• Lennart Bach, Institute for Marine and Antarctic Studies, University of Tasmania, Australia

 

Re-thinking Ocean Alkalinity Enhancement

Ocean Alkalinity Enhancement (OAE) is an umbrella term for a range of marine CO2 removal methods that sequester CO2 in seawater via the addition of alkaline substances to the ocean. Upon its invention, OAE was described as a geoengineering tool to counteract climate change. Early research with numerical models generally adopted OAE’s initial geoengineering conceptualization, asking what would happen if humankind were to somehow mobilize billions of tonnes of alkaline material per year and dissolve it in the ocean. While addressing this question revealed the theoretical carbon removal potential of OAE, it also consolidated the general view of OAE as a geoengineering tool out of reach of local communities.  However, the idea of a centrally coordinated roll-out of OAE to confront global warming is not only unlikely to generate necessary public support, it is also inconsistent with how OAE research and development is currently evolving. Rather than a singular, coordinated, global top-down process, there is a diversification of OAE methods currently under study, each of which are highly adapted to specific local environments. Innovations are often driven by scientists and engineers who are designing OAE methods around local constraints and opportunities with regard to geographical settings, infrastructure, and community demands. In this presentation we challenge the geoengineering narrative long used to discuss OAE and, instead, try to develop a vision for future research and deployment of OAE as a flexible tool for the management of local CO2 sinks and sources that is governed and managed by communities.

• Phil Williamson, School of Environmental Sciences, University of East Anglia, Norwich, United Kingdom

 

Environmental impacts of mCDR

Many environmental impacts, both negative and positive from a human perspective, could arise as a consequence of marine CO2 removal (mCDR). These impacts are highly method-specific, as will be briefly reviewed. The relative importance of such environmental effects is likely to be scale-dependent, although with uncertainties associated with model assumptions and upscaling from experimental studies to operational deployment. In addition to this variability and uncertainty, evidence will be presented for four other general conclusions. First, that a greater range of ecological and biogeochemical impacts would result from biologically-based mCDR than from chemically-based approaches; second, that the climatic effectiveness of many mCDR methods may be greatly reduced by feedback impacts, including ‘nutrient robbing’ and methane release; third, that public and political concerns regarding adverse environmental impacts could be crucial in determining whether mCDR will ever be used at climatically-meaningful scale; and fourth, that the cost of monitoring environmental impacts (likely to be extremely challenging for far-field effects) could significantly decrease the cost-effectiveness of using mCDR for climate mitigation

• Dariia Atamanchuk, Ocean Science and Technology Group (CERC.Ocean), Department of Oceanography, Dalhousie University, Halifax, Canada

 

MRV and environmental monitoring 

Among the various proposed marine carbon dioxide removal (mCDR) methods, ocean alkalinity enhancement (OAE) stands out as a promising technology capable of removing 10-15 gigatons of CO2 from the atmosphere annually. For OAE to be considered a viable and safe carbon removal solution, it requires robust Monitoring, Reporting, and Verification (MRV) tools to quantify carbon dioxide removal (CDR) and the increase in ocean alkalinity above natural levels. Accurate measurements of the carbonate system and other parameters over relevant timescales allow for direct observation of perturbations and provide data to inform and validate biogeochemical models, thereby offering a broader spatiotemporal context for the measurements.

This talk will focus on the multi-year, large-scale OAE field trials conducted in Halifax Harbour in collaboration with the climate tech company Planetary 

• Laurent Bopp, Institut Pierre-Simon Laplace | Ecole normale supérieure, Paris, France

 

State of the climate and the potential need for (m)CDR 

The global climate is undergoing rapid changes due to anthropogenic greenhouse gas (GHG) emissions. Observations show that, for the 2014–2023 decade average, global surface warming was 1.2 °C, of which ~100% is human-induced. The remaining carbon budget to limit warming to 1.5°C is estimated at around 200 GtCO₂, while for 2°C, the budget is approximately 1100 GtCO₂. Given current emission rates of roughly 40 GtCO₂ per year, these thresholds are at risk of being exceeded within years without immediate and significant mitigation efforts.

To achieve the Paris Agreement targets, most emission pathways rely on carbon dioxide removal (CDR) to offset residual emissions and potentially enable net-negative emissions in the latter half of the century. According to the IPCC, achieving 1.5°C without overshoot would require removing hundreds of gigatons of CO₂ cumulatively by 2100. Even some scenarios stabilizing at 2°C project substantial reliance on CDR to balance emissions from hard-to-abate sectors.

Among the CDR options, ocean-based CDR (mCDR) methods, classified into biotic and abiotic approaches, offer opportunities due to the vast size of the ocean, its large carbon reservoir, the long time scales involved in carbon sequestration, and the potential for long-term durability. However, they also present numerous risks and uncertainties. Biotic methods include ocean fertilization, ocean afforestation with macroalgal cultivation, and blue carbon ecosystem enhancement (e.g., seagrass restoration, mangrove expansion). These approaches face significant limitations in scalability, effectiveness, and permanence, as well as concerns about unintended environmental impacts such as ecosystem disruption and ocean deoxygenation. Abiotic methods, including ocean al kalinization and deep-sea CO₂ injection, offer more durable carbon sequestration solutions. Ocean alkalinization, by enhancing the ocean's natural ability to absorb CO₂, may provide long-term storage benefits, while deep-sea CO₂ injection could isolate carbon from the atmosphere for extended timescales. Nevertheless, these approaches still face critical challenges regarding their real-world effectiveness, scalability, monitoring, reporting, and verification (MRV) mechanisms, as well as potential ecological consequences such as shifts in ocean chemistry and impacts on marine life.

Despite these uncertainties, mCDR could complement other CDR approaches in achieving climate targets, particularly if technological advancements improve efficiency and reduce unintended ecological consequences. Future research should prioritize assessing the environmental trade-offs, scalability, and governance frameworks needed to integrate mCDR into global mitigation strategies effectively.`

• Romany Webb, Sabin Center for Climate Change Law, Columbia Law School, USA

 

Governing Marine Carbon Dioxide Removal: Past, Present, and Future

The existing governance framework for marine carbon dioxide removal is highly complex. A number of international agreements and rules of customary international law could have implications for the conduct of marine carbon dioxide removal, as could various domestic laws (including, in some cases, both national and subnational laws). This presentation will survey the most relevant legal instruments and discuss recent efforts to apply them to new marine carbon dioxide removal activities. As we shall see, while some progress has been made, there remain many gaps and shortcomings in the existing governance framework. We will explore ways of addressing those issues and ensuring robust and effective governance of marine carbon dioxide removal.

• Catriona Hurd, Institute for Marine and Antarctic Studies, Hobart, Australia
  

Challenges of ocean afforestation: can seaweed open ocean aquaculture enhance the biological pump?   

Seaweeds (marine macroalgae) are crucial primary producers of coastal systems, particularly in temperate regions where they provide a range of ecosystem services including habitat creation and food for higher trophic levels, carbon and nitrogen cycling, and support substantial global industries.  Both natural beds and aqua-cultured seaweed systems have been proposed as biological methods of marine CDR (mCDR).  ‘Ocean afforestation’ is the deliberate introduction of seaweeds into the open ocean, with the goal of enhancing the ocean’s natural microalgal-driven biological pump via sinking the seaweed biomass that is produced.  In this talk, I will discuss: 1. Monitoring, Reporting and Verification that seaweed carbon originates from the atmosphere (CO2 or methane) is stored permanently (> 100 years) such that it does not return to the atmosphere; 2. Additionality i.e. that the carbon removed by seaweed biomass is additional to that already being removed and stored by existing (natural) phytoplankton systems; 3. Iron limitation of seaweed growth in the open ocean, and 4. Ensuring that ocean afforestation is environmentally safe with minimal (or at least known/quantifiable) negative impacts on natural systems. 

• Adrien Comte, French Research Institute for Sustainable Development (IRD), LEMAR laboratory, Plouzané, France

 

Pannel introduction

Current policies and actions to combat climate change are insufficient to keep the planet on track to reach the objectives of the Paris Agreement. A range of mCDR options is now being discussed to scale-up action to reach these objectives. The lack of ambition is also driven by a finance gap, as the mobilization of public and privde funding is largely insufficient. Carbon markets have been institutionalized over the past decades to mobilize private finance on mitigation action. In this talk, we will review the current funding landscape on mitigation, the mechanisms of the carbon markets, with a specific focus on blue carbon, and provide an overview of the possible funding mechanisms towards mCDR development and deployment

• Terre Satterfield, Institute for Resources, Environment and Sustainability, University of British Columbia, Vancouver, Canada

 

Contemplating ocean futures: Thinking at scale and the many social uncertainties across climate solutions

The ocean is a home to coastal communities, a place of wonder that feeds our imagination, and provides unparalleled natural gifts. It is also central to planetary health and to the global carbon cycle that so links land and marine systems. Increasingly, its potential as the central space for all manner of climate solutions is also evident. This is because the sheer volume of ocean space might provide our best hope for addressing excess atmospheric carbon dioxide, which requires its removal and storage long term. Prominent among options are: increasing the flow of alkaline inputs to enhance the ocean’s ability to uptake carbon dioxide; using the seabed’s basalt formations to store and convert carbon dioxide to solid rock; or cultivating large volumes of marine plants to store or enhance carbon uptake. And yet any one of these options might well succeed or fail from the point of view of the complicated decision logics people use to judge new technologies and to the scale of carbon dioxide removal needed to meet its climate targets. This lecture will provide an overview of recent studies that explain why people are both drawn to or remain resistant to different options given the moral conundrums involved, the enduring salience of perceiving options as natural versus technical, the cognitive burden of grasping the scale of the problem itself, and the social trust as well as novel governing and financial systems any such technology might require. Final reflections will address the interdisciplinary challenges that persist across social, physical and engineering scientists seeking answers, alongside some cautions on how to move forward when we know that the need is urgent and public thinking uncertain.

• Spyros Foteinis, Heriot-Watt University, Edinburgh, UK

 

Life cycle assessment in the carbon dioxide removal ecosystem: unraveling the net-negativity and environmental trade-offs of (m)CDR.

To limit global temperature rise to well below 2 °C above pre-industrial levels, drastic emissions reductions should be complemented by significant emissions removals, i.e., by carbon dioxide removal (CDR). This is a Herculean task, given that by as early as 2050 tens of billions of tons (Gt) of excess (anthropogenic) carbon dioxide (CO₂) will need to be removed each year and safely stored. However, the industry remains nascent and, therefore, CDR sustainable scaling up -particularly of marine-based approaches (mCDR) which are characterized by low technology readiness levels- remains challenging. Ocean alkalinity enhancement (OAE) has emerged as a promising mCDR strategy due to its high carbon removal capacity and durability. Yet, concerns about its overall net-negativity and environmental sustainability have been raised. In this talk, the main issues surrounding the net-negativity and environmental sustainability of OAE will be examined through the lenses of life cycle assessment (LCA). Insight into the main environmental hotspots, as well as of different spatial and temporal constraints, will be given, along with avenues to improve environmental sustainability. For example, for coastal enhanced weathering to be meaningful for climate change mitigation, both olivine’s overland transportation distance and particle size should be carefully considered, as it may take decades or even centuries before this technology becomes net-negative. Furthermore, the nickel content in olivine poses potential toxicity risks, suggesting trade-offs between carbon removal and environmental and human toxicity. On the other hand, even though electrochemical approaches for OAE are promising, these are currently water and energy intensive, which greatly affects their net-negativity and therefore their potential for CDR. These aspects, and more, will be discussed, along with opportunities for future research on the environmental sustainability of (m)CDR.