It would take about 365 million Hiroshima-sized atomic bombs to release the amount of energy the world’s oceans absorbed as additional heat by 2025. This figure comes from the latest global ocean heat content report, which found the oceans to have warmed the most on record for nine years in a row.
The 55 scientists who contributed to the report, published Friday in the journal Advances in Atmospheric Sciences, warn that the ongoing trend is fueling extreme weather, sea level rise, and the collapse of marine ecosystems. They say the biggest driver is undoubtedly the increase in greenhouse gases.
We are already seeing the catastrophic effect that ocean warming is having on communities around the world, particularly through severe storms and extreme flooding. Researchers point to a number of deadly events in 2025, from unprecedented heavy rains that killed more than 1,300 people in Southeast Asia to floods that killed at least 138 people—including 27 summer youth and counselors—in Central Texas.
“In the long term, in line with projections from advanced climate models, globally [ocean heat content] it is expected to continue breaking records until net-zero greenhouse gas emissions are achieved,” the authors write.
Reaching the boiling point
The world’s oceans act as its primary sink for heat energy, absorbing 90% of the excess heat trapped by greenhouse gases in the atmosphere. The tropical ocean therefore plays an important role in controlling global and regional climate impacts, particularly through increased atmospheric moisture and the presence of energy, which causes hurricane intensification.
To calculate how much heat the world’s oceans have gained by 2025, the researchers measured temperature changes in 6,500 feet (2,000 meters) of the world’s oceans. They found that ocean temperatures were about 23 zettajoules higher than they were at the end of 2024, making 2025 the hottest year on record according to the OHC.
About 14% of the world’s oceans reached their warmest state on record last year, especially in the Southern Ocean, the tropical South Atlantic Ocean, the Mediterranean Sea, and the North Indian Ocean. About one-third of the world’s oceans are ranked among their three hottest years on record, and more than half are ranked among their five hottest. This emphasizes the widespread and cumulative nature of ocean heat gain, according to the researchers.
It never stops trending
That extra heat is stronger storms, but it also accelerates the melting of sea ice and sea ice. By 2025, Arctic sea ice extent reached the lowest annual extent since satellite observations began, and Antarctic sea ice extent fell to the third lowest annual extent.
Year-to-date observations show that both the Greenland and Antarctic ice sheets have reached record lows, and new research suggests parts of them have crossed climate critical points, according to 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 ocean temperatures exceed the temperature of these delicate 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 warming event, according to NOAA, with red-hot temperatures affecting about 84% of the world’s coral reefs by January 2023. Extensive coral bleaching has been documented in at least 83 countries and regions.
Researchers don’t expect this nine-year streak of record ocean warming to end anytime soon. Until the world achieves net-zero carbon emissions, global sea temperatures will continue to rise.
We are nowhere near reaching that goal, but immediate emissions reductions can still limit future impacts. To better understand and prepare for these impacts, the authors emphasize the importance of strengthening ocean monitoring and uncovering processes of ocean heat redistribution. These efforts will help communities around the world adapt to the new climate and build resilience.
