A new United Nations report has raised concern about the ocean’s ability to keep absorbing carbon dioxide as the climate warms, placing the issue in focus before the COP31 climate conference.
The ocean still plays a major role in limiting the build-up of heat and carbon dioxide in the climate system. It absorbs roughly 30 per cent of human-caused carbon dioxide emissions and around 90 per cent of the excess heat associated with greenhouse gases. The new assessment says, however, that climate change has left the ocean taking up about six per cent less carbon than it otherwise could.
That figure does not mean the ocean has stopped absorbing carbon, nor does it describe a single uniform change in every sea. Carbon uptake differs between places and seasons, and it depends on linked physical, chemical and biological processes. The assessment’s warning is that those processes are already under growing pressure, reducing the strength of a natural service that has helped moderate warming.
Why carbon uptake matters
When the ocean absorbs carbon dioxide, less of it remains in the atmosphere. A weaker ocean carbon sink therefore leaves more carbon dioxide in the air than would otherwise be the case, making efforts to meet climate goals more difficult and potentially more costly.
The report identifies warming, acidification, deoxygenation, pollution and ecosystem degradation as pressures that can disrupt the ocean’s climate-regulating role. These pressures do not operate in isolation. Changes in water temperature and chemistry can affect the movement of carbon through the ocean, while the condition of coastal and marine ecosystems also matters to the wider picture.
The assessment does not present the six per cent estimate as a forecast of a sudden, fixed loss in future years. Instead, it describes an existing reduction in the amount of carbon the ocean could otherwise have absorbed. That distinction matters when interpreting a figure drawn from a complex system whose behaviour varies across regions and over time.
A call for better observation
At the centre of the report is a proposal described as Ocean Carbon Intelligence. It brings together ocean science, monitoring technologies, data infrastructure and analytical tools in an effort to give governments, researchers and investors a clearer view of how carbon moves through the ocean.
The proposed approach is not a single technology or a substitute for cutting emissions. It is an argument for stronger long-term observation, better data systems and improved modelling. The report says gaps in monitoring and scientific understanding can make it harder to assess risks, judge the likely effects of proposed interventions and take informed decisions.
Those gaps have practical consequences. Reliable observation is needed to distinguish a temporary change from a longer-term shift, to understand regional differences and to evaluate whether climate-related projects are delivering their stated aims. The report also highlights the importance of access to finance, technology, data and capacity-building for coastal states and small island developing states.
Limits on ocean-based solutions
Interest in ocean-based climate action is growing, including work to restore coastal ecosystems that store carbon and emerging approaches intended to remove carbon dioxide from seawater or the atmosphere. The report says such measures may have a role, but only as a complement to rapid and sustained reductions in greenhouse-gas emissions.
It also sets conditions for responsible development: a strong scientific basis, transparency, environmental safeguards and effective governance. That emphasis reflects the need to understand potential benefits and risks before interventions are scaled up. A project designed to help the climate cannot be assessed solely by its intended carbon outcome if it may also affect marine ecosystems or communities that depend on them.
The timing gives the report added relevance ahead of COP31. Its central message is that the ocean cannot be treated as an unlimited buffer for continued emissions. Protecting its ability to regulate the climate depends both on reducing the pressures already affecting it and on building the knowledge needed to understand how the system is changing.
