What Is the Role of the Ocean in Global Warming?
The ocean is often described as a victim of global warming. That is true, but it is only half the story.
The ocean is also one of Earth’s most important climate regulators. It absorbs enormous amounts of heat and carbon dioxide (CO₂), moves heat around the planet, influences rainfall and storms, and helps keep global temperatures from rising even faster.
But there is a serious problem: the ocean has limits.
As humans continue to add greenhouse gases to the atmosphere, the ocean is absorbing more heat and CO₂. This protects us from even faster atmospheric warming, but it is also changing the ocean itself.
The latest evidence makes this especially clear. The World Meteorological Organization (WMO) reports that ocean heat content reached a new record in 2025, and the rate of ocean warming during 2005–2025 was more than twice the rate observed during 1960–2005.
Understanding the ocean is therefore essential to understanding global warming.
Why Is the Ocean So Important to Earth’s Climate?
The ocean covers about 71% of Earth’s surface and contains an enormous amount of water.
Water can absorb and store large quantities of heat. This gives the ocean a powerful ability to moderate Earth's climate.
Think of the ocean as a giant heat reservoir.
When greenhouse gases trap additional energy in the Earth system, most of that extra energy does not immediately stay in the atmosphere. The ocean absorbs the vast majority of it.
Scientists estimate that around 90% of the excess heat in the climate system is stored in the ocean.
This has slowed the rate at which the atmosphere and land have warmed.
But it does not mean the problem has disappeared.
The heat is still in the Earth system. The ocean is storing it.
1. The Ocean Absorbs Most of the Excess Heat
This is probably the ocean's most important role in global warming.
Greenhouse gases such as carbon dioxide, methane and nitrous oxide trap heat in Earth's climate system. The ocean absorbs much of that extra energy.
According to NOAA, upper-ocean heat content reached a record high in 2025. It was the fifth consecutive year in which a new record was set.
WMO's 2025 assessment also found that ocean heat content reached its highest level in the 66-year observational record.
That matters because ocean heat is not simply a number for scientists.
More ocean heat can contribute to:
- Stronger marine heatwaves
- Coral bleaching
- Changes in fish populations
- Melting of floating and polar ice
- Sea-level rise
- Changes in rainfall
- More energy available to some storms
- Stress on marine ecosystems
In other words, ocean warming connects directly to things people experience on land.
2. The Ocean Absorbs Carbon Dioxide
The ocean does more than absorb heat.
It also absorbs a substantial amount of human-produced CO₂.
The IPCC estimates that the surface ocean has absorbed about one-quarter of human-caused CO₂ emissions. NOAA gives a broader estimate of around 30% of the CO₂ released into the atmosphere.
This is extremely important.
If the ocean did not absorb so much CO₂, atmospheric CO₂ levels would be even higher.
That would increase the greenhouse effect and lead to more warming.
But there is a cost.
More CO₂ means greater ocean acidification
When seawater absorbs CO₂, chemical reactions occur. They increase hydrogen-ion concentration and reduce the availability of carbonate ions.
This makes seawater more acidic.
Ocean acidification can make it harder for some organisms to build and maintain shells and skeletons.
This includes organisms such as:
- Corals
- Oysters
- Clams
- Some plankton
- Other shell-forming marine organisms
This is an important example of how the ocean can protect the atmosphere while simultaneously experiencing damage itself.
3. The Ocean Delays Some Effects of Global Warming
The ocean's ability to store heat has created a kind of natural buffer.
Without this enormous heat-storage capacity, Earth's surface temperature would respond much more quickly to the energy imbalance created by greenhouse gases.
Ocean currents also move heat around the planet.
Warm water can travel from tropical regions toward higher latitudes. Cold water can move toward warmer regions.
This circulation affects:
- Regional temperatures
- Rainfall
- Storms
- Marine ecosystems
- Seasonal weather patterns
The ocean therefore acts like a huge global climate transportation system.
However, this buffering effect should not be misunderstood.
The ocean is not eliminating global warming. It is slowing part of the warming response while accumulating heat and undergoing major changes.
4. Warmer Oceans Are Raising Sea Levels
Ocean warming is one of the important causes of sea-level rise.
There are two major processes involved.
Thermal expansion
Warm water takes up more space than cold water.
As the ocean warms, seawater expands. This causes sea levels to rise.
Melting land ice
Global warming is also melting glaciers and ice sheets.
The water from melting land ice eventually enters the ocean and increases sea level.
According to NOAA's 2025 climate assessment, global mean sea level reached a record high for the 14th consecutive year.
This creates serious risks for coastal communities.
Potential impacts include:
- Coastal flooding
- Erosion
- Saltwater intrusion
- Damage to infrastructure
- Loss of wetlands
- Greater storm-surge risk
- Threats to low-lying islands
This is not only an environmental issue.
It is also an economic and social issue.
5. Ocean Warming Creates Marine Heatwaves
A marine heatwave is a period when ocean temperatures are unusually high for a particular location.
These events can last days, weeks or even months.
Marine heatwaves are becoming an increasingly important climate concern.
They can cause:
- Coral bleaching
- Fish migration
- Seagrass damage
- Kelp forest loss
- Changes in food webs
- Fisheries problems
- Stress on marine animals
NOAA reported that approximately 87% of the global ocean surface experienced at least one marine heatwave during 2025.
That is a striking number.
It shows why looking only at average global temperature can hide important changes happening beneath the surface.
6. Warmer Oceans Can Influence Extreme Weather
Warm ocean water provides energy and moisture to the atmosphere.
This can influence tropical cyclones and other weather systems.
A warmer ocean does not automatically mean every storm will become stronger. Storm behavior depends on many factors, including atmospheric conditions and wind patterns.
But warmer oceans can provide more energy and moisture that may contribute to more intense rainfall and stronger tropical cyclones in suitable conditions.
This is one reason scientists monitor ocean temperatures closely before and during tropical storms.
The ocean and atmosphere are not separate systems.
They constantly exchange heat, water and energy.
7. Ocean Warming Threatens Coral Reefs
Coral reefs are particularly sensitive to changes in ocean temperature.
When corals experience prolonged heat stress, they can expel the microscopic algae that normally live within their tissues.
This causes coral bleaching.
Bleached coral is not necessarily immediately dead. But prolonged or repeated heat stress can cause serious damage and mortality.
Coral reefs support extraordinary biodiversity and provide food, tourism income, coastal protection and other benefits to millions of people.
This makes ocean warming a human issue as well as an ecological one.
8. The Ocean Is Part of the Carbon Cycle
The ocean is a major part of Earth's natural carbon cycle.
Carbon enters the ocean through several processes.
These include:
- Direct absorption of atmospheric CO₂
- Biological activity by marine organisms
- Transport of carbon into deeper waters
- Circulation and mixing
Some carbon can eventually be transported into the deep ocean and stored for long periods.
Marine organisms also play a role.
Tiny organisms such as phytoplankton use CO₂ during photosynthesis. When organisms die or are consumed, some of that carbon can move through marine food webs and into deeper waters.
This is sometimes described as the biological carbon pump.
It is a remarkable natural process.
But it should not be treated as an unlimited solution to human carbon emissions.
9. What Happens If the Ocean Keeps Warming?
This is where the future becomes particularly important.
The ocean will continue to respond to the greenhouse gases already added to the climate system.
WMO reports that ocean warming can persist for very long periods, even if emissions decline substantially.
A warmer ocean can lead to a chain of connected effects:
Greenhouse gases increase → more heat is trapped → ocean absorbs more heat → seawater warms → ecosystems are stressed → sea level rises → weather and coastal risks increase.
The exact outcome will vary by region.
Some places will experience stronger impacts than others.
But the overall direction is clear.
10. Why 2025 Was an Important Ocean Climate Year
The latest data give us an important warning.
According to WMO, 2025 was one of the two or three warmest years on record, depending on the dataset. The organization also reported that the past 11 years, from 2015 through 2025, were the 11 warmest years on record.
At the same time, ocean heat content reached another record.
A 2026 study reported that the heat stored in the upper 2,000 meters of the ocean increased by approximately 23 ± 8 zettajoules from 2024 to 2025.
A zettajoule is an enormous amount of energy.
The number is difficult to imagine, but the message is simple:
The ocean is continuing to accumulate huge amounts of heat.
Ocean vs. Global Warming: Is the Ocean Helping or Hurting?
The answer is both.
The ocean helps by:
- Absorbing most excess climate heat
- Absorbing a large share of human CO₂
- Moving heat around the planet
- Supporting the global carbon cycle
- Moderating Earth's climate
But warming is harming the ocean through:
- Rising ocean temperatures
- Marine heatwaves
- Ocean acidification
- Coral bleaching
- Biodiversity loss
- Changing fisheries
- Sea-level rise
- Deoxygenation in some regions
This apparent contradiction is one of the most important things to understand about climate change.
The ocean is protecting us from some of the immediate effects of global warming, but the protection comes at a cost to the ocean itself.
What Can We Do?
There is no single solution.
Protecting the ocean requires action on several levels.
Governments and industries can:
- Reduce fossil-fuel use
- Expand clean energy
- Improve energy efficiency
- Protect marine ecosystems
- Reduce overfishing
- Establish and effectively manage marine protected areas
- Improve coastal planning
- Monitor ocean temperatures and chemistry
- Strengthen early-warning systems
Individuals can:
- Reduce unnecessary energy use
- Choose lower-carbon transportation when practical
- Reduce food waste
- Support sustainable seafood choices
- Avoid unnecessary single-use products
- Reduce consumption where possible
- Support credible climate and ocean conservation efforts
- Learn how local climate policies affect their community
Individual action alone cannot solve climate change.
But individual choices can support broader changes in markets, politics and culture.
The biggest gains will come when personal action and large-scale policy work together.
My Perspective: The Ocean Is a Climate Warning System
In my view, one of the biggest mistakes we can make is to think of the ocean as something far away.
For someone living hundreds of miles from a coastline, ocean warming may seem unrelated to daily life.
It is not.
The ocean influences the air we breathe, rainfall, food systems, storms, coastal economies and global climate.
More importantly, ocean heat tells us something that air temperature alone cannot.
Earth is continuing to accumulate energy.
The fact that ocean heat keeps reaching record levels means we should not become complacent when a particular year is slightly cooler than the previous one.
Short-term variations such as El Niño and La Niña can move temperatures up and down.
The long-term trend is what matters.
And that trend remains deeply concerning.
What Does the Future Hold?
The future of the ocean depends heavily on what happens with greenhouse-gas emissions.
If emissions remain high, we can expect greater warming, more marine heatwaves, continued sea-level rise, increasing acidification and growing pressure on marine ecosystems.
If humanity rapidly reduces greenhouse-gas emissions, the future can be less damaging.
Some ocean changes will continue for a long time because the ocean responds slowly.
But every fraction of a degree of avoided warming matters.
That is why reducing emissions is not only about protecting the atmosphere.
It is also about giving the ocean a better chance to recover and continue supporting life.
Final Thoughts
The ocean is one of Earth's greatest climate protectors.
It absorbs enormous amounts of heat.
It removes a significant amount of human-produced CO₂ from the atmosphere.
It distributes heat around the planet.
But we are asking too much of it.
Record ocean heat in recent years is a clear reminder that the ocean cannot absorb unlimited amounts of our climate pollution without consequences.
The most important lesson is simple:
The ocean is not hiding global warming. It is storing much of it.
Protecting the ocean therefore means protecting the climate, food systems, coastal communities and future generations.
The best time to reduce pressure on the ocean is not when its limits have been reached. It is now.
What You Can Do Today
Lasting change can begin with a few small steps rather than trying to change everything overnight.
Start with a few practical steps:
- Learn where your energy comes from.
- Reduce unnecessary energy consumption.
- Waste less food.
- Choose sustainable seafood when reliable information is available.
- Avoid unnecessary plastic use.
- Support credible environmental policies and conservation projects.
- Share accurate climate information, rather than sensational claims.
Small actions become more meaningful when they support larger changes in technology, business and government.
Frequently Asked Questions
Does the ocean cause global warming?
No. Today’s global warming is mainly driven by the increasing concentration of greenhouse gases released through human activities. The ocean absorbs much of the resulting excess heat and CO₂, which changes the ocean itself.
How much excess heat does the ocean absorb?
Approximately 90% of the excess heat in Earth's climate system is stored in the ocean.
Does the ocean absorb CO₂?
Yes. The ocean has absorbed roughly one-quarter to around 30% of human-produced atmospheric CO₂, depending on the period and accounting method used.
Does ocean warming cause sea-level rise?
Yes. Warmer seawater expands, contributing to sea-level rise. Melting glaciers and ice sheets add additional water to the ocean.
Can the ocean stop global warming?
No. The ocean can slow some aspects of atmospheric warming, but it cannot permanently absorb unlimited heat and CO₂ without consequences.
Disclaimer
Disclaimer: This article is for general educational purposes. Climate conditions, scientific estimates and regional impacts can change as new observations become available. For scientific, policy or professional decisions, consult current reports from authoritative organizations such as WMO, NOAA and the IPCC.
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