The world’s oceans absorbed a record amount of heat in 2025, highlighting the accelerating impact of climate change on marine ecosystems, sea levels and coastal communities.

A new assessment from Mercator Ocean International estimates that the oceans absorbed around 23 zettajoules of excess heat during 2025. The amount is roughly 40 times the total annual energy consumption of humanity, underlining the enormous role the oceans play in absorbing excess heat generated by a warming planet.

Oceans Continue to Take Up Most Excess Heat

The oceans have absorbed the vast majority of the additional heat associated with greenhouse gas emissions.

This has helped slow the rate at which atmospheric temperatures would otherwise rise, but the process is producing significant changes beneath the ocean surface. Rising water temperatures are affecting marine ecosystems, weather patterns and the physical properties of seawater.

Scientists are increasingly monitoring ocean heat as one of the key indicators of the changing climate.

Sea Levels Continue to Rise

The warming of the oceans is also contributing to rising global sea levels.

Seawater expands as it becomes warmer, while melting glaciers and ice sheets add additional water to the world’s oceans. The assessment indicates that the rate of sea-level rise has accelerated compared with earlier decades.

The continued rise poses growing challenges for low-lying coastal regions and small island states, where even relatively small increases in sea level can increase exposure to flooding and coastal erosion.

Ocean Acidification Adds Further Pressure

Another major concern is the increasing acidity of seawater.

The oceans absorb a significant amount of carbon dioxide from the atmosphere. As carbon dioxide enters seawater, chemical reactions reduce ocean pH and make the water more acidic.

Ocean acidity has increased substantially since the pre-industrial era, creating additional pressure on organisms such as corals and shell-forming marine species.

Marine Heatwaves Affect Ecosystems

Rising ocean temperatures are also contributing to more intense marine heatwaves.

Exceptionally warm waters can disrupt marine food chains, alter fish distribution and damage sensitive ecosystems. Coral reefs are particularly vulnerable to prolonged periods of unusually high temperatures because extreme heat can cause coral bleaching.

Recent years have seen extensive marine heat stress in several parts of the world’s oceans.

Polar Regions Facing Rapid Changes

The effects of ocean warming are particularly visible around the polar regions.

Shrinking sea ice is changing habitats for marine species and affecting ecosystems that depend on seasonal ice conditions. Changes in Arctic and Antarctic waters can also influence ocean circulation and wider climate patterns.

Scientists are closely monitoring these changes because polar ecosystems are highly sensitive to temperature and sea-ice variations.

Climate Change and Ocean Health

The latest findings underline the close relationship between atmospheric warming and ocean conditions.

Ocean warming, sea-level rise, acidification and marine heatwaves are not isolated developments. They interact with one another and can compound pressure on marine ecosystems and coastal societies.

The continuing accumulation of heat in the oceans therefore remains a major indicator of the scale of the global climate challenge.

A Growing Global Challenge

The record heat absorbed by the oceans demonstrates that climate change is affecting the planet far beyond land-based temperature records.

As greenhouse gas concentrations remain elevated, scientists expect the world’s oceans to remain under significant pressure. Protecting marine ecosystems will require efforts to reduce greenhouse-gas emissions while strengthening adaptation measures for vulnerable coastal communities.

The latest assessment provides another indication that changes in the world’s oceans are becoming an increasingly important part of the global climate story.