It would take roughly 365 million Hiroshima-sized atomic bombs to release the amount of energy the world’s oceans absorbed as additional heat in 2025. This figure stems from the latest report on the global ocean heat content, which found record-high marine warming for the ninth year in a row.
The 55 scientists who contributed to the report, published Friday in the journal Advances in Atmospheric Sciences, warn that the continued trend is exacerbating extreme weather, sea-level rise, and the destabilization of marine ecosystems. The main driver is unequivocally rising greenhouse gas emissions, they say.
We’re already seeing the catastrophic effect marine warming has on communities around the world, particularly through stronger storms and more severe flooding. The researchers point to a slew of deadly events in 2025, from the unprecedented monsoon rains that killed more than 1,300 people across Southeast Asia to flash floods that killed at least 138 people—including 27 young summer campers and counselors—in Central Texas.
“In the long term, consistent with projections from state-of-the-art climate models, global [ocean heat content] is expected to continue breaking records until net-zero greenhouse gas emissions are achieved,” the authors write.
Reaching a boiling point
Earth’s oceans act as its primary thermal energy sink, absorbing 90% of the excess heat trapped by greenhouse gases in the atmosphere. The thermal state of the ocean therefore plays an important role in regulating global and regional climate impacts, mainly through increased atmospheric moisture and energy availability, which fuel storm intensification.
To calculate how much heat the world’s oceans absorbed in 2025, the researchers measured temperature fluctuations in the upper 6,500 feet (2,000 meters) of the global ocean. They found that the ocean heat content was about 23 zettajoules higher than it was at the end of 2024, making 2025 the hottest year on record in terms of OHC.
Roughly 14% of the global ocean area reached its warmest state on record last year, particularly in the Southern Ocean, tropical and South Atlantic Ocean, Mediterranean Sea, and North Indian Ocean. About one-third of the global ocean area ranked among its top three hottest years on record, and more than half ranked among its five hottest. This underscores the pervasive and accumulated nature of ocean heat gain, according to the researchers.
No end to the trend
That extra thermal energy is supercharging storms, but it’s also speeding the already rapid melting of glaciers and sea ice. In 2025, Arctic sea ice extent reached the lowest annual maximum since satellite observation began, while Antarctic sea ice extent fell to its third-lowest annual maximum.
Year-to-date observations also show that both the Greenland and Antarctic ice sheets reached record-low mass levels, and new studies suggest portions of them have crossed critical climate tipping points, according to the researchers. These findings indicate continued long-term contributions to global sea-level rise.
Ocean warming poses a deadly threat to coral reefs as well. When seawater temperatures exceed the thermal tolerance threshold of these fragile organisms, they expel the symbiotic algae that provide them with nutrients and energy, resulting in bleaching.
The world is currently in the midst of its fourth global coral bleaching event, according to NOAA, with bleaching-level heat stress impacting roughly 84% of the world’s coral reef area since January 2023. Mass coral bleaching has been documented in at least 83 countries and territories.
The researchers don’t expect this nine-year streak of record-breaking marine warming to end anytime soon. Until the world achieves net-zero carbon emissions, the global ocean heat content will continue to rise.
We’re nowhere near reaching that goal, but rapid emissions reductions can still limit future impacts. To better understand and prepare for these impacts, the authors stress the importance of strengthening ocean monitoring and uncovering the mechanisms behind ocean heat redistribution. These efforts will help communities around the world adapt to a new climate reality and build resilience.
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