The world's oceans are absorbing heat at a record rate and new research has shown that the sea environment is changing so quickly. In 2025 the oceans absorbed an additional 23 zettajoules of heat, a third record for ocean heat content. That is equivalent, in energy terms, to about 12 Hiroshima-scale atomic bombs hitting every second throughout the year. Scientists use the comparison to illustrate the enormous amount of energy being stored by the world's oceans.
So it is part of an increasingly clear picture that ocean warming is one of the most convincing indicators of long-term climate change. The new Copernicus Ocean State Report is based on decades of data collected by satellites, ocean sensors, buoys and other monitoring systems, and looks at global sea levels, sea ice and marine ecosystems. Data on ocean temperature, sea levels, acidity, sea ice and ocean ecology are all linked.
Oceans are absorbing most of the excess heat
The ocean acts as a major buffer against global warming, absorbing more than 90 per cent of the excess heat trapped by greenhouse gases. The atmosphere itself has cooled at a reduced rate, but enormous amounts of energy have moved from the atmosphere to the marine environment.
The 23 zettajoule increase in 2025 was the largest annual increase in ocean heat content since modern measurements were started and the ninth consecutive year in which a new ocean heat record was set. The estimate focuses on heat stored through the upper 2,000 metres of the ocean, where a significant amount of excess heat accumulates.
A zettajoule is an incredibly large unit of energy of one sextillion joules. Because the number is hard to visualize scientists and climate communicators have used the Hiroshima-bomb comparison to describe the scale. The comparison is only for the equivalent energy and does not mean that there is an atomic explosion in the ocean.
Why Rising Ocean Heat Matters
Increasing ocean temperatures have consequences far beyond the environment of the ocean. Warmer seawater can cause more frequent and persistent marine heatwaves, changes in ocean circulation and temperature of the atmosphere.
Ocean temperatures also influence evaporation and the amount of moisture in the atmosphere. Water and heat from the ocean can be transferred to air more easily as oceans warm, and so rainfall and extreme weather events can be more erratic. Ocean warming is a big part of the climate system for storms, precipitation and other aspects.
Marine ecosystems are most vulnerable because many species depend on relatively stable temperature conditions. Longer-term marine heatwaves can lead to stress on coral reefs, fish populations and other organisms, and ocean conditions can disrupt food webs and the distribution of marine species.
Rising seas add to the risk
Ocean warming is also closely linked to rising sea levels. As seawater becomes warmer, it expands and so sea levels rise. Melting glaciers and ice sheets add more water to the oceans.
Global sea levels have been rising for decades and are increasing the risks for coastal cities, low-lying cities and islands worldwide. Higher seas can be especially damaging for storm surges and coastal flooding, particularly during powerful storms.
For countries and communities situated close to the coast, these changes can affect housing, transport infrastructure, agriculture, freshwater supplies and livelihoods. Small island developing states are particularly vulnerable because even small sea level increases have a big impact on their land and infrastructure.
Ocean Acidification Creates Another Threat
Heat is not the only major change taking place beneath the waves. The world’s oceans are also becoming more acidic as they absorb carbon dioxide from the atmosphere.
Ocean acidification can interfere with the ability of organisms such as corals, oysters and other shell-forming species to build and maintain their shells or skeletons. This poses risks to marine ecosystems and industries and communities that depend on fisheries and healthy coastal environments.
All of these factors, warming, acidification, and other environmental changes can put multiple pressures on marine organisms at the same time. So scientists monitor these changes together, rather than treating ocean warming as a single problem.
The polar regions have changed drastically.
The effects of a warming ocean can be seen most clearly in polar regions. Sea ice coverage changes affect ecosystems dependent on frozen marine environments, such as the ones that feed, breed and shelter on sea ice.
Antarctic sea ice fluctuations have also been associated with major ecological implications. Extremely low sea ice conditions have caused problems for species such as Emperor penguins, whose breeding cycle depends heavily on stable sea ice conditions.
The Arctic is at a moment when environmental changes are also going fast with warming temperatures and falling sea ice that change habitats and ocean circulation and ecosystems.
What The Ocean Heat Record Means For The Future
The latest measurements underline the need to do long-term ocean monitoring. Scientists rely on satellites, buoys, ships and underwater instruments to gauge how rapidly conditions are changing and how different parts of the ocean respond.
The record heat stored in 2025 does not represent a temporary change confined to one year. Ocean heat content is an important measure of the Earth's long-term energy imbalance because water can store enormous quantities of heat.
The continuing rise in ocean temperatures, combined with sea-level rise, acidification and changes in polar ecosystems, shows why oceans are central to understanding the planet's changing climate. For coastal communities, marine industries and ecosystems, the consequences are already becoming increasingly severe.
As scientists continue to monitor the ocean more and more, that data will serve as a guide to governments and communities for the risks as well as for preparations for changes in marine environments. The record set in 2025 is a marker both for a longer-term trend in the warming ocean and the huge amount of energy that the planet’s seas are absorbing.
