese Academy of Sciences.
"The idea comes from the 'shrimp soldiers and crab generals' guarding the underwater palace in Journey to the West," said Cheng. "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."
The collection will include various aspects of OHC changes and in-depth analyses of the mechanisms—for instance, regional OHC changes in the seas around China , the South Pacific, and the Indian Ocean. As leading climate scientist Kevin Trenberth wrote in the preface of the special issue, it is structured as an ongoing collection, reflecting the evolving nature of climate science.
While the science continues to evolve, one conclusion remains clear: 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.
