Copernicus report: oceans absorbed record 23 zettajoules of heat in 2025

The world's oceans absorbed a record 23 zettajoules of additional heat in 2025, roughly 40 times all the energy humanity consumed that year, according to the 10th Copernicus Ocean State Report published on September 30, 2026. The report, produced by Mercator Ocean International for the European Union's Copernicus Marine Service, marks a decade of authoritative ocean science and describes an ocean that is hotter, higher, more acidic and less hospitable than at any point in the modern observational record.

The 23 zettajoule figure is the ninth consecutive annual record for ocean heat content, and it represents an extraordinary concentration of thermal energy. CNN reported on September 30 that the addition was equivalent to 12 Hiroshima bombs detonating every second for a full year. A joule is the electricity needed to run a one watt device for one second, and a zettajoule is a billion trillion joules, a unit that becomes meaningful only when applied to planetary scales.

The report draws on more than four decades of data collected by satellites, buoys and ocean sensors, and it was assembled by more than 100 international scientists. It arrives at a moment when the ocean is already absorbing roughly 90 percent of the excess heat trapped by greenhouse gases and about one third of the carbon dioxide humanity has emitted. Those two services have shielded the atmosphere from far more severe warming, but the bill is now being presented in the form of rising seas, marine heatwaves and collapsing polar habitats.

"The ocean is transforming before our eyes," said Pierre Bahurel, director general of Mercator Ocean International, which published the report. "Warming, rising seas and ecosystem degradation are interconnected signs of significant and long term change." Bahurel's warning is echoed across the report's findings, which span from the Mediterranean to the Kara Sea and from the Arctic to Antarctica.

Key Facts

The Guardian reported on September 30 that the record breaking addition of 23 zettajoules of heat left ocean waters hotter, higher and less hospitable. Sea levels have risen by about 3.8 millimeters per year since 1999, accelerating to 4.2 millimeters per year between 2012 and 2025, a total increase of nearly four inches. The global ocean has also become about 17 percent more acidic over the last 40 years, a chemical shift that weakens the shells and skeletons of many marine organisms.

The Copernicus Marine Service said on September 30 in its official release that the total amount of heat stored in the ocean reached its highest level on record in 2025. The report notes that the rate of ocean warming has doubled over the past 20 years, and that sea surface temperatures reached record highs in 2023, 2024 and again in 2026. A monster El Nino is adding further pressure, and it is on track to be the most intense on record.

Regional extremes are equally stark. In 2024 the Mediterranean and Black Seas experienced record breaking marine heatwaves, with temperatures in some regions exceeding 4.6 degrees Celsius above average and some events lasting more than 30 days. The entire Mediterranean has warmed by 0.32 degrees Celsius per decade, and Europe as a whole is heating twice as fast as the global average. The Mediterranean also recorded the highest number of marine heatwave days over the past two decades, with the Eastern Mediterranean seeing the highest number, the longest average duration and the most intense events.

The Japan Times reported on September 30 that polar oceans are being transformed. The Arctic's combined sea surface and sea ice surface temperature rose 4.2 degrees Celsius between 1982 and 2024, or about 1 degree Celsius per decade, compared with just 0.12 degrees Celsius per decade for global sea surface temperatures. In the Kara Sea north of Siberia, temperatures have climbed 6.7 degrees Celsius since 1982, a rate of 1.68 degrees Celsius per decade.

Arctic sea ice has shrunk by nearly half a million square kilometers per decade since 1979, equivalent to losing an area the size of Spain every ten years. The Copernicus release puts the figure at roughly 490,000 square kilometers per decade between 1979 and 2025. In Antarctica, record low sea ice in 2022 coincided with the first ever recorded large scale breeding failure of emperor penguins, and four colonies saw total breeding failures that year. In April, emperor penguins were upgraded from near threatened to endangered by the IUCN, and more than 90 percent of colonies could be near extinct by 2100 if sea ice loss continues at its current rate.

Analysis

The numbers in OSR10 are not isolated anomalies. They describe a system that has been absorbing an ever larger share of humanity's waste heat for decades, and the pace is accelerating. The warming rate has doubled in 20 years, sea level rise has accelerated from 3.8 to 4.2 millimeters per year, and the ocean has added a record amount of heat for nine consecutive years. Each of these trends reinforces the others, because warmer water expands, melts ice sheets from below, and holds less dissolved oxygen and carbon dioxide.

What this really means is that the ocean's role as a climate buffer is being converted into a climate amplifier. The same water that has spared the atmosphere from roughly 90 percent of the extra heat is now driving stronger marine heatwaves, more frequent coral bleaching and the collapse of polar habitats. Between January 2023 and September 2025, 84 percent of the world's coral reefs experienced bleaching level heat stress. Over the past 30 years, nearly 5 percent of the ocean has seen a persistent shift in phytoplankton growth patterns, and the report notes that the most recent decade stands out from the rest.

The polar findings deserve particular attention. The Arctic has warmed at about 1 degree Celsius per decade in combined sea surface and sea ice temperature since 1982, while the global sea surface has warmed at 0.12 degrees Celsius per decade, and the retreating ice exposes coastlines to waves and worsening erosion. Karina von Schuckmann, director of the oceans report, said the Arctic sea ice has been declining since the early 1980s, leaving around 60 percent of coastal infrastructure vulnerable to erosion and sea level rise. The Barents Sea experienced an unprecedented freshening between 2016 and 2022, temporarily pausing the longer term Atlantification of that Arctic sea, a reminder that regional shifts can be complex even as the overall direction of travel is unambiguous.

Small Island Developing States face multiple overlapping climate driven pressures, and the report singles them out. For them, and for coastal communities everywhere, sea level rise is not an abstraction but a direct threat to freshwater supplies, infrastructure and territorial integrity.

Why It Matters

Ocean heat content is one of the most reliable indicators of planetary warming because the ocean integrates energy over time. A record 23 zettajoules in a single year is a signal that the Earth is accumulating heat at a rate that far exceeds anything in the instrumental record. Because the ocean absorbs about 90 percent of the excess heat produced by humans burning fossil fuels, that energy is effectively committed for centuries.

The consequences are already visible. Sea level rise of 3.8 millimeters per year since 1999 may sound small, but the acceleration to 4.2 millimeters per year between 2012 and 2025 compounds over decades. The 17 percent increase in ocean acidity since 1985 makes it harder for marine organisms to build shells and skeletons, with knock on effects for fisheries and food security. Marine heatwaves lasting more than 30 days and reaching 4.6 degrees Celsius above average can trigger mass mortality events in the Mediterranean and Black Seas.

The report also matters because it is a decade long baseline. The Copernicus Marine Service said on September 30 that OSR10 marks a decade of authoritative ocean science and highlights how the most recent decade stands out. With a monster El Nino on track to be the most intense on record, the coming years will test whether the ocean can continue to buffer the climate system without paying an even higher ecological price.

Next Up

The Copernicus Marine Service and Mercator Ocean International will continue tracking the 2026 El Nino through their satellite, buoy and sensor networks, and the next Ocean State Report will assess its full impact on ocean heat content, sea level and marine ecosystems. Researchers will continue to monitor Antarctic sea ice, coral reef bleaching risk across the tropics, and Mediterranean marine heatwaves as Europe continues to warm at twice the global average.

Policymakers face a narrowing window. The report says its findings on ecosystems, communities and coastal infrastructure are directly relevant to decisions being taken now. As Pierre Bahurel put it, the message is very clear: the ocean is changing rapidly, and the time to act on that knowledge is now.