Oceans absorb most of the excess heat caused by global warming
Oceans cover more than 70% of the Earth’s surface and play a fundamental role in regulating global temperatures. They absorb much of the excess heat trapped in the atmosphere by greenhouse gases, helping to slow the rate of atmospheric warming.
According to the Intergovernmental Panel on Climate Change (IPCC), the oceans have absorbed more than 90% of the excess heat accumulated in the Earth’s climate system over recent decades.
While this process has limited atmospheric temperature increases, it has also caused significant changes in marine environments. Ocean temperatures have risen across much of the planet, with warming affecting both surface waters and deeper ocean layers.
The consequences extend far beyond temperature changes. Warmer waters influence ocean circulation, oxygen availability and the distribution of nutrients essential to marine ecosystems.
Marine heatwaves, defined as prolonged periods of unusually high ocean temperatures, have also become more frequent and intense. These events can cause sudden disruptions to marine habitats, sometimes with effects lasting several years.
Coral reefs face an unprecedented threat
Coral reefs are among the ecosystems most vulnerable to rising ocean temperatures. Although they cover only a small fraction of the ocean floor, they provide habitats for an extraordinary diversity of marine organisms.
Corals depend on microscopic algae living within their tissues. These algae provide much of the energy needed for coral growth and survival.
When water temperatures remain excessively high, corals become stressed and may expel these algae, causing a phenomenon known as coral bleaching. Without their usual energy source, corals become weaker and may eventually die if stressful conditions persist.
The scale of this problem has increased considerably. According to the US National Oceanic and Atmospheric Administration (NOAA), the fourth global coral bleaching event, which began in 2023, exposed approximately 84% of the world’s coral reef area to bleaching-level heat stress by mid-2025.
The destruction of coral reefs has consequences for thousands of marine species that depend on them for food, shelter and reproduction.
It also affects coastal populations, as healthy coral reefs help reduce wave energy and protect shorelines from erosion and storm damage.
Rising temperatures disrupt marine biodiversity
As ocean temperatures increase, many marine species are forced to adapt to changing environmental conditions.
Fish, marine mammals and other organisms often have specific temperature ranges within which they can survive and reproduce. When surrounding waters become too warm, some species migrate towards cooler regions or move into deeper waters.
These movements can disrupt established ecosystems. Predators may encounter unfamiliar prey, while species that depend on particular habitats may struggle to survive.
The distribution of commercially important fish populations is also changing. Some fishing regions may experience declining stocks, while other areas could see the arrival of species previously found in warmer waters.
For coastal communities that depend on fishing, these changes can create considerable economic uncertainty.
Not all marine organisms can move to more suitable habitats. Species living in coral reefs, coastal wetlands or isolated ecosystems may be particularly vulnerable when environmental conditions change too rapidly.
Ocean warming can therefore contribute to biodiversity loss and alter the structure of marine food chains.
Warmer oceans contain less oxygen
One of the less visible consequences of ocean warming is the decline in dissolved oxygen.
As seawater becomes warmer, its capacity to hold oxygen decreases. Rising temperatures also strengthen the separation between warmer surface waters and colder deep waters, a process known as ocean stratification.
This can reduce the circulation of oxygen and nutrients between different ocean layers.
For marine organisms, lower oxygen concentrations can create serious difficulties. Fish and other animals need sufficient oxygen to survive, and prolonged exposure to oxygen-poor waters can affect their growth, reproduction and behavior.
In extreme situations, oxygen levels may become too low to support many forms of marine life, contributing to the expansion of so-called dead zones.
According to the IPCC, ocean warming and declining oxygen levels are already affecting marine ecosystems, with further changes expected as global temperatures continue to rise.
These developments are particularly concerning because they can combine with other environmental pressures, including chemical pollution and excessive nutrient runoff from agricultural activities.
Rising sea levels threaten coastal environments
Ocean warming is also a major contributor to global sea-level rise.
As seawater becomes warmer, it expands, increasing the volume of the oceans. This process, known as thermal expansion, is one of the principal causes of rising sea levels, alongside the melting of glaciers and ice sheets on land.
Higher sea levels increase the risk of coastal flooding, shoreline erosion and saltwater intrusion into freshwater resources.
Low-lying islands, river deltas and densely populated coastal regions are particularly exposed.
Coastal ecosystems are also under pressure. Mangrove forests, salt marshes and wetlands provide important habitats for wildlife and help protect coastlines against storms.
When sea levels rise too quickly, these ecosystems may struggle to adapt, especially where buildings, roads or other infrastructure prevent them from moving inland.
Their degradation can further increase the vulnerability of coastal communities to extreme weather events.
Ocean warming can intensify extreme weather events
The temperature of the ocean surface influences weather systems around the world.
Warm ocean waters provide energy and moisture that can contribute to the development and intensification of tropical cyclones. When atmospheric conditions are favorable, higher sea surface temperatures can support more intense storms and heavier rainfall.
Ocean warming can also influence regional precipitation patterns and large-scale climate systems.
Changes in ocean circulation are another source of concern. Major currents transport heat around the planet and influence temperatures and weather conditions across continents.
Scientists are studying how continued warming and changes in seawater salinity could affect these circulation systems over the coming decades.
While the relationship between ocean warming and individual weather events is complex, the broader trend is clear: warmer oceans can amplify several climate-related risks and contribute to greater environmental instability.
Ocean acidification adds another pressure on marine life
Ocean warming is occurring alongside another major environmental change: ocean acidification.
The two processes are closely connected through rising atmospheric carbon dioxide concentrations, although they are not the same phenomenon.
While greenhouse gases trap heat and cause ocean temperatures to increase, the ocean also absorbs a significant proportion of the carbon dioxide released by human activities.
When CO₂ dissolves in seawater, it triggers chemical reactions that lower the water’s pH.
This change makes it more difficult for certain marine organisms, including corals, shellfish and some plankton species, to build and maintain their calcium carbonate shells or skeletons.
Combined with rising temperatures and declining oxygen levels, acidification places additional stress on marine ecosystems.
These multiple pressures can weaken the ability of marine environments to recover from disturbances and maintain their ecological functions.
Protecting the oceans requires global action
Addressing ocean warming requires efforts at both international and local levels.
Reducing greenhouse gas emissions remains the most important long-term measure. Limiting the use of fossil fuels, developing low-carbon energy systems and improving energy efficiency can help slow the accumulation of heat in the oceans.
Protecting marine ecosystems is another priority. Marine protected areas, sustainable fishing practices and the restoration of coastal habitats can strengthen the resilience of marine biodiversity.
Reducing plastic pollution, chemical contamination and other sources of environmental degradation can also help ecosystems better withstand rising temperatures.
Scientific monitoring is equally essential. Ocean temperature measurements, satellite observations and research into marine biodiversity provide valuable information for understanding changes and anticipating future risks.
Governments and coastal communities must also develop adaptation strategies, particularly in regions exposed to sea-level rise, declining fish populations and increasingly severe marine heatwaves.
The oceans are essential to life on Earth, influencing the climate, supporting biodiversity and providing food and economic resources for billions of people. Their warming represents one of the most significant environmental challenges of the coming decades. Preserving marine ecosystems will require sustained efforts to reduce emissions, protect vulnerable habitats and adapt to changes that are already underway.
