The Ocean Didn’t ‘Cool’ in 2025—It Hid 24 Zettajoules of Heat Instead (and the Argo-Float Fight Is Your New Climate Signal)
2025 looked calmer at the surface, and a comforting narrative took off. But peer‑reviewed ocean heat content shows the ocean stored ~24 ZJ anyway—just deeper than the charts people share.
By TheMurrow Editorial
April 3, 2026
Key Points
- 1Reject the “2025 cooling” narrative: ocean heat content still jumped by ~23–24 ZJ even as sea-surface temperatures eased.
- 2Distinguish metrics: SST is a noisy surface snapshot; OHC is the integrated energy balance sheet through depth (often 0–2000 m).
- 3Interpret “hidden heat” literally: mixing and subduction can push warmth downward, muting SST charts while stored heat keeps rising.
In 2025, the ocean played a familiar trick on public attention.
Sea-surface temperatures—the numbers that feed nightly weather graphics and viral charts—looked a little less alarming than the highs of 2023 and 2024. Some people took that as a sign the planet’s overheating had eased. The story was comforting, tidy, and wrong.
What the surface seemed to say—and what the ocean actually did
Beneath the surface, the ocean kept taking on energy. A lot of it. A peer‑reviewed analysis published in Advances in Atmospheric Sciences on January 9, 2026 reported that global upper‑2000 meter ocean heat content rose by about 23 ± 8 zettajoules (ZJ) from 2024 to 2025. A Nature Reviews Earth & Environment research highlight published March 31, 2026 summarized the full‑depth change as +24 ± 6 ZJ over the same period—and noted that full‑depth ocean heat content has increased by 481 ± 48 ZJ since 1960 (as of 2025).
So no, the ocean didn’t “cool” in 2025. What happened is more subtle—and more unsettling: the ocean continued to store heat, even as surface signals wobbled.
So no, the ocean didn’t “cool” in 2025. What happened is more subtle—and more unsettling: the ocean continued to store heat, even as surface signals wobbled.
“Sea-surface temperature can take a breather. Ocean heat content rarely does.”
— — TheMurrow Editorial
The headline that’s almost right: “the ocean hid ~24 zettajoules of heat”
The phrase “hid heat” is rhetorically loaded, but the underlying science is close to the truth—if you translate it properly.
The 2026 Advances in Atmospheric Sciences paper (led by researchers associated with the Institute of Atmospheric Physics, Chinese Academy of Sciences—often abbreviated IAP/CAS in summaries) reports an upper‑ocean (0–2000 m) increase of ~23 ± 8 ZJ from 2024 to 2025. A separate Nature Reviews Earth & Environment highlight describes full‑depth ocean heat content increasing by 24 ± 6 ZJ from 2024 to 2025, putting the annual jump in the context of a long trend: +481 ± 48 ZJ since 1960 (as of 2025).
Those are not small numbers, but zettajoules don’t come with built‑in intuition. One widely cited comparison comes via the EurekAlert summary of the AAS paper: 23 ZJ is roughly 37 years of global primary energy consumption at the 2023 level (~620 exajoules per year). That comparison can be misread if it’s not stated carefully: it does not mean humans used 37 years of energy in one year. It means the extra heat the upper ocean gained in that single year is comparable in magnitude to decades of current human energy use.
“Hidden,” in this context, does not imply secrecy or a missing measurement. It describes vertical redistribution—heat mixing downward or being subducted below the surface where it doesn’t show up as dramatically in sea-surface temperature (SST) charts.
The 2026 Advances in Atmospheric Sciences paper (led by researchers associated with the Institute of Atmospheric Physics, Chinese Academy of Sciences—often abbreviated IAP/CAS in summaries) reports an upper‑ocean (0–2000 m) increase of ~23 ± 8 ZJ from 2024 to 2025. A separate Nature Reviews Earth & Environment highlight describes full‑depth ocean heat content increasing by 24 ± 6 ZJ from 2024 to 2025, putting the annual jump in the context of a long trend: +481 ± 48 ZJ since 1960 (as of 2025).
Those are not small numbers, but zettajoules don’t come with built‑in intuition. One widely cited comparison comes via the EurekAlert summary of the AAS paper: 23 ZJ is roughly 37 years of global primary energy consumption at the 2023 level (~620 exajoules per year). That comparison can be misread if it’s not stated carefully: it does not mean humans used 37 years of energy in one year. It means the extra heat the upper ocean gained in that single year is comparable in magnitude to decades of current human energy use.
“Hidden,” in this context, does not imply secrecy or a missing measurement. It describes vertical redistribution—heat mixing downward or being subducted below the surface where it doesn’t show up as dramatically in sea-surface temperature (SST) charts.
+23 ± 8 ZJ
Estimated increase in global upper‑2000 m ocean heat content from 2024 to 2025 (Advances in Atmospheric Sciences, Jan. 9, 2026).
+24 ± 6 ZJ
Estimated full‑depth ocean heat content increase from 2024 to 2025 (Nature Reviews Earth & Environment highlight, Mar. 31, 2026).
What “hidden” really means: muted surface signals, rising stored heat
Surface temperatures respond quickly to winds, clouds, and ocean circulation. Heat content responds to the long‑term energy imbalance of the planet. A year can be “less record‑breaking” at the surface while the ocean keeps accumulating energy through depth.
That distinction—surface vs. stored—is where much of the 2025 confusion began.
That distinction—surface vs. stored—is where much of the 2025 confusion began.
“Calling it ‘hidden heat’ isn’t a conspiracy claim. It’s a shorthand for heat moving below the surface.”
— — TheMurrow Editorial
Why people thought the ocean cooled in 2025
The confusion wasn’t irrational. It was a category error, encouraged by the way climate information is packaged.
Many reports described 2025 global average sea-surface temperature as lower than 2023–2024, often framed as “third warmest,” with coverage pointing to the transition from El Niño toward La Niña conditions in the tropical Pacific. EurekAlert’s reporting reflects that narrative: surface patterns shift with El Niño and La Niña, and those shifts can show up in global averages.
The problem is that SST is not the same thing as ocean heat content. SST is the “skin” temperature, sensitive to short‑term changes in winds and currents. OHC measures the integrated heat stored through depth—most commonly reported for 0–2000 meters, where observational coverage is stronger.
A one‑year dip or plateau in SST does not mean the ocean stopped taking up heat. It can mean heat moved into deeper layers, or that the atmosphere temporarily stopped transferring as much heat into the topmost layer—even while the net energy imbalance continued.
Many reports described 2025 global average sea-surface temperature as lower than 2023–2024, often framed as “third warmest,” with coverage pointing to the transition from El Niño toward La Niña conditions in the tropical Pacific. EurekAlert’s reporting reflects that narrative: surface patterns shift with El Niño and La Niña, and those shifts can show up in global averages.
The problem is that SST is not the same thing as ocean heat content. SST is the “skin” temperature, sensitive to short‑term changes in winds and currents. OHC measures the integrated heat stored through depth—most commonly reported for 0–2000 meters, where observational coverage is stronger.
A one‑year dip or plateau in SST does not mean the ocean stopped taking up heat. It can mean heat moved into deeper layers, or that the atmosphere temporarily stopped transferring as much heat into the topmost layer—even while the net energy imbalance continued.
A tale of two metrics: SST vs. OHC
Readers deserve a clear mental model:
- Sea-surface temperature (SST) tells you what’s happening at the top layer where the ocean meets the atmosphere.
- Ocean heat content (OHC) tells you how much energy the ocean has stored, integrated over depth.
SST is noisy; OHC is steadier. That’s why multiple secondary writeups—including Time and others—could truthfully say: 2025 set a record for ocean heat content even though sea‑surface temperatures were not record‑breaking.
- Sea-surface temperature (SST) tells you what’s happening at the top layer where the ocean meets the atmosphere.
- Ocean heat content (OHC) tells you how much energy the ocean has stored, integrated over depth.
SST is noisy; OHC is steadier. That’s why multiple secondary writeups—including Time and others—could truthfully say: 2025 set a record for ocean heat content even though sea‑surface temperatures were not record‑breaking.
Two climate metrics that get conflated
Before
- Sea-surface temperature (SST)
- top “skin” layer
- highly responsive to winds/currents
- can swing year-to-year
After
- Ocean heat content (OHC)
- integrated heat through depth (often 0–2000 m or full depth)
- steadier record of accumulated energy
Ocean heat content, explained without the math headache
Ocean heat content is conceptually simple: it’s the heat stored in seawater compared to a baseline, calculated from temperature profiles across the ocean’s volume and the heat capacity of seawater.
The reason scientists and climate services emphasize OHC is straightforward. According to the Copernicus Marine “Ocean Heat Content” explainer, the ocean absorbs nearly 90% of the excess heat associated with global warming. That framing matters: the ocean is the planet’s main heat reservoir, and changes in that reservoir carry consequences.
If the atmosphere is where we feel heat day to day, the ocean is where the climate system banks it.
The reason scientists and climate services emphasize OHC is straightforward. According to the Copernicus Marine “Ocean Heat Content” explainer, the ocean absorbs nearly 90% of the excess heat associated with global warming. That framing matters: the ocean is the planet’s main heat reservoir, and changes in that reservoir carry consequences.
If the atmosphere is where we feel heat day to day, the ocean is where the climate system banks it.
Nearly 90%
Share of excess heat associated with global warming absorbed by the ocean (per Copernicus Marine’s OHC explainer, as referenced in the article).
Why the “upper 2000 meters” keeps showing up
Most public OHC reporting focuses on the upper 2000 meters. The reason is not that the deep ocean is unimportant; it’s that the observations are richest and confidence is higher in the upper layers. The AAS paper’s headline number—~23 ± 8 ZJ (0–2000 m) from 2024 to 2025—fits that established practice.
The Nature Reviews Earth & Environment highlight adds another lens: full‑depth OHC. That matters because heat doesn’t stop at 2000 meters. Over time, circulation can transport energy deeper, and full‑depth accounting aims to capture the whole system.
The key point for non-specialists is not the exact depth boundary. It’s the direction: up.
The Nature Reviews Earth & Environment highlight adds another lens: full‑depth OHC. That matters because heat doesn’t stop at 2000 meters. Over time, circulation can transport energy deeper, and full‑depth accounting aims to capture the whole system.
The key point for non-specialists is not the exact depth boundary. It’s the direction: up.
“The ocean is where global warming goes to live.”
— — TheMurrow Editorial
The 2025 numbers in context: a record year that didn’t look like one
The year-to-year jump is striking on its own: +23 ± 8 ZJ in the upper 2000 m (AAS), or +24 ± 6 ZJ full‑depth (Nature Reviews highlight), from 2024 to 2025.
But the bigger story is what that jump sits atop. The Nature Reviews Earth & Environment highlight reports +481 ± 48 ZJ of full‑depth ocean heat content since 1960 (as of 2025). That number is best understood as accumulated change: decades of a persistent planetary energy imbalance, with the ocean doing most of the absorbing.
Coverage from outlets like Carbon Brief and Time emphasizes the same paradox that confused readers: OHC can hit a record even when SST doesn’t. A surface‑focused narrative tends to interpret “not a new SST record” as “cooling.” A heat‑content narrative reads it as: “the system kept accumulating energy.”
But the bigger story is what that jump sits atop. The Nature Reviews Earth & Environment highlight reports +481 ± 48 ZJ of full‑depth ocean heat content since 1960 (as of 2025). That number is best understood as accumulated change: decades of a persistent planetary energy imbalance, with the ocean doing most of the absorbing.
Coverage from outlets like Carbon Brief and Time emphasizes the same paradox that confused readers: OHC can hit a record even when SST doesn’t. A surface‑focused narrative tends to interpret “not a new SST record” as “cooling.” A heat‑content narrative reads it as: “the system kept accumulating energy.”
+481 ± 48 ZJ
Reported increase in full‑depth ocean heat content since 1960 (as of 2025), per the Nature Reviews Earth & Environment highlight.
The energy comparison—useful, but easy to misuse
EurekAlert’s summary offers a memorable comparator: 23 ZJ is about 37 years of global primary energy consumption at the 2023 level (~620 EJ/year). Carbon Brief offers another framing, describing the annual jump as roughly dozens of times global energy use in 2023 (their figure is often cited as ~39×).
These comparisons can anchor the scale, but readers should treat them as “order-of-magnitude intuition,” not as a one-to-one equivalence. Ocean heat uptake is distributed through a vast mass of water. It doesn’t behave like electricity on a grid or fuel in a tank. The usefulness of the comparison is emotional calibration: the ocean’s heat gain is enormous.
These comparisons can anchor the scale, but readers should treat them as “order-of-magnitude intuition,” not as a one-to-one equivalence. Ocean heat uptake is distributed through a vast mass of water. It doesn’t behave like electricity on a grid or fuel in a tank. The usefulness of the comparison is emotional calibration: the ocean’s heat gain is enormous.
Key Insight
“Hidden heat” is about where the energy went (downward), not whether it exists. OHC keeps rising even when SST charts look calmer.
How heat gets “moved” downward: the mechanics behind the mismatch
If SST wasn’t record-shattering in 2025, where did the heat go? The most responsible answer, given the research at hand, is: into the ocean—often below the immediate surface layer—through mixing and redistribution.
The AAS analysis and related explanations emphasize that OHC is the metric that keeps the score even when surface conditions shift. The “hid heat” framing maps onto processes like:
- Vertical mixing, where winds and waves stir warmer water downward.
- Subduction, where surface waters slide beneath other layers along density surfaces.
- Circulation changes tied to large-scale climate patterns, including El Niño and La Niña transitions, that can redistribute heat without eliminating it.
Readers do not need a fluid dynamics degree to grasp the implication. A lower global average SST in a given year can coincide with continued heat accumulation because the surface is not the whole ocean.
The AAS analysis and related explanations emphasize that OHC is the metric that keeps the score even when surface conditions shift. The “hid heat” framing maps onto processes like:
- Vertical mixing, where winds and waves stir warmer water downward.
- Subduction, where surface waters slide beneath other layers along density surfaces.
- Circulation changes tied to large-scale climate patterns, including El Niño and La Niña transitions, that can redistribute heat without eliminating it.
Readers do not need a fluid dynamics degree to grasp the implication. A lower global average SST in a given year can coincide with continued heat accumulation because the surface is not the whole ocean.
Multiple perspectives: what skeptics get right—and where they go wrong
Skeptics and contrarians often point to a cooler‑than‑last‑year SST and argue the climate system is “recovering.” They are right about one narrow observation: SST can fluctuate, and those fluctuations can be meaningful for weather.
Where that argument collapses is the inference. A single metric over a short period is not a full energy budget. Ocean heat content—especially measured through the upper 2000 meters and, where possible, full depth—tracks the integrated warming more reliably.
A scientifically honest take holds two ideas at once: the surface can vary, and the system can still be accumulating heat.
Where that argument collapses is the inference. A single metric over a short period is not a full energy budget. Ocean heat content—especially measured through the upper 2000 meters and, where possible, full depth—tracks the integrated warming more reliably.
A scientifically honest take holds two ideas at once: the surface can vary, and the system can still be accumulating heat.
Real-world implications: why OHC is not an academic statistic
Ocean heat content sounds abstract until you connect it to what people live through. Stored ocean heat influences sea level (through thermal expansion), marine heatwaves, and the energy available to weather systems. The research notes provided here don’t quantify those downstream impacts, so we should avoid pretending the 2025 OHC increase translates into a specific number of storms or a precise amount of sea-level rise.
Still, the direction of travel has practical meaning: more heat in the ocean increases the baseline energy in Earth’s climate system.
Still, the direction of travel has practical meaning: more heat in the ocean increases the baseline energy in Earth’s climate system.
Case study: the “quiet surface, hot interior” year
2025 is a useful case study precisely because it frustrates simplistic narratives. By many accounts, global average SST in 2025 ranked below 2023–2024, likely influenced in part by the Pacific’s shift from El Niño toward La Niña. People watching surface charts could convince themselves the system had cooled.
Yet the peer‑reviewed OHC accounting says the opposite for stored heat: a record or near‑record heat gain—~23 ± 8 ZJ in the upper 2000 m and ~24 ± 6 ZJ full‑depth.
The lesson is not “ignore SST.” The lesson is to treat SST as a headline and OHC as the balance sheet.
Yet the peer‑reviewed OHC accounting says the opposite for stored heat: a record or near‑record heat gain—~23 ± 8 ZJ in the upper 2000 m and ~24 ± 6 ZJ full‑depth.
The lesson is not “ignore SST.” The lesson is to treat SST as a headline and OHC as the balance sheet.
Practical takeaways for readers, voters, and communicators
If you want to stay oriented when climate claims start ricocheting across social media:
- Ask which metric is being cited. If the claim is “the ocean cooled,” check whether it’s SST or OHC.
- Look for depth. “Upper 2000 m” is a standard benchmark; “full depth” adds context.
- Favor multi‑year context over single-year rankings. OHC’s value is its stability compared with surface wiggles.
- Treat energy analogies as scale cues, not proofs. “37 years of energy use” is an intuition pump, not a direct equivalence.
- Reward communicators who cite uncertainty. Numbers like 23 ± 8 ZJ and 24 ± 6 ZJ tell you scientists are quantifying confidence, not just declaring certainty.
- Ask which metric is being cited. If the claim is “the ocean cooled,” check whether it’s SST or OHC.
- Look for depth. “Upper 2000 m” is a standard benchmark; “full depth” adds context.
- Favor multi‑year context over single-year rankings. OHC’s value is its stability compared with surface wiggles.
- Treat energy analogies as scale cues, not proofs. “37 years of energy use” is an intuition pump, not a direct equivalence.
- Reward communicators who cite uncertainty. Numbers like 23 ± 8 ZJ and 24 ± 6 ZJ tell you scientists are quantifying confidence, not just declaring certainty.
A quick checklist for evaluating “the ocean cooled” claims
- ✓Identify the metric: SST or OHC
- ✓Check the depth range: 0–2000 m vs full depth
- ✓Compare multi‑year trends, not a single-year rank
- ✓Treat energy comparisons as intuition, not literal equivalence
- ✓Look for uncertainty ranges (± ZJ) and sourced studies
Reading climate signals like an adult: what 2025 should teach us
The ocean did not “take a year off” in 2025. The best-available synthesis says it continued to absorb heat—and did so at record levels—while surface temperatures failed to deliver the dramatic visual reinforcement many people have come to expect.
That mismatch is not a minor technicality. It is a reminder that climate change is an energy story, not a vibes story. Surface temperature is where we feel the day-to-day, but ocean heat content is where the system keeps the long-term receipts.
The next time a chart suggests the ocean “cooled,” the more intelligent question is: cooled where, and measured how?
Because in 2025, by the measures that matter most for the climate’s trajectory, the ocean did not cool. It kept warming—quietly, deeply, and at a scale large enough to make a zettajoule feel like a small unit.
That mismatch is not a minor technicality. It is a reminder that climate change is an energy story, not a vibes story. Surface temperature is where we feel the day-to-day, but ocean heat content is where the system keeps the long-term receipts.
The next time a chart suggests the ocean “cooled,” the more intelligent question is: cooled where, and measured how?
Because in 2025, by the measures that matter most for the climate’s trajectory, the ocean did not cool. It kept warming—quietly, deeply, and at a scale large enough to make a zettajoule feel like a small unit.
1) Did the ocean cool in 2025?
Not according to the peer‑reviewed estimates summarized here. A paper in Advances in Atmospheric Sciences (Jan. 9, 2026) reports upper‑2000 m OHC increased by ~23 ± 8 ZJ from 2024 to 2025. A Nature Reviews Earth & Environment highlight (Mar. 31, 2026) reports full‑depth OHC increased by 24 ± 6 ZJ over the same period.
2) Why did some reports say 2025 sea-surface temperatures were lower than 2023–2024?
Because they were discussing sea-surface temperature (SST), which can fluctuate year to year. Coverage linked 2025’s SST ranking (often described as below 2023–2024) partly to the transition from El Niño toward La Niña in the tropical Pacific. SST is a real signal, but it’s not the same as total heat stored in the ocean.
3) What’s the difference between SST and ocean heat content (OHC)?
SST is the temperature at the ocean’s surface. OHC is the total heat stored through a layer of the ocean (often 0–2000 meters) or even full depth. OHC is generally a steadier indicator of the planet’s energy imbalance because it integrates changes through depth rather than reflecting short-term surface variability.
4) What does it mean to say the ocean “hid” heat?
It’s a narrative shorthand for heat being redistributed below the surface, through processes such as mixing and subduction. When more heat is stored below the surface, SST may look less extreme even as OHC continues to rise. “Hidden” doesn’t mean unmeasured; it means not obvious if you only watch surface charts.
5) How big is 23–24 zettajoules, really?
It’s enormous. One comparison cited in coverage (via EurekAlert’s summary of the AAS paper) is that 23 ZJ is roughly 37 years of global primary energy consumption at the 2023 level (~620 EJ/year). The comparison helps convey scale, but it’s not a literal statement about energy use in one year—it’s a magnitude analogy.
6) Why do scientists often report OHC for the upper 2000 meters?
Because observations are more abundant and reliable in the upper ocean, making 0–2000 m a common benchmark for tracking change. That said, deeper layers matter too, which is why the Nature Reviews Earth & Environment highlight also discusses full‑depth ocean heat content.
7) What’s the long-term trend in ocean heat content?
The Nature Reviews Earth & Environment highlight reports that full‑depth OHC has risen by 481 ± 48 ZJ since 1960 (as of 2025). That long-term accumulation underscores why OHC is treated as a core indicator of ongoing global warming, even when surface temperatures fluctuate year to year.
T
About the Author
TheMurrow Editorial is a writer for TheMurrow covering science.
Frequently Asked Questions
Did the ocean cool in 2025?
Not according to the peer‑reviewed estimates summarized here. A paper in Advances in Atmospheric Sciences (Jan. 9, 2026) reports upper‑2000 m OHC increased by ~23 ± 8 ZJ from 2024 to 2025. A Nature Reviews Earth & Environment highlight (Mar. 31, 2026) reports full‑depth OHC increased by 24 ± 6 ZJ over the same period.
Why did some reports say 2025 sea-surface temperatures were lower than 2023–2024?
Because they were discussing sea-surface temperature (SST), which can fluctuate year to year. Coverage linked 2025’s SST ranking partly to the transition from El Niño toward La Niña in the tropical Pacific. SST is a real signal, but it’s not the same as total heat stored in the ocean.
What’s the difference between SST and ocean heat content (OHC)?
SST is the temperature at the ocean’s surface. OHC is the total heat stored through a layer of the ocean (often 0–2000 meters) or even full depth. OHC is generally a steadier indicator of the planet’s energy imbalance because it integrates changes through depth rather than reflecting short-term surface variability.
What does it mean to say the ocean “hid” heat?
It’s shorthand for heat being redistributed below the surface via mixing and subduction. When more heat is stored below the surface, SST may look less extreme even as OHC continues to rise. “Hidden” doesn’t mean unmeasured; it means less visible on surface charts.
How big is 23–24 zettajoules, really?
It’s enormous. One comparison cited in coverage is that 23 ZJ is roughly 37 years of global primary energy consumption at the 2023 level (~620 EJ/year). It’s a magnitude analogy, not a literal statement about annual human energy use.
What’s the long-term trend in ocean heat content?
The Nature Reviews Earth & Environment highlight reports that full‑depth OHC has risen by 481 ± 48 ZJ since 1960 (as of 2025), underscoring persistent long-term heat accumulation even when SST fluctuates.
