The scientific data collected over the past year leaves little room for doubt: in In 2025, the oceans once again set a temperature record.accumulating more thermal energy than at any other time since modern measurements have been available. The finding reinforces the scientific community's concern about the speed at which the climate system is transforming.
This new record is not simply a temporary spike in sea surface area; behind it lies a sustained increase in heat stored in the deep layerswhich act as a veritable "storage" for the excess energy generated by greenhouse gases. The consequences are already being felt in extreme weather events, rising sea levels, and the health of marine ecosystems in Europe and the rest of the planet.
A global record for ocean heat
An international consortium of more than 50 scientists from 31 institutions has concluded that by 2025 the ocean heat content The Earth's Hydrocarbons Concentration (OHC) reached its highest value in the entire available series, which dates back to the mid-20th century. This indicator measures the energy accumulated in the first 2.000 meters of depth, a key reference for monitoring the evolution of the Earth's crust. long-term global warming.
According to estimates, the seas added around 23 zettajoules of energy, an enormous amount that the authors equate to global energy consumption of about 37 yearsThis is based on 2023 levels (around 620 exajoules annually). In other words, the amount of extra heat absorbed by the oceans in 2025 is equivalent to several decades of all the energy used by humanity.
The work, published in the scientific journal Advances in Atmospheric SciencesIt also confirms a acceleration trend since the 1990sIn the last nine years, each year has marked a peak or remained very close to the upper limit of the series, indicating that the ocean system continues to accumulate energy almost uninterruptedly.
The analysis integrates databases from different observation programs, including the Institute of Atmospheric Physics of the Chinese Academy of Sciences, the European service Copernicus Marine and the NOAA/NCEI American, in addition to an ocean meta-analysis (CIGAR-RT) that combines information from Asia, Europe, and the Americas. This convergence of sources, which includes a fleet of thousands of buoys and autonomous robots, reinforces the robustness of the heating signal.
The authors emphasize that the ocean absorbs over 90% of the excess heat trapped by greenhouse gases, so the OHC has become established as one of the best indicators of the planet's energy balanceAs long as the Earth continues to gain energy, they explain, ocean heat content will continue to set new historical records.
Uneven warming: hotspots in the Atlantic, Mediterranean and Southern Ocean
The report makes it clear that the The increase in ocean temperature is not distributed equally.In 2025, around 16% of the world's ocean surface It reached record values of heat content and approximately one 33% were among the three highest levels of their records. That is, in a large part of the world's seas, historical highs are being reached or exceeded.
Among the most affected regions are the South Atlantic and tropical, the North Pacific, the Southern Ocean and wide swathes of the tropical oceansIn the European context, the Mediterranean Sea and the northern Indian Ocean These are cited among the hotspots where the increase in heat has been especially intense, with direct impacts on the coasts of southern Europe, North Africa and the Middle East.
Scientists point out that areas have also been observed with relative coolings, such as parts of the equatorial Pacific or the western Indian Ocean, linked above all to dynamic adjustments at basin scale The transition to La Niña conditions is already underway. These regional variations do not contradict global warming, but rather are part of the ocean's response to the combination of human forcing and natural variability.
In the first 2.000 depth metersThe signal is more homogeneous: virtually all basins show a steady energy gain since the 1990s, with a slight increase in the rate in the last part of the series. The underlying message is that the deep heat reservoir It continues to grow, although on the surface the effect may vary from year to year.
The authors insist that this uneven pattern implies that Certain regions concentrate higher risksat marine heat waves These effects can be more persistent, even causing severe disruptions to currents. For Europe, the focus is on the Atlantic basin and the Mediterranean, where episodes of exceptionally high water temperatures have already been documented in recent summers.
Sea surface temperature: third highest value recorded
Although the biggest leap is in the heat accumulated at depth, the global average sea surface temperature (TSM) also remained at very high levels. In 2025 it was the third warmest on record since reliable data has been available, settling around 0,5°C above the 1981-2010 average.
This value represents a slight decrease compared to 2023 and 2024, a difference that researchers attribute mainly to the transition from an intense El Niño episode to La Niña conditions in the tropical Pacific. These natural phenomena modulate the heat at the ocean surface on a scale of a few years, but do not alter the underlying upward trend associated with climate change.
Sea surface temperature (SST) is a particularly sensitive parameter for climate because It influences evaporation and the amount of moisture available in the atmosphere. Warmer waters result in air more laden with water vapor, which translates into heavier rains and more powerful storms when the right conditions are met.
During 2025, notable episodes were observed of extreme weather events linked, at least in part, to this excess of ocean energy: large-scale flooding in areas of Southeast Asia and Mexico, episodes of exceptional rainfall in the Pacific Northwest and periods of severe drought in the Middle EastAlthough these impacts are concentrated outside of Europe, the same physical mechanisms that trigger them influence Atlantic storms, heat waves, and gales that affect the continent.
Experts point out that with a warmer ocean, the probability of more intense tropical cyclones and prolonged marine heatwaves. When these situations reach mid-latitudes, they can lead to torrential rains, strong winds, and coastal flooding in European countries, especially along the Atlantic and Mediterranean coasts.
Impacts: sea level, extreme weather events and marine ecosystems
One of the most direct effects of increased ocean heat is the sea level riseAs water heats up, it expands, a process known as Thermal expansionAdded to this is the contribution of water from the melting of glaciers and ice sheets, so the increase in the OHC translates into a gradual but persistent rise in sea levels.
For European coastal areasFrom the Atlantic coast to the Mediterranean and the North Sea, this rise poses an added risk to natural erosion and storm surges. Organizations such as the German Federal Maritime and Hydrographic Agency have already issued warnings. unusually high temperatures in the North Sea and the Baltic SeaThis reinforces calls to strengthen defenses and long-term coastal planning.
The additional heat in the ocean also prolongs marine heat wavesThese are episodes in which the water temperature remains well above normal for weeks or months. These situations can cause mass die-offs of marine species, from corals and Posidonia seagrass meadows to fish and mollusks of fishing interest, with direct economic impacts for coastal communities.
The study indicates that, by 2025, processes such as the coral reef bleachingThis is a clear symptom of thermal stress and acidification caused by carbon dioxide absorption. Although large tropical reefs are far from Europe, their deterioration is an indicator of the general vulnerability of marine ecosystems in the face of the warming.
Furthermore, the increase in ocean heat content increases the humidity and the energy available in the atmosphereThis tends to intensify extreme rainfall and storms. Floods, landslides, and infrastructure damage become more likely when saturated soils, spring tides, and storms fueled by a warmer ocean coincide.
The science of ocean heat and the role of Europe
The advance in the knowledge of OHC has been made possible thanks to the combination of in situ observations, satellite sensors, and reanalysis modelsThe thousands of floating robots that are part of the international Argo network, capable of descending to depths of up to 2.000 meters and measuring temperature and salinity, have become an essential tool for monitoring the evolution of marine heat.
Europe plays a central role in this surveillance through programs such as Copernicus Marinewhich integrates data from satellites, buoys, ships, and numerical models to provide an up-to-date picture of the oceans. This information is key to meteorological services, fisheries management, coastal planning and climate risk assessment in the countries of the European Union and its surrounding area.
The new special collection of Advances in Atmospheric Sciences dedicated to changes in ocean heat content It will also address regional studies in nearby seas, such as seas of China, the South Pacific or the Indian Oceanbut includes analyses with direct implications for the North Atlantic and the Mediterranean. The goal is to refine the understanding of the mechanisms that redistribute heat between basins and depths.
Researchers such as Kevin Trenberth or Lijing Cheng emphasize the evolutionary nature of climate scienceEach year, better data and methods are incorporated, allowing for adjustments to estimates and a reduction in physical uncertainties. However, the overall picture emerging from the latest reports is consistent: the ocean continues to warm and act as a buffer against global warming, at the cost of accumulating stress on the climate system.
For Europe, this reality translates into the need to integrating ocean information into adaptation and mitigation policiesFrom coastal and port protection plans to the regulation of maritime activities and marine renewable energy, understanding how and where the oceans are warming is a prerequisite for anticipating impacts and preparing effective responses.
The authors of the study agree that the main unknown no longer lies so much in the physics of the climate system as in the decisions that society will make in the coming decadesThe ocean will continue to absorb heat as long as greenhouse gas concentrations continue to rise; the question is to what extent this will be achieved. rapidly reduce emissions and strengthen adaptation to limit damage to ecosystems, infrastructure and ways of life linked to the sea.
