Record global temperatures have pushed the amount of heat stored in the ocean to a new high, according to the latest data collected by the UN’s World Meteorological Organization (WMO).
The WMO’s State of the Global Climate 2025 report confirms that the global mean near-surface temperature (both land and sea surfaces) was higher during 2023-2025 than at any point since observational records began, 176 years ago. Last year was either the second or third warmest on record, depending on which dataset is referenced.
Humanity’s thirst for fossil fuels continues to drive this warming, causing sea levels to rise, ocean acidification, and heatwaves. Most of this excess energy in the Earth system is being taken up by the ocean. As such, ocean heat content reached a record high in 2025.
“Ocean heat content has been changing from 2005 to 2025 at about 11 zettajoules per year. And that’s roughly equivalent to about 18 times the annual human energy use,” John Kennedy, lead author of the report, explained during its press launch.
The heat produced by human activities is absorbed by different parts of the Earth system. Only 1% of this heat remains in the atmosphere, while 3% is absorbed by ice, causing it to melt. Slightly more – around 5% – warms the land. But the rest of that heat, approximately 91%, ends up in the ocean.
Karina von Schuckmann was a senior adviser to the report, and is a researcher at Mercator Ocean International, an intergovernmental body headquartered in southern France. She said ocean heat content is a fundamental indicator for climate scientists.
“This has been calculated from different observational data sets for the upper 2,000 metres of the ocean, from 1960 to 2025, and the results have shown that the ocean heat content in the year 2025 was the highest on record,” she added.
This heat is also penetrating further down into the ocean than in the past. It is therefore warming not just the near-surface waters but deeper, colder waters too. This traps it for centuries, or longer, locking in the impact of humanity’s greenhouse gas emissions for a similar time period.
“The more we have heat captured away from communication with the atmosphere, the more we are moving also to timescales of committed climate change for hundreds to thousands of years,” explained von Schuckmann.
The WMO report finds Arctic sea-ice extent was at its lowest or second lowest on record in 2025, with Antarctic sea-ice extent third lowest after 2023 and 2024. Ocean acidification also continues, albeit with large regional variations. The Indian, Southern and some parts of the Pacific and Atlantic oceans are acidifying faster than the average speed. Meanwhile, global sea levels in 2025 were comparable to the record high levels seen in 2024.
Out of balance
This year, the WMO report includes a new indicator of planetary stress: Earth’s energy imbalance. This is the difference between how much energy the planet gets from the sun (incoming sunlight minus reflected sunlight) and how much is sent back into space via radiation.
“There’s less outgoing energy due to the increased concentrations of greenhouse gases,” explained Kennedy. “More energy coming in than going out means that energy is accumulating in the Earth’s system.”
This accumulation was at its highest level in 2025, too. It is this excess energy that is mostly ending up in the ocean.
The UN secretary general, António Guterres, pointed out that as well as the past three years being the hottest on record, the past 11 years were also the hottest on record.
Speaking at the press launch via video link, Guterres added: “Planet Earth is being pushed beyond its limits. Every key climate indicator is flashing red.”
