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GESAMP Working Group 41: HIGH LEVEL REVIEW OF A WIDE RANGE OF PROPOSED MARINE GEOENGINEERING TECHNIQUES

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The report provides an initial high-level review of twenty-seven proposed marine geoengineering techniques - with its potential subsets - for climate mitigation that focuses on their efficacy, practicality, side-effects, knowledge gaps, verification and potential environmental and socio-economic impacts.
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... To identify these experts for interviews, we used a mix of purposive and convenience sampling, in order to achieve breadth of technologies covered as well as depth on specific mCDR approaches (Boyd and Vivian, 2019; National Academies of Sciences Engineering and Medicine, 2021; "Ocean Visions | Catalyzing Solutions For Ocean Health," n. d.). Purposive sampling involved identifying mCDR projects. ...
... GESAMP, for example, describes ocean-based CDR as interventions that "involve the enhancement of natural sinks" (pg. 17, Boyd and Vivian, 2019). The 2021 report of the U.S. National Academies on mCDR grounded its assessments in the role the oceans play as "a large natural reservoir for CO₂," where "natural ocean processes" already remove large amounts of anthropogenic CO₂ emissions, and includes numerous references to 'natural' carbon cycles as the relevant context of interpretation for new technological options (National Academies of Sciences Engineering and Medicine, 2021). ...
... Both D) seaweed restoration and E) sinking seaweed in the deep sea do not appear to be viable for carbon credits in the short term due to concerns raised in Challenges 3, 6 and 7, as well as challenges of attribution of seaweed carbon sequestered beyond the habitat boundaries to a particular action (Challenge 2). Likewise, due to concerns around traceability and quantification of carbon in Challenge 2, (B) transport of POC and DOC offshore from seaweed aquaculture is unlikely to be viable in the short term, as issuing of carbon credits requires evidence that the carbon is in a stable form and remains sequestered for a significant time period -usually over 100 years (Boyd et al., 2019, Bach et al., 2021, Siegel et al., 2021. However, our review suggests there is enough evidence to support a development towards a global standardised methodology for the accumulation of carbon beneath sediments of seaweed farms (A) and for seaweed product carbon abatement credits (C), providing that sustainability standards for seaweed aquaculture are developed and adhered to. ...
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Seaweed (macroalgae) has attracted attention globally given its potential for climate change mitigation. A topical and contentious question is: Can seaweeds' contribution to climate change mitigation be enhanced at globally meaningful scales? Here, we provide an overview of the pressing research needs surrounding the potential role of seaweed in climate change mitigation and current scientific consensus via eight key research challenges. There are four categories where seaweed has been suggested to be used for climate change mitigation: 1) protecting and restoring wild seaweed forests with potential climate change mitigation co-benefits; 2) expanding sustainable nearshore seaweed aquaculture with potential climate change mitigation co-benefits; 3) offsetting industrial CO2 emissions using seaweed products for emission abatement; and 4) sinking seaweed into the deep sea to sequester CO2. Uncertainties remain about quantification of the net impact of carbon export from seaweed restoration and seaweed farming sites on atmospheric CO2. Evidence suggests that nearshore seaweed farming contributes to carbon storage in sediments below farm sites, but how scalable is this process? Products from seaweed aquaculture, such as the livestock methane-reducing seaweed Asparagopsis or low carbon food resources show promise for climate change mitigation, yet the carbon footprint and emission abatement potential remains unquantified for most seaweed products. Similarly, purposely cultivating then sinking seaweed biomass in the open ocean raises ecological concerns and the climate change mitigation potential of this concept is poorly constrained. Improving the tracing of seaweed carbon export to ocean sinks is a critical step in seaweed carbon accounting. Despite carbon accounting uncertainties, seaweed provides many other ecosystem services that justify conservation and restoration and the uptake of seaweed aquaculture will contribute to the United Nations Sustainable Development Goals. However, we caution that verified seaweed carbon accounting and associated sustainability thresholds are needed before large-scale investment into climate change mitigation from seaweed projects.
... Covering 70% of Earth's surface, the ocean contains untapped storage potential, and avoids pressures land base that may limit scalability of terrestrial CDR (Gattuso et al., 2018;National Academies of Sciences Engineering and Medicine, 2022). Ocean-based CDR encompasses a diverse range of NETs: further approaches might store CO 2 , for example, as bicarbonates dissolved in seawater, or deposits in sub-seafloor geological structures (Boyd & Vivian, 2019; National Academies of Sciences Engineering and Medicine, 2022). They can be applied at diverse sites in varying proximity to coasts and oceanic depths. ...
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In the face of mounting global climatic pressures, negative emission technologies (NETs) for carbon dioxide removal (CDR) are increasingly proposed as necessary for meeting climate targets. While initial work has identified the potential of terrestrial NETs, a diverse set of marine/ocean-based NETs are gaining new and particular attention. Emerging studies on the feasibility of marine NETs are urgently needed, especially to explore the logics that public groups use to judge different approaches, and to ensure that design and governance of these technologies align with public values and priorities. This study explores factors of interest in understanding public views on four marine NETs, both perceptions of climate severity and urgency, and beliefs about marine environments. It uses a quantitative survey to explore how a representative sample of people in British Columbia, Canada and Washington state, United States evaluate four marine NETs: coastal restoration; ocean alkalinity enhancement; ocean fertilization; and offshore direct air carbon capture and storage. We find that perceived severity and urgency of climate change predicts greater comfort with all NETs studied, and views of marine environments as adaptable, fragile and manageable vary in predicting both greater and lesser comfort. Drawing upon these insights, the paper offers reflections on the conditional thinking linked with emerging views of marine NETs, concluding with methodological suggestions for future research on public perceptions as concerns the deployment of ocean-based CDR near and long term. Incorporating these insights into policy for ocean-based CDR will be important to ensuring responsible governance of these technologies. Key policy insights • Incorporating research on public perceptions will be important to the design of marine NETs and accompanying policies. • Public groups in both British Columbia and Washington expressed high levels of comfort with coastal restoration, some comfort with offshore direct air carbon capture and storage, and some discomfort with ocean alkalinity enhancement and ocean fertilization. • Perceived severity and urgency of climate change predicted greater comfort with all approaches; this evidence aligns with a small but growing body of scholarship indicating openness to environmental intervention amongst public groups concerned with climate impacts. • Beliefs about marine environments, namely whether they are ‘adaptable’, ‘manageable’ or ‘fragile’, also predicted comfort, suggesting that CDR in ocean contexts requires further examination regarding public perceptions.
... The ocean offers many potential opportunities for enhanced mitigation [9,10]. Proposals typically focus on either enhancing biological ocean carbon sinks through marine afforestation and fertilisation or chemically increasing the inorganic ocean carbon sink through techniques such as ocean alkalinization [11]. ...
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In combination with drastic emission reduction cuts, limiting global warming below 1.5 °C or 2 °C requires atmospheric carbon dioxide removal (CDR) of up to 16 GtCO2 yr⁻¹ by 2050. Among CDR solutions, ocean afforestation through macroalgae cultivation is considered promising due to high rates of productivity and environmental co-benefits. We modify a high-resolution ocean biogeochemical model to simulate the consumption of dissolved inorganic carbon and macronutrients by idealised macroalgal cultivation in Exclusive Economic Zones. Under imposed macroalgal production of 0.5 PgC yr⁻¹ with no nutrient feedbacks, physicochemical processes are found to limit the enhancement in the ocean carbon sink to 0.39 PgC yr⁻¹ (1.43 GtCO2 yr⁻¹), corresponding to CDR efficiency of 79%. Only 0.22 PgC yr⁻¹ (56%) of this air–sea carbon flux occurs in the regions of macroalgae cultivation, posing potential issues for measurement, reporting, and verification. When additional macronutrient limitations and feedbacks are simulated, the realised macroalgal production rate drops to 0.37 PgC yr⁻¹ and the enhancement in the air–sea carbon flux to 0.21 PgC yr⁻¹ (0.79 GtCO yr⁻¹), or 58% of the macroalgal net production. This decrease in CDR efficiency is a consequence of a deepening in the optimum depth of macroalgal production and a reduction in phytoplankton production due to reduced nitrate and phosphate availability. At regional scales, the decrease of phytoplankton productivity can even cause a net reduction in the oceanic carbon sink. Although additional modelling efforts are required, Eastern boundary upwelling systems and regions of the Northeast Pacific and the Southern Ocean are revealed as potentially promising locations for efficient macroalgae-based CDR. Despite the CDR potential of ocean afforestation, our simulations indicate potential negative impacts on marine food webs with reductions in phytoplankton primary production of up to −40 gC m⁻² yr⁻¹ in the eastern tropical Pacific.
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The Paris Agreement to limit global warming to well below 2°C requires drastic reductions in greenhouse gas emissions and the balancing of any remaining emissions by carbon dioxide removal (CDR). Due to uncertainties about the potential and durability of many land-based approaches to deliver sufficient CDR, marine CDR options are receiving more and more interest. We present the current state of knowledge regarding the potentials, risks, side effects as well as challenges associated with technical feasibility, governance, monitoring, reporting and accounting of marine CDR, covering a range of biotic and geochemical approaches. We specifically discuss to what extent a comparison with direct injection of CO 2 into seawater, which had been proposed decades ago and is now prohibited by international agreements, may provide guidance for evaluating some of the biotic marine CDR approaches.
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The model pathways of the Intergovernmental Panel for Climate Change (IPCC) for the timely achievement of global climate targets, especially the target of limiting global warming to 1.5°C compared to pre-industrial levels, suggest the need for safeguarding and enhancing the global carbon sink. Experts argue that the deployment of so-called negative emissions technologies for large-scale carbon dioxide removal holds potential for keeping the temperature in line with limits set by the Paris Agreement. Ocean-based negative emissions technologies (ONETs) intend to enhance carbon sequestration and storage in the ocean, e.g., by changing the ocean’s physical or biogeochemical properties. But in addition to these intended effects, ONETs may also cause unintentional impacts on the ocean’s condition and on related coastal and marine ecosystem services that are relevant for the attainment of a range of global policy goals. This article links potential direct and indirect, intentional and unintentional impacts of eight ONETs on the marine environment to the regulations and policy goals of international environmental agreements of the current global ocean governance regime. The results thereof outline a direct, implicit and indirect governance framework of ONETs. Hereby, a broader perspective of the concept of (global) ocean governance is adopted to outline a wider network that goes beyond the explicit regulation of ONETs within the realm of ocean governance. This first-order assessment derives gaps and challenges in the existing governance framework, as well as needs and opportunities for comprehensive governance of the technologies. It is determined that while the inclusion of ONETs in the global climate strategy may be deemed necessary for reaching net zero emission targets in the future, a range of potential trade-offs with other policy goals may need to be considered or dealt with when deploying ONETS for climate mitigation. Further, foresight-oriented and adaptive governance mechanisms appear imperative to bridge gaps resulting from extensive uncertainties and unknowns linked to ONET deployment in a changing ocean and. The identified ONET governance framework reiterates current challenges in ocean governance, for instance related to fragmentation, but also represents an opportunity for a synergistic and integrated approach to future governance.
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