New Study Highlights Ocean's Role in Carbon Emissions
The ocean has quietly become more important in slowing the pace of climate change. Since 1990, it has absorbed a growing fraction of carbon dioxide from fossil fuels, while forests and soils have shown no similar increase. That shift matters because it exposes the limits of what natural systems can do, and it changes the balance of risk in the global carbon cycle.
A study published in Science in November 2024 used atmospheric oxygen measurements to track where carbon goes after it leaves power stations, vehicles and industrial sites. The findings show the ocean taking up more carbon each decade, even as the land sink reached a peak around 2009 and then weakened. For UK businesses working on carbon reduction, supply chain emissions or net zero targets, the research underlines a fundamental point. Natural carbon sinks are not expanding in line with emissions, and the systems we rely on to buffer atmospheric CO₂ are under growing strain.
Oxygen measurements reveal diverging carbon sinks
Researchers at the Scripps Institution of Oceanography analysed more than three decades of data from the Scripps O₂ Program. Atmospheric oxygen provides an independent way to distinguish carbon absorbed by the ocean from carbon taken up or released by land ecosystems. When fossil fuels burn, they consume oxygen and release carbon dioxide. Photosynthesis does the reverse. By measuring both gases together, scientists can estimate how much carbon each natural sink is removing.
Between 1990 and 2019, the ocean absorbed an average of 2.8 billion metric tons of CO₂ each year. That represented roughly 35 per cent of fossil fuel emissions during the period. Ocean uptake grew by approximately 0.63 billion metric tons of carbon per decade. Meanwhile, land ecosystems absorbed an average of about 1 billion metric tons of carbon annually, equivalent to around 13 per cent of fossil fuel emissions. Crucially, the land sink showed no significant long term growth over those three decades.
The land sink is a net figure. It reflects carbon absorbed by vegetation and soils, minus emissions from deforestation, wildfires, decomposition and land use change. A stable or declining land sink means terrestrial ecosystems are not increasing their contribution as emissions rise. The Scripps study found that the land sink reached its strongest level around 2009, then generally weakened.
Why this divergence between ocean and land uptake matters
Most global carbon budget estimates combine atmospheric CO₂ measurements with fossil fuel data, ocean chemistry, satellite observations and ecosystem models. The Scripps research adds a long term oxygen record to that mix. The approach is valuable because land carbon estimates are difficult to calculate directly. Forest growth, drought, fires, soil respiration and land use change vary widely across regions and from year to year.
The multi decade oxygen record helps distinguish short term fluctuations from longer trends. It confirms what recent carbon budget reports have suggested. Natural sinks together still absorb about half of human caused emissions, but their performance is uneven and increasingly affected by climate change itself.
The latest Global Carbon Budget estimates cover a slightly different period and accounting framework. For the decade from 2014 through 2023, the ocean absorbed an average of 10.5 billion metric tons of CO₂ annually, or about 26 per cent of total emissions. The land sink absorbed approximately 11.7 billion metric tons per year, or around 29 per cent. Those figures include emissions from both fossil fuels and land use change, whereas the Scripps study focused mainly on fossil fuel emissions and natural sinks.
Despite the different methods, both assessments point to the same broad conclusion. Natural carbon sinks remove a substantial fraction of emissions, but climate conditions are already weakening them. The Global Carbon Budget estimates that warming and reduced rainfall cut the land sink by roughly 27 per cent during the past decade. It also suggests climate change has reduced the efficiency of land and ocean sinks by approximately 20 per cent and 7 per cent, respectively.
El Niño, drought and fire weakened the land sink sharply in 2023
Recent carbon budget estimates show why the long term land trend is a concern. In 2023, the global land sink was estimated at 2.3 billion metric tons of carbon, down from the previous year and the lowest estimate since 2015. Analysts linked the decline mainly to the 2023 to 2024 El Niño event and extensive wildfires in Canada.
El Niño suppresses rainfall across parts of the tropics, increases heat stress and reduces plant growth. Fires then release carbon that forests and soils had stored for decades, potentially turning ecosystems from carbon sinks into carbon sources. Preliminary assessments for 2024 suggested an even sharper weakening. One low latency analysis estimated that the global land system may have become a small net carbon source in 2024, though other approaches still indicated a small net sink and emphasised substantial uncertainty.
The divergence reflects the difficulty of measuring rapid changes caused by drought, fire and extreme heat. It also illustrates a broader risk. As warming intensifies, the land sink may not only fail to grow but could shrink or reverse in some years. That would leave a larger fraction of each additional ton of CO₂ in the atmosphere, accelerating climate change.
How ocean carbon uptake works and where it leads
The ocean absorbs carbon dioxide through physical and biological processes. Colder seawater dissolves more CO₂, and ocean circulation transports carbon into deeper waters. Marine organisms use dissolved carbon during photosynthesis, moving some of it through the food web and eventually into deeper ocean layers. The ocean's increasing uptake is beneficial in one immediate sense. It leaves less CO₂ in the atmosphere than would otherwise be present.
However, the process is not harmless. Additional dissolved CO₂ increases seawater acidity, threatening organisms that build shells or skeletons from calcium carbonate. Coral reefs, shellfish and plankton are all affected. Changes to ocean chemistry can alter marine ecosystems in ways that reduce biodiversity, disrupt fisheries and weaken the biological pump that moves carbon into deep storage.
Nor does a growing ocean sink guarantee it will continue indefinitely. Warming reduces the solubility of CO₂ in seawater, changes circulation patterns and can disrupt marine biological processes. A 2025 study in Nature revised estimates of the relative contributions of land and ocean. After accounting for biases in ocean models and effects involving the warm surface layer, researchers estimated an ocean sink of 3.1 billion metric tons of carbon per year for 2014 to 2023, about 15 per cent larger than the revised land sink. The authors also concluded that climate change has contributed to an atmospheric CO₂ increase equivalent to 8.3 parts per million since 1960 by weakening natural sinks.
What UK businesses need to understand about weakening carbon sinks
For organisations working on carbon reduction, the findings have several practical implications. First, natural carbon sinks cannot substitute for cutting emissions. Their capacity is limited, variable and vulnerable to the same climate changes they help to buffer. Businesses that rely on offsetting strategies tied to forestry or land restoration need to account for the risk that those sinks may weaken or fail over time.
Second, supply chain emissions are becoming harder to offset through nature based solutions alone. Many UK businesses use carbon credits linked to forestry projects to balance residual emissions. The weakening land sink suggests those credits may become less reliable or more expensive as project performance deteriorates in a warmer climate. Consequently, reducing emissions at source becomes more important than compensating for them later.
Third, emissions accounting standards are tightening. Public sector suppliers face requirements under Procurement Policy Note 06/21 to publish carbon reduction plans and demonstrate progress toward net zero. The Environment Agency, Companies House and the Financial Conduct Authority are all expanding disclosure rules. As natural sinks weaken, the carbon intensity of business operations will rise faster than activity levels alone would suggest, making year on year reductions harder to achieve without operational change.
Fourth, ocean acidification creates supply chain risks for businesses that depend on marine resources. Fisheries, aquaculture, coastal tourism and marine biotechnology are all exposed. Businesses that source seafood, fish oils, algae based ingredients or coastal services need to consider how ocean chemistry changes may affect availability, quality and price.
Finally, the shifting balance between land and ocean sinks affects the credibility of corporate net zero claims. Investors, regulators and customers are scrutinising carbon reduction strategies more closely. A business that assumes continued growth in natural carbon uptake may find its targets become unachievable or its disclosures challenged as the evidence base shifts.
Key facts about ocean and land carbon uptake since 1990
- The ocean absorbed an average of 2.8 billion metric tons of CO₂ per year between 1990 and 2019, representing roughly 35 per cent of fossil fuel emissions during that period.
- Ocean carbon uptake increased by approximately 0.63 billion metric tons of carbon per decade over the same period.
- Land ecosystems absorbed an average of about 1 billion metric tons of carbon annually, equivalent to around 13 per cent of fossil fuel emissions, but showed no significant long term growth.
- The global land sink reached its approximate peak around 2009 and generally declined afterward.
- In 2023, the land sink was estimated at 2.3 billion metric tons of carbon, the lowest level since 2015, largely due to El Niño and wildfires.
- Climate change has already weakened the land sink by roughly 27 per cent during the past decade and reduced ocean sink efficiency by about 7 per cent.
- Fossil fuel and cement emissions reached approximately 10.1 billion metric tons of carbon in 2023, with total human caused emissions at around 40.6 billion metric tons of CO₂.
Limits of natural carbon storage and implications for emission targets
The study challenges the assumption that land ecosystems will automatically absorb an increasing share of emissions as atmospheric CO₂ rises. Elevated CO₂ can stimulate plant growth under some conditions, but that benefit is often offset by heat, water stress, fires, nutrient limitations, deforestation and ecosystem degradation. In practice, the carbon fertilisation effect is weaker than early models suggested, and it cannot compensate for the combined pressures of climate change and land use.
This has direct implications for businesses setting science based targets. The Science Based Targets initiative requires companies to align emissions reductions with pathways that limit warming to 1.5 degrees Celsius. Those pathways assume natural sinks will continue to function at roughly current levels. If sinks weaken further, the remaining carbon budget shrinks and the required pace of emissions reduction accelerates. Businesses that have set long term targets may need to revise them sooner than planned.
It also affects the viability of net zero strategies that rely heavily on offsetting. If the land sink is stagnating or declining, the supply of high quality, permanent carbon removals will not keep pace with demand. Prices for verified removals are likely to rise, and the regulatory acceptance of certain offset types may narrow. Businesses should therefore prioritise reductions in operational and supply chain emissions over reliance on offsetting, particularly for Scope 1 and Scope 2 emissions.
For organisations involved in public sector supply chains, the findings reinforce the importance of demonstrable progress. Procurement Policy Note 06/21 requires suppliers to publish carbon reduction plans and show evidence of action. Buyers are increasingly sceptical of plans that depend on future carbon credits or unproven removal technologies. A credible plan focuses on energy efficiency, renewable energy, low carbon materials, sustainable logistics and supplier engagement. Support for carbon reduction planning is available through our net zero program for carbon reporting compliance, which helps businesses meet PPN 06/21 requirements and prepare for tightening disclosure rules.
Protecting existing carbon stocks becomes more urgent
If natural sinks are not expanding, protecting what remains becomes more important. Preventing deforestation, restoring degraded ecosystems and improving land management can preserve existing carbon storage, even if they cannot fully compensate for continued emissions. The UK government has committed to protecting 30 per cent of land and sea by 2030, and businesses are under growing pressure to demonstrate that their supply chains do not contribute to deforestation or habitat loss.
For UK businesses, this means greater scrutiny of commodities such as timber, palm oil, soy, cocoa and beef. The Environment Act 2021 gives the government powers to introduce due diligence requirements for forest risk commodities, similar to the European Union Deforestation Regulation. Businesses that source these materials need to map their supply chains, assess deforestation risk and engage suppliers on land use practices. Guidance on sustainable procurement and supply chain due diligence is available through our sustainable procurement support for public sector suppliers.
Restoring degraded land can also create co benefits. Peatland restoration, for example, reduces emissions, improves water quality and supports biodiversity. Woodland creation can provide timber, recreation and flood management alongside carbon storage. Businesses with land holdings or rural supply chains should consider how restoration projects align with operational needs and stakeholder expectations.
However, restoration is not a like for like substitute for preventing loss. Newly planted forests take decades to accumulate the carbon held in mature ecosystems, and they are vulnerable to fire, drought and pests during establishment. Protecting existing stocks is therefore more reliable and more urgent than creating new ones.
Ocean acidification and marine ecosystem risks
Carbon absorbed by the ocean contributes to acidification. Since the start of the industrial era, ocean surface pH has dropped by approximately 0.1 units, representing a 30 per cent increase in acidity. That change is faster than at any point in the past 300 million years. Marine organisms that build calcium carbonate structures, including corals, molluscs, echinoderms and some plankton, are particularly affected. Acidification reduces the availability of carbonate ions needed for shell and skeleton formation, making it harder for these organisms to grow and survive.
The ecological consequences extend beyond individual species. Coral reefs support fisheries, coastal protection and tourism. Plankton form the base of marine food webs. Disruption to these systems can affect fish stocks, aquaculture productivity and the livelihoods of coastal communities. For UK businesses that source seafood, marine ingredients or coastal services, ocean acidification represents a material supply chain risk.
Fisheries and aquaculture businesses should monitor scientific assessments of stock vulnerability and consider diversification or adaptation strategies. Food manufacturers that rely on marine ingredients may face higher prices or supply interruptions. Businesses involved in coastal tourism or marine conservation need to account for reef degradation and ecosystem shifts. According to the Department for Environment, Food and Rural Affairs, the UK government is working on marine protection measures, but business adaptation will also be necessary.
Carbon accounting uncertainty and the need for multiple methods
The Scripps study highlights the value of using multiple measurement approaches to track the global carbon cycle. Atmospheric observations, ocean measurements, ecosystem models and satellite data each capture different parts of the cycle. Their uncertainties are particularly large for land use emissions and the global land sink. A 2023 assessment by the Global Carbon Project noted that land use change emissions remain the largest source of uncertainty in the global carbon budget, with estimates ranging from 3.7 to 8.6 billion metric tons of CO₂ per year depending on the method used.
For businesses, this uncertainty complicates carbon accounting. Scope 3 emissions from land use change, agriculture and forestry are difficult to measure accurately. Supplier data is often incomplete or inconsistent. Businesses that report these emissions need to document their methodology, state assumptions clearly and update estimates as new data becomes available. Training on Scope 3 emissions accounting and supply chain measurement is available through SBS Academy training on carbon reporting and supply chain emissions.
Uncertainty also affects the credibility of carbon removal claims. Businesses that purchase forest carbon credits need to verify additionality, permanence and leakage. Additionality means the carbon removal would not have happened without the project. Permanence means the carbon will remain stored for a meaningful period. Leakage means the project does not simply displace emissions or deforestation elsewhere. As the land sink weakens, the risk of non permanence increases, and verification standards are tightening in response.
Climate feedbacks may accelerate atmospheric CO₂ growth
As warming weakens natural sinks, a larger fraction of each additional ton of CO₂ may remain in the atmosphere. This is a positive climate feedback. It means emissions have a greater impact on atmospheric concentrations than they did in the past, and it reduces the remaining carbon budget for any given temperature target. The Nature study estimated that sink weakening has already contributed to an atmospheric CO₂ increase of 8.3 parts per million since 1960.
For businesses setting long term targets, this feedback effect means the remaining budget for 1.5 degrees Celsius is smaller than previously thought. The Intergovernmental Panel on Climate Change estimated in 2021 that the remaining carbon budget for a 50 per cent chance of limiting warming to 1.5 degrees was around 500 billion metric tons of CO₂. Global emissions are currently about 40 billion metric tons per year, leaving roughly a decade at current rates. If natural sinks weaken further, that timeline shortens.
Businesses should therefore plan for steeper emissions reduction trajectories than they might have considered a few years ago. Linear reductions may no longer be sufficient. Accelerated cuts in the next five to ten years, particularly in high emission sectors such as energy, transport and heavy industry, become more important. For many UK SMEs, this means prioritising energy efficiency, electrification, renewable energy procurement and low carbon logistics sooner rather than later.
Further information and authoritative sources
The Scripps Institution of Oceanography published the full study in Science, and the findings are discussed in detail on the Scripps website. The Global Carbon Project publishes annual carbon budget estimates, available through the Global Carbon Project website, which provide comprehensive data on emissions, sinks and trends. The UK government's approach to net zero, land use and marine protection is set out by the Department for Energy Security and Net Zero.
Businesses working on carbon reduction, supply chain emissions or net zero compliance can find guidance on measurement, reporting and target setting through our ESG compliance and carbon reporting services. The Science Based Targets initiative provides frameworks for setting emissions reduction targets aligned with climate science, and the UK government's net zero strategy outlines policy measures and support available to businesses.
Understanding how natural carbon sinks are changing is essential for setting realistic targets, managing supply chain risks and building credible net zero strategies. The evidence shows that neither the ocean nor the land will compensate for continued high emissions. Reducing emissions at source remains the only reliable path to meeting climate goals, and the weakening of natural sinks makes that path narrower and more urgent.