1417. Ocean Heat Content Sets Record in 2025

The oceans play a critical role in regulating the Earth's climate, absorbing 90% of the excess heat released into the atmosphere by human-caused greenhouse gas emissions, including carbon dioxide. All of this additional energy sets off powerful chain reactions. Warmer oceans increase the amount of moisture in the atmosphere, leading to tropical cyclones and destructive rainfall. Rising ocean temperatures also directly contribute to sea level rise, and prolonged marine heatwaves can kill corals.

A paper published in Advances in Atmospheric Sciences found that in 2025, the world's oceans accumulated approximately 23 zettajoules of heat, the highest since modern record-keeping began in the early 1950s.

Ocean warming is not uniform, with different regions warming at different rates. Tropical oceans, the South Atlantic, the Mediterranean, the northern Indian Ocean, and the Antarctic Ocean were among the regions that absorbed record amounts of heat in 2025. This occurred despite a slight decrease in average sea surface temperatures in 2025 due to a transition from the strongly warming El Niño phenomenon that occurred in 2023-2024 to a La Niña phenomenon, which typically involves temporary ocean cooling. Sea surface temperatures remained the third-highest on record.

Over the long term, the rate of ocean warming is accelerating due to persistent increases in atmospheric greenhouse gas concentrations, primarily caused by the burning of fossil fuels. Researchers say that as long as global warming is not addressed and the amount of heat trapped in the atmosphere continues to increase, the oceans will continue to break records.

The researchers argue that the greatest uncertainty in the climate system is no longer physics but human choices, and that rapid emissions reductions can help limit future impacts and protect a climate in which societies and ecosystems can thrive.

(Reference)
Pan, Y., Cheng, L., Abraham, J. et al. Ocean Heat Content Sets Another Record in 2025. Adv. Atmos. Sci. (2026). https://doi.org/10.1007/s00376-026-5876-0

Contributor: IIYAMA Miyuki, Information Program