Worried that the sea is quietly absorbing the planet’s warming? You’re right to look closer. Ocean heat content, the heat stored in seawater, set a record again in 2025, as in each of the past nine years, per a January 2026 analysis.
This explainer sits within the wider marine science section, where related ocean topics are collected.
| Item | Value |
|---|---|
| Upper 2000 m heat gain, 2025 versus 2024 | about 23 ± 8 ZJ |
| Share of excess greenhouse-gas heat absorbed by the ocean | more than 90% |
| Record-high ocean heat content streak | each of the past nine years |
| Warming rate of the upper 2000 m, 2005-2025 | 0.32 ± 0.14 W m-2 (10 yr)-1 |
| Global mean sea surface temperature, 2025 | about 0.5 °C above the 1981-2010 average |
| Ocean area ranked among its top three warmest | about 33% |
These figures come from the study abstract and the research team’s news release. They can change as datasets are revised, so check the sources before you rely on them.
What ocean heat content actually measures
Ocean heat content (OHC) is the heat stored in the ocean. Because it reflects the accumulation of that heat over time, the news release describes it as one of the best indicators of long-term climate change.
The reason is the ocean’s role as a reservoir. The same release says the ocean absorbs more than 90% of the excess heat trapped by greenhouse gases, which makes it the main heat reservoir of the climate system.
You will meet OHC reported in a few different ways, and it helps to know which one a headline is using:
- A total change: the yearly gain in stored heat, given in zettajoules (ZJ).
- A rate: how fast heat is accumulating, given in watts per square metre per decade.
- A ranking: the share of ocean area that falls among its warmest years on record.
How much heat did the ocean gain in 2025?
According to IAP/CAS estimates in the study abstract, global upper 2000 m OHC rose by about 23 ± 8 ZJ relative to 2024. The plus-or-minus is the uncertainty, so the result is best read as a range rather than one exact number.
The news release puts 23 ZJ in context: it is equivalent to about 37 years of global primary energy consumption at the 2023 level. That comparison is a way to picture the scale, not a measure of how the heat is used.
The result rests on more than one dataset. According to the same release, more than 50 scientists from 31 research institutions took part, combining three observational products (from the Institute of Atmospheric Physics at the Chinese Academy of Sciences, Copernicus Marine and NOAA/NCEI) with an ocean reanalysis called CIGAR-RT. All of them confirmed that 2025 reached the highest OHC on record, as of the analysis published in January 2026.
This is not a one-off spike. Oceanographic Magazine’s report describes 2025 as the ninth consecutive year in which the ocean set a new heat record.
Is the ocean warming faster than before?
The study’s rate figures suggest yes. The abstract reports that warming of the upper 2000 m increased from 0.14 ± 0.03 W m-2 (10 yr)-1 over 1960-2025 to 0.32 ± 0.14 W m-2 (10 yr)-1 over 2005-2025 (IAP/CAS). The authors note the latter is consistent with estimates of Earth’s Energy Imbalance within uncertainties.
The same pattern shows up in the longer view. Oceanographic Magazine reports that since the 1990s, heat accumulation in the upper 2,000 metres has increased more rapidly than in previous decades.
As for the cause, the abstract links continued ocean warming in 2025 to increased greenhouse gas concentrations and recent reductions in sulfate aerosols.
Why surface temperature and heat content can tell different stories
Surface readings can dip while the total heat keeps climbing. The abstract says global annual mean sea surface temperature (SST) in 2025 was 0.49°C above the 1981-2010 baseline, yet 0.12 ± 0.03°C lower than in 2024.
It still ranked as the third-warmest year on record. The news release attributes the small dip from 2023 and 2024 mainly to the transition from El Niño to La Niña in the tropical Pacific.
So the two measures answer different questions. SST describes the surface, while OHC totals the heat held in the upper 2000 m, which is why OHC could still hit a record in a year when the surface cooled slightly.
Where the warming shows up
Ocean warming is not uniform. The news release says about 16% of the global ocean area reached a record-high OHC in 2025, and about 33% ranked among its top three warmest values.
The abstract adds that about 57% fell within the top five over the 1958-2025 record. Regions named across the two sources include:
- the tropical and South Atlantic Ocean
- the North Pacific Ocean
- the Southern Ocean
- the Mediterranean Sea
- the North Indian Ocean
Heat does not stay where it enters, because water moves between depths. A companion explainer on what is upwelling in the ocean covers one way that happens.
On the largest scale, the movement of water is covered in the guide to thermohaline circulation.
Why ocean heat content matters for sea level and weather
The news release states that rising ocean heat drives global sea-level rise through thermal expansion. It also strengthens and prolongs heatwaves and intensifies extreme weather by adding heat and moisture to the atmosphere.
The release ties warmer surface waters to more evaporation and heavier rain. It links that pattern to events in 2025:
- widespread flooding and disruption across much of Southeast Asia
- drought in the Middle East
- flooding in Mexico and the Pacific Northwest
Its conclusion is plain: as long as the Earth’s heat continues to increase, ocean heat content will continue to rise and records will continue to fall.
How to read an ocean heat headline
Numbers about the ocean can sound alike while meaning different things. Before you compare two claims, run through this short checklist, using the study abstract as the model:
- Check the depth. The 2025 figures cover the upper 2000 m, not the whole water column.
- Check the period. A rate for 1960-2025 differs from a rate for 2005-2025.
- Check the baseline. SST anomalies are measured against a reference, here 1981-2010.
- Check the uncertainty. A figure like 23 ± 8 ZJ is a range.
- Check the dataset. The abstract names IAP/CAS as the source of its headline estimate.
Frequently asked questions
What is ocean heat content?
Show answer
It is the amount of heat stored in the ocean. The news release says it reflects the accumulation of heat in the ocean over time, which is why it is a key indicator of long-term climate change.
Did ocean heat content set a record in 2025?
Show answer
Yes, according to the analysis published in January 2026. The news release says it reached a record high in 2025 as it has in each of the past nine years.
Why was sea surface temperature lower in 2025 than in 2024?
Show answer
The news release attributes the slightly lower SST mainly to the transition from El Niño to La Niña in the tropical Pacific. Even so, 2025 was the third-warmest year in the instrumental record.
How much excess heat does the ocean absorb?
Show answer
The news release says the ocean absorbs more than 90% of the excess heat trapped by greenhouse gases.
Does a warmer ocean raise sea level?
Show answer
Yes. Warmer water takes up more space, and the news release names this thermal expansion as one driver of global sea-level rise.
