We have crossed a carbon-climate threshold. The global average temperature is breaching 1.5°C of warming above pre-industrial levels as a sustained reality. The Paris Agreement‘s most ambitious guardrail, once described as a target to strive towards, is now a line we are crossing.
The question is no longer how to avoid overshoot but how to minimise it and whether we can chart a credible climate-restoration course that will involve clawing back CO2 emitted beyond the 1.5°C threshold, regionally and globally.
The physics of climate change is unforgiving. Every fraction of human-linked CO2 emissions leads to additional warming, which amplifies climate risks — more frequent and lethal winds, heatwaves, droughts and floods, accelerated ice loss, deeper coral bleaching and ocean acidification (the ocean becoming more acidic as it absorbs excess carbon dioxide from the atmosphere). The remaining carbon budget to limit warming to 1.5°C has effectively been exhausted, which means that reaching net-zero emissions, while essential, is no longer sufficient on its own.
To stabilise and ultimately bring climate systems out of the danger zone, we must make deep cuts to CO2 emissions and scale up carbon dioxide removal (CDR) — actively drawing CO2 out of the atmosphere and storing it durably. It is an urgent scientific problem and a governance and policy imperative.
The most familiar CDR approaches are biological and land-based, including intertidal wetlands, or blue carbon, reforestation, afforestation, soil carbon sequestration and bioenergy with carbon capture and storage. But their limitations are real and increasingly well understood. Land resources are finite and the competition for them is fierce — between food production, biodiversity conservation, water security and human settlement.
Many land-based CDR approaches also carry impermanence risk: forests burn, soils release carbon with warming, sea levels rise and ecosystems are disturbed. The land-based interventions play an important but, in the long term, uncertain role in national climate commitments, including South Africa’s own revised nationally determined contributions (NDCs).
The Intergovernmental Panel on Climate Change‘s estimates suggest that land-based biological CDR can contribute meaningfully but cannot alone deliver the gigatonne-scale removal that the climate system requires over the coming decades. We need to look beyond the coastline — to the ocean.
Ocean carbon sink
On World Oceans Day celebrated on 8 June each year, it’s important to recognise that the ocean does extraordinary work. It stores 98% of all CO2 on the planet, absorbs at least a quarter of all human-made CO2 emissions annually and soaks up at least 90% of the excess heat generated by those emissions. It is the flywheel of the global climate.
Marine carbon dioxide removal (mCDR) is a deliberate intervention to enhance the ocean’s capacity to capture and store CO2 from the atmosphere (carbon sequestration). Among the most scientifically promising approaches is ocean alkalinity enhancement (OAE) — adding alkaline minerals to seawater to change ocean carbon chemistry in ways that increase and neutralise the absorption of CO2 from the atmosphere.
The underlying geochemistry is well established and OAE is effectively a sped-up version of the geochemical rock weathering that has set present-day ocean alkalinity. By accelerating natural rock weathering processes at scale, OAE could, in principle, remove gigatonnes of CO2 durably — potentially over timescales of thousands of years.
But five critical questions govern whether OAE — and mCDR more broadly — can move from theory to deployment in South Africa and globally:
- Efficiency — How much CO2 is removed per unit of alkalinity addition?
- Effectiveness — Does it lead to an equivalent uptake of atmospheric CO2?
- Durability — Is the carbon genuinely and permanently sequestered?
- Scalability — Can it be implemented at the gigatonne scale required?
- Ecosystem implications, societal costs and benefits — Does it still work when a full life-cycle analysis is applied?
These are not merely technical questions. They are the frontier of an emerging field of science that requires coordination across disciplines, observations and modelling, governance and society, including policies that make clear who benefits from the interventions.
The transition from laboratory experiments to open-ocean field trials is where OAE and mCDR research now stand globally. Small-scale laboratory, mesocosm (a small, controlled ecosystem created for scientific research) and field trials have begun in coastal and offshore environments in North America, Europe and parts of Asia, testing alkalinity addition, macroalgae cultivation and ocean iron fertilisation under real-world conditions.
What has become immediately clear is that monitoring, reporting and verification — the backbone of any credible carbon accounting system — is enormously complex in a marine environment and depends strongly on models in which we need confidence. This framework is also not just about carbon but ecosystem services, ecology and biodiversity.
The medium-term limitation of mCDR is that it is unlikely to scale to more than 10% of present-day emissions — about 4 billion to 5 billion tonnes of CO2 a year. This means that mCDR will be effective only if we make rapid, deep and sustained cuts in global CO2 emissions as well. Its short- to medium-term impact will be greatest in offsetting hard-to-abate emissions as we approach net zero.
South Africa’s opportunity
South Africa sits at the meeting point of three great ocean systems — the Atlantic, Indian and Southern oceans. They are linked by powerful ocean circulation systems that regulate Southern and Northern Hemisphere carbon and climate and possess extraordinary marine biodiversity, unique oceanographic conditions and a growing base of marine science expertise.
Yet scientific potential alone is insufficient. Marine carbon dioxide removal raises several questions that require integrative, cross-disciplinary thinking:
- Should mCDR-derived carbon be tradeable as offsets in voluntary or compliance carbon markets and, if so, under what standards and governance frameworks?
- What economic models could finance field trials and eventual deployment?
- How do we distinguish between using mCDR to enable continued emissions elsewhere — a carbon-offset logic — and deploying it as a climate intervention in the public interest to reduce actual atmospheric CO2 concentrations and their associated effects?
The questions carry profoundly different ethical, economic and governance implications for science, government and business.
South Africa has the coastline, institutional capacity and climate vulnerability to be a serious contributor to answering the questions. What is needed now is coordinated investment in ocean science, robust engagement between researchers and policymakers and the intellectual courage to treat mCDR not as a distant global conversation but as our next research frontier.
A multi-institutional ocean carbon-climate network that spans the science-society nexus across universities and research institutions, together with a coordination platform for the responsible development of ocean-climate solutions, is being developed to lead regional capacity in assessing mCDR alongside government and business.
With good governance, societal benefits could extend from communities to complex industrial value chains.
A panel session on ‘Lab to Life: Using Marine Carbon Dioxide Removal as a Case for Climate-Positive Coastal Infrastructure’ will take place at the SRI Conference on 26 June.
Pedro Monteiro is Professor of Ocean Carbon–Climate Studies at the School for Climate Studies at Stellenbosch University, where he also leads the emerging research area of carbon dioxide removal.
On World Oceans Day, celebrated on 8 June each year, it’s important to recognise that the ocean does extraordinary work. It stores 98% of all CO2 on the planet, absorbs at least a quarter of all human-made CO2 emissions annually and soaks up at least 90% of the excess heat generated by the emissions. It is the flywheel of the global climate
