Executive Summary
A new peer-reviewed assessment finds that global ocean heat content (OHC) in the upper 2000 meters reached its highest level on record in 2025, rising by about 23 ± 8 zettajoules (ZJ) compared with 2024 (Pan et al., 2026; IAP/CAS). The same paper reports that 33% of the global ocean area ranked among the historical top three warmest conditions (1958–2025) and 57% ranked among the top five, underscoring how widespread heat accumulation has become (Pan et al., 2026). At the surface, global mean sea surface temperature (SST) cooled slightly relative to 2024 (about 0.12 ± 0.03°C lower), consistent with a shift toward La Niña conditions, yet still ranked as the third-warmest year on record and remained 0.49°C above the 1981–2010 baseline (Pan et al., 2026; IAP/CAS and other SST datasets). Because the oceans absorb more than 90% of the excess heat trapped by greenhouse gases, OHC provides one of the most robust indicators of long-term climate change and helps explain why climate impacts can intensify even when surface temperatures temporarily fluctuate (Trenberth et al., 2014; von Schuckmann et al., 2020/2023; Pan et al., 2026).
What Is Ocean Heat Content, and Why Do Scientists Track It?
Ocean heat content measures the amount of heat energy stored in the ocean over a defined depth range (here, 0–2000 m). Conceptually, it is the climate system’s heat “savings account”: it integrates changes over depth and time, making it less sensitive to short-term atmospheric variability and ENSO swings than global mean surface temperature (Pan et al., 2026). The link to Earth’s Energy Imbalance (EEI) is direct. EEI is the net surplus of incoming over outgoing energy at the top of the atmosphere, and it has remained strongly positive in recent decades (Loeb et al., 2021/2022; Hakuba et al., 2024; Pan et al., 2026). With greenhouse gases at record levels, the paper notes that the ocean absorbs more than 90% of the excess heat trapped by these gases, so the thermal state of the ocean becomes central to understanding climate impacts (Pan et al., 2026; Trenberth et al., 2014; von Schuckmann et al., 2020/2023).
What Happened in 2025: The Headline Numbers
The study’s main result is straightforward: upper-2000 m OHC increased by about 23 ± 8 ZJ in 2025 relative to 2024 in the IAP/CAS estimate, and independent products (CIGAR-RT and Copernicus Marine) also confirm continued ocean heat gain (Pan et al., 2026). The spatial footprint of this warming is equally striking. Regionally, about 33% of the global ocean area fell in its historical top three warmest conditions and about 57% fell in the top five, with notable warmth in the tropical and South Atlantic, the Mediterranean Sea, the North Indian Ocean, and the Southern Ocean (Pan et al., 2026). A complementary ranking analysis finds that approximately 14% of the global ocean area reached its warmest state on record in 2025, reinforcing the notion that many regions are now approaching their local historical maxima (Pan et al., 2026).
If the Ocean Set a Heat Record, Why Did Sea Surface Temperature Cool?
This is one of the most common “answer engine” questions about the 2025 climate signal, and the paper directly addresses it. The global annual mean SST in 2025 was 0.49°C above the 1981–2010 baseline but 0.12 ± 0.03°C lower than in 2024, consistent with evolving La Niña conditions, yet still ranking as the third warmest year on record (Pan et al., 2026). The key mechanism is redistribution and air–sea coupling. As the climate system transitions toward La Niña, stronger trade winds and enhanced upwelling cool parts of the tropical Pacific surface, even while the ocean continues to take up heat overall (Pan et al., 2026). The study emphasizes that OHC changes are far less sensitive to short-term variability than global mean surface temperature, which is why OHC can keep rising even as SST temporarily dips (Pan et al., 2026).
Is Ocean Warming Accelerating? The Paper’s Evidence
The assessment reports a clear acceleration in ocean warming rates. In the abstract, the authors summarize that the 0–2000 m OHC warming rate increased from 0.14 ± 0.03 W m⁻² per decade (1960–2025) to 0.32 ± 0.14 W m⁻² per decade (2005–2025) in the IAP/CAS product, and that the recent rate is consistent with EEI estimates within uncertainties (Pan et al., 2026; IAP/CAS; EEI). In the main results, the authors quantify long-term and recent trends using multiple datasets. The IAP/CAS record shows an average heat gain of 6.6 ± 0.3 ZJ per year over 1958–2025, increasing from 2.9 ± 0.5 ZJ per year (1958–1985) to roughly 9.2 ± 0.4 ZJ per year after 1986, described as more than a threefold increase (Pan et al., 2026; IAP/CAS). Over 2007–2025, multiple products converge on even higher rates (roughly 11–13 ZJ per year), reflecting improved observational coverage in the modern era (IAP/CAS, Copernicus Marine, NCEI/NOAA, CIGAR-RT; Pan et al., 2026). The study also notes that OHC reached a new record for 9 consecutive years (2017–2025) in both IAP/CAS and CIGAR-RT, the longest streak of consecutive annual OHC records in the observational era (Pan et al., 2026).
Where Did the Heat Go? Regional Hotspots and Counter-Signals
The ranking map perspective clarifies which regions have been pushed toward their historical maxima by sustained warming. The paper highlights the Southern Ocean, the tropical and South Atlantic Oceans, the Mediterranean Sea, and the North Indian Ocean as regions with the highest rankings in 2025 (Pan et al., 2026). The regional breakdown also shows that year-to-year changes can differ by basin even in a record OHC year. For example, the Mediterranean Sea warmed by 0.11 GJ m⁻² (0.28 ZJ) in 2025 relative to 2024, exceeding its long-term trend over 2004–2024, and the paper points to independent observations consistent with continued intermediate-water warming (Pan et al., 2026). By contrast, the Indian Ocean experienced the strongest cooling among the eight regions analyzed, with upper-2000 m OHC decreasing by 0.22 GJ m⁻² (6.5 ZJ) relative to 2024—yet still ranking as the third highest in the historical record, illustrating how strong the long-term warming baseline has become (Pan et al., 2026).
How Do Scientists Estimate OHC and Validate It Against EEI?
For OHC, the study synthesizes three gridded observational products and one ocean reanalysis: IAP/CAS, Copernicus Marine, NOAA/NCEI, and the CIGAR reanalysis (Pan et al., 2026). The IAP/CAS analysis is based on subsurface temperature profiles from the World Ocean Database and post-quality-controlled Argo data, plus 107,623 additional non-WOD profiles that improve coverage in historically undersampled regions (Pan et al., 2026). Copernicus Marine (2005–2025) relies on Argo temperature profiles from the CORA dataset using a weighted box-averaging approach (Pan et al., 2026). CIGAR-RT is an ensemble reanalysis system; its real-time extension expanded from four to eight ensemble members, helping characterize uncertainty and improve near-real-time updates (Pan et al., 2026).
Earth’s Energy Imbalance Measurement and Trend Estimation Methods
For EEI, the authors use the CERES EBAF Ed4.2 product, noting that CERES has provided continuous broadband radiation measurements since March 2000, and that this study uses March 2000 through September 2025 due to data availability (Pan et al., 2026; CERES EBAF Ed4.2). They also report an EEI-implied ocean heat uptake increase of 0.37 ± 0.20 W m⁻² per decade over 2005 through September 2025, under the common assumption that more than 90% of the excess energy is absorbed by the ocean (Pan et al., 2026; CERES). Methodologically, the paper uses LOWESS smoothing to reduce the influence of high-frequency variability (including ENSO) on long-term trend estimates, and it quantifies internal uncertainty using Monte Carlo surrogate realizations (Pan et al., 2026).
Why the 2025 OHC Record Matters for Climate Risk and Adaptation
The paper connects rising OHC to three major impact pathways that matter for climate risk intelligence and decision-making: sea-level rise via thermal expansion, intensified marine heatwaves, and stronger extremes through increased heat and moisture exchange with the atmosphere (Pan et al., 2026). It also argues that global OHC is expected to keep breaking records until net-zero greenhouse gas emissions are achieved, because a positive EEI implies ongoing heat accumulation (Pan et al., 2026). A practical takeaway for climate monitoring is that continuity of both ocean observing systems and space-based radiation measurements is essential for constraining the global energy budget and detecting shifts in the pace of warming (Pan et al., 2026).
Frequently Asked Questions (FAQs)
- What is ocean heat content, and why is it important? Ocean heat content (OHC) measures the total amount of heat stored in the ocean over a given depth. Because the ocean absorbs more than 90% of excess heat from greenhouse gas emissions, OHC is one of the most reliable indicators of long-term climate change.
- Why did ocean heat content reach a record in 2025 even though sea surface temperatures cooled slightly? Short-term climate variability, such as a shift toward La Niña conditions, can temporarily cool sea surface temperatures while the ocean as a whole continues to accumulate heat at depth. OHC integrates heat over time and depth, making it less sensitive to year-to-year fluctuations.
- How much did global ocean heat content increase in 2025? The upper 2000 meters of the global ocean gained approximately 23 ± 8 zettajoules of heat in 2025 compared with 2024, marking the highest level on record and continuing a multi-year trend of accelerating ocean warming.
- Is ocean warming accelerating, and what evidence supports this? Yes. Observations show that the rate of ocean heat uptake in recent decades is more than double the long-term average since the mid-20th century, consistent with measurements of Earth’s persistent energy imbalance.
- Why does rising ocean heat content matter for climate risk and adaptation? Increasing OHC drives sea-level rise through thermal expansion, intensifies marine heatwaves, and fuels stronger weather extremes. These impacts directly affect infrastructure, ecosystems, and economies, making OHC a critical metric for climate risk assessment and adaptation planning.
(Source: Ocean Heat Content Sets Another Record in 2025 (Advances in Atmospheric Sciences; Pan et al., 2026; DOI: 10.1007/s00376-026-5876-0).)
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