Ocean’s Ability to Absorb Carbon Is Weakening as Climate Change Intensifies, UN Warns

The world’s oceans, which play a crucial role in regulating the Earth’s climate by absorbing large amounts of carbon dioxide from the atmosphere, are showing signs of weakening as global temperatures continue to rise, the United Nations has warned.

The development has raised fresh concerns among climate scientists, as the ocean acts as one of the planet’s most important natural carbon sinks. Its ability to absorb and store carbon dioxide helps slow the pace of global warming. However, rising sea temperatures, changing ocean circulation patterns and increasing acidification are putting this vital function under growing pressure.

The warning highlights the need for stronger climate action, reduced greenhouse gas emissions and better protection of marine ecosystems.

Oceans Play a Critical Role in Fighting Climate Change

Oceans cover more than 70 per cent of the Earth’s surface and absorb a substantial share of the carbon dioxide released by human activities.

They take up carbon dioxide through physical and biological processes. Colder waters can absorb more carbon dioxide, while marine organisms such as phytoplankton use carbon during photosynthesis. Some of this carbon eventually sinks into deeper ocean layers, where it can remain stored for long periods.

This natural process helps limit the amount of carbon dioxide accumulating in the atmosphere.

However, scientists warn that climate change is disrupting the conditions that make the ocean such an effective carbon sink. As the planet warms, the capacity of seawater to absorb additional carbon dioxide can decline, potentially leaving more greenhouse gases in the atmosphere.

UN Report Finds Ocean’s Carbon Absorption Capacity Has Fallen by 6%

A new 2026 UN report warns that climate change has already reduced the ocean’s capacity to absorb carbon by approximately 6 per cent.

According to the report, the ocean absorbs around 30 per cent of human-caused carbon dioxide emissions and approximately 90 per cent of the excess heat trapped by greenhouse gases.

The assessment was developed through collaboration between the United Nations Environment Programme (UNEP) and the UN Framework Convention on Climate Change (UNFCCC), ahead of the COP31 climate conference scheduled for November.

Sara Traerup, the report’s lead author, warned that the ocean’s ability to provide this essential climate service is coming under increasing pressure.

Rising temperatures and changes in ocean chemistry, circulation patterns and marine ecosystems are affecting the natural processes responsible for absorbing and storing carbon.

Scientists fear that these changes could accelerate in the coming years, making it increasingly difficult to limit global warming.

One of the biggest concerns is the continuous rise in ocean temperatures.

Normally, colder seawater can absorb more carbon dioxide than warmer water. As global temperatures increase, the ability of ocean waters to take up additional carbon dioxide can weaken.

Climate change is also altering ocean circulation, which plays an important role in transporting carbon from surface waters into deeper layers.

Another major concern is ocean acidification. When seawater absorbs carbon dioxide, chemical reactions increase its acidity, affecting marine organisms such as corals, shellfish and other species that depend on calcium carbonate to build their shells and structures.

The combination of warming, acidification and ecosystem degradation is placing additional pressure on the ocean’s carbon storage systems.

The weakening of the ocean’s carbon absorption capacity is also threatening marine biodiversity and coastal ecosystems.

Coral reefs, mangrove forests, seagrass meadows and salt marshes play important roles in storing carbon and maintaining ecological balance.

According to the UN report, so-called blue carbon ecosystems, including mangroves, seagrasses and salt marshes, already capture approximately 407 million tonnes of carbon dioxide annually.

These ecosystems also provide several other environmental benefits:

• Protect coastal communities from storms, flooding and erosion.

• Provide habitats for fish and other marine species.

• Support fisheries and the livelihoods of coastal communities.

• Store carbon in vegetation and underwater sediments.

However, rising sea levels, pollution, coastal development and increasing temperatures are putting these natural carbon storage systems at risk.

The UN report emphasises the importance of strengthening ocean research, monitoring systems and climate technologies to understand how marine carbon systems are changing.

It highlights three major areas requiring attention.

1. Improved ocean monitoring

Scientists need better access to reliable information about ocean temperatures, carbon absorption, acidity and circulation patterns.

The report identifies major gaps in monitoring data, particularly across the Southern Ocean, Indian Ocean, Arctic and several coastal regions.

2. Protection of blue carbon ecosystems

Restoring mangroves, seagrass meadows and salt marshes could strengthen natural carbon storage while protecting biodiversity and coastal communities.

3. Development of marine carbon removal technologies

Researchers are exploring technologies that could remove carbon dioxide from the ocean or atmosphere and store it over extended periods.

However, these approaches remain uncertain and require further scientific research, environmental safeguards and effective regulation.

The report also highlights the vulnerability of Small Island Developing States (SIDS), which contribute less than 1 per cent of global greenhouse gas emissions but face significant threats from rising sea levels, coastal erosion and ocean warming.

These countries often lack the financial resources and technological infrastructure needed to monitor and protect their marine ecosystems.

The UN has called for increased climate financing, technology transfers and capacity-building programmes to help developing countries strengthen ocean conservation efforts.

Examples include initiatives in Fiji, Papua New Guinea, Solomon Islands and Vanuatu, where local communities have received support to map mangrove forests and seagrass meadows and assess their carbon storage potential.

The report also stresses the importance of incorporating Indigenous and local knowledge into marine conservation policies.

The weakening of the ocean’s ability to absorb carbon could have serious consequences for global climate targets.

If the ocean stores less carbon dioxide, more of the greenhouse gas could remain in the atmosphere, adding to global warming and increasing pressure on already vulnerable ecosystems.

Martin Krause, Director of the Climate Change Division at UNEP, warned that a weaker ocean carbon sink would make achieving climate goals more difficult and expensive.

The report makes clear that ocean-based technologies and conservation programmes cannot replace reductions in greenhouse gas emissions. Instead, they must complement efforts to reduce fossil fuel consumption and accelerate the transition towards cleaner energy.

As governments prepare for COP31, protecting the ocean’s climate-regulating role is expected to remain an important part of international climate discussions.