A common challenge for carbon dioxide removal (CDR) approaches, including those falling under ocean alkalinity enhancement (OAE), is the need to develop new global industries and supply chains to underpin their sustainable scale-up. To facilitate roll-out, OAE approaches could be incorporated into existing industries, notably by: i) partially substituting silica sand in existing beach nourishment projects with (ultra)mafic (e.g., olivine-rich) sand for carbon dioxide drawdown through coastal enhanced weathering; ii) using the spare capacity of the cement industry to produce slaked lime for ocean liming; and iii) producing sodium hydroxide from waste desalination brines through electrochemical brine splitting. Here, the gross CDR capacities of these OAE approaches were examined for Spain by considering spare capacities, technical constraints, and engineering restrictions and challenges. Prospective life cycle assessment (pLCA) revealed their net-negativity and their sensitivity to future climate scenarios and particularly technology maturity. Spain's gross/net CDR capacity via OAE amounted to 13.52/8.27, 38.35/26.50, and 40.54/38.13 Mt CO2 yr−1 in 2030, 2040, and 2050, respectively. Future decarbonisation and improved technology readiness levels and spare capacities drive this increase. Therefore, technological maturity and decarbonised economies, particularly decarbonised energy, are needed for CDR technologies to achieve their full potential. Finally, we considered how complex social, regulatory, and governance factors may impinge on the feasibility of these three OAE approaches. Regulatory clarity and innovation, governance frameworks, and social acceptance are needed, which also requires robust efforts to consult and engage communities. Overall, OAE could underpin national long-term strategies and form an essential part of climate-neutrality strategies.

The deployment of a wide range of carbon dioxide removal approaches is essential to limit global warming to well below 2 °C. Ocean liming (OL) combines CO2 removal with the co-benefit of ocean acidification mitigation. This study evaluates four configurations of a process for removing atmospheric CO2 including OL. They combine two energy sources (i.e. woody biomass and renewable electricity) to produce slaked lime, and two methods for the storage of CO2 generated by the process (i.e. in underground geological formations and as bicarbonates in the sea via pH-equilibrated ocean alkalinization). The environmental impacts of the different configurations are evaluated through the life cycle assessment methodology. Results reveal efficiencies of carbon dioxide removal (i.e. the ratio between the net CO2 equivalent removal and the gross amount of removed CO2) in the range 85–92% for scenarios using the Italian electricity mix projected for 2050, and 93–96% for those using biomass. Therefore, all scenarios achieve net CO2 removal and the carbon penalty due to materials and energy used in the processes does not undermine the feasibility of OL. However, trade-offs are noted between climate change and other impact categories, such as land use.The proposed process configurations capitalize on existing technologies but require the integration of effective CO2 capture and storage to prevent emissions during calcination. Among these, the pH-equilibrated approach emerges as a promising alternative for CO2 storage, especially in regions where geological storage is not available.

Alkaline feedstocks for ocean alkalinity enhancement (OAE) must guarantee efficient alkalinity release while having limited impact on marine ecosystems and carbonate mineral saturation levels (Omega(CaCO3)). When considering mineral powder addition as a deployment option, currently considered feedstocks either exhibit slow dissolution kinetics or may require additional water treatment to limit rapid pH changes. Carbonate minerals, on the other hand, feature fast dissolution when undersaturated, accompanied with a reduced impact on pH and Omega(CaCO3). However, traditional (non-hydrated) carbonate minerals such as calcite and aragonite are insoluble in seawater and therefore impractical as direct-to-use feedstocks for OAE. Here, we examine the dissolution kinetics and alkalinity release efficiency of ikaite (CaCO3 & centerdot;6H(2)O) - a hydrated carbonate mineral producible from limestone and dissolvable in seawater - across a range of global sea surface temperatures through a series of controlled laboratory-scale experiments. The main focus lay on determining the alkalinity release efficiency at temperatures where ikaite usually becomes thermodynamically unstable (>6 degrees C). The conducted experiments showed that ikaite dissolved at temperatures of 10 - 26 degrees C with initial dissolution rates comparable to calcite at highly undersaturated conditions (7.62 x 10(-6) - 2.17 x 10(-5) mol m(-2) s(-1)) but exhibited a limited alkalinity release efficiency (50 - 80 % of potential alkalinity added). The reduced alkalinity release was likely caused by partial transformation of ikaite into non-soluble carbonate minerals before complete alkalinity release could be reached. An additional experiment at 3 degrees C resulted in a higher alkalinity release efficiency (85 - 100 %) but was accompanied by a two magnitudes lower initial dissolution rate (1.43 - 1.63 x 10(-7) mol m(-2) s(-1)) revealing a tradeoff between the dissolution rate and alkalinity release efficiency as a function of temperature. The results emphasize that efficient ikaite dissolution is limited to cold waters (<10 degrees C). It also stresses that when rates vary with the environmental conditions they must comply with suitable particle size distributions to maintain sinking rates and effective CO2 uptake. Therefore, we integrated derived dissolution rates and alkalinity release efficiencies together with different practical particle size distributions into a non-turbulent stokes formulation. The model output suggests that in warmer seawater (>10 degrees C) the alkalinity disperses within 10 m below the sea surface, but due to reduced alkalinity release efficiency, CO2 would be lowered to similar to 0.7 - 1.3 mol per mole of added Ca2+. At 3 degrees C, on the other hand, the CO2 uptake efficiency is less affected by incomplete dissolution of ikaite but likely varies more with mean particle size, emphasizing its critical role in cold waters. It remains to be tested whether these results also apply on a larger scale, for example by conducting tank or mesocosm experiments.

Ocean alkalinity enhancement is a promising carbon dioxide removal (CDR) approach, but scaling up to gigatonnes (Gt) of CO2 per year will require safe, sustainable, and abundant alkaline feedstocks. Here, we propose the use of a relatively unexplored resource for OAE, namely naturally occurring sodium (bi)carbonates. We identified and mapped 109 such deposits globally, although quantitative resource information is available for only 16. Quantified deposits collectively contain >200 Gt of sodium (bi)carbonate-rich minerals and brines, dominated by trona (Na2CO3 & centerdot;NaHCO3 & centerdot;2H(2)O) and nahcolite (NaHCO3) mainly concentrated in the USA, China, Turkey, and Africa. We then assessed three OAE pathways using trona as a feedstock, i.e., 1) Mining, crushing, and ocean dispersal of trona (gross CDR capacity 0.16 tCO(2) t(-1)); 2) Calcining trona (with carbon capture and storage) to produce soda ash (Na2CO3) (0.31 tCO(2) t(-1)) prior to dispersal; and 3) Purification of soda ash via dissolution, crystallisation, and drying prior to dispersal. Using Green River, Wyoming, USA (similar to 116 Gt of bedded trona) as a case study, life cycle assessment informed on the net-negativity of each pathway. Provided that mining and transportation have largely decarbonised, all pathways are net-negative (carbon penalties in the range 29-41%), with pathway 2 achieving the highest net CDR capacity. Solution mining and reserves nearer the coastline can optimise net-negativity. Overall, naturally occurring sodium (bi)carbonate resources emerge as a promising future feedstock for soluble, safe, and scalable OAE.

Carbon dioxide removal (CDR) is essential for achieving net zero and net-negative emissions, yet robust monitoring, reporting, and verification (MRV) remains a major challenge. Current accounting practices are fragmented, with inconsistent system boundaries and a narrow focus on carbon, overlooking wider environmental impacts and co-benefits. Drawing on five years of research and demonstration under the UK GGR-D Programme, the most long-term global MRV effort, with multiple years of monitoring across multiple CDR technologies, this study proposes a harmonized framework for defining system boundaries across six key CDR approaches: biochar, bioenergy with carbon capture and storage (BECCS), direct air capture with storage (DACCS), peatland restoration, enhanced rock weathering, and afforestation. We map data availability for evidencing net removal across full supply chains, including the capture of CO2 from the atmosphere and its final storage, and assess gaps in environmental impact data. Our findings show that harmonization is feasible across diverse CDR methods—land-based, engineered, and hybrid land-based - engineered—but data coverage is uneven, particularly for non-carbon metrics. These gaps pose risks for sustainability assessments and the credibility of CDR claims, with implications for emerging policy frameworks and carbon markets. This work provides actionable insights for developing robust MRV systems that support transparent, sustainable CDR deployment at scale.

Purpose: Human activities in the ocean are putting growing pressure on marine ecosystems. Life cycle assessment (LCA) is used to assess these activities environmentally but faces limitations in capturing marine impacts. Improving LCA requires a detailed understanding of the marine environment impact pathways of these technologies to develop sub-compartmentalised and regionalised characterisation factors (CFs). We demonstrate how such pathways can be identified, illustrating with ocean alkalinity enhancement (OAE). Methods: We build on Woods et al. (2021), who propose a qualitative framework to identify key components of impact pathways, and Richter et al. (2024), who provide guidance on framework development in a multidisciplinary context. We develop a methodological approach that allows to qualitatively identify the marine environmental impact pathways of marine technologies and determine which components are integrated in LCIA models or missing, as an initial phase toward developing CFs for life cycle impact assessment (LCIA). We then apply the approach to OAE as an illustrative case. Results and Discussion: Our methodological approach includes: (1) the selection of literature on a studied marine technology and its impacts on marine ecosystems; (2) the identification of the marine elementary flows in the life cycle inventory and the fate, exposure, and effect processes; (3) the inputs from LCIA, oceanography, and technology experts; (4) the review of LCIA models to examine which elements of the marine environmental impact pathways are represented or lacking; (5) the definition of research priorities to advance the assessment of the marine technology’s environmental impacts within LCA. We identified three impact categories associated with OAE marine environmental impacts pathways: marine ecotoxicity, marine eutrophication, and ocean acidification. Existing LCIA models only partially capture these pathways and require adaptation for assessing comprehensively OAE. Research priorities include conducting additional experiments on the ecotoxicological and eutrophic effects of OAE deployment in marine environments, and the effect of added alkalinity on a broader range of marine calcifiers. Several of our recommendations are also relevant to enhance marine technologies’ assessments in LCA more broadly, such as improving the ocean’s representation in models, modelling direct release to the marine compartment, and broadening the elementary flows’ coverage for marine eutrophication. Conclusion: We present a methodology to identify marine environmental impact pathways of marine technologies, providing a first phase toward developing CFs. The methodological approach can be applied to other marine technologies, where identifying impact pathways require a multidisciplinary approach that combines the LCA field with oceanography and engineering expertise.

Abstract Lime carbonation direct air capture (DAC) systems remove atmospheric carbon dioxide (CO 2 ) by carbonating calcium hydroxide (Ca(OH) 2 ) to produce calcium carbonate (CaCO 3 ), which can release CO 2 for durable storage. Accurate and precise measurement of generated CaCO 3 is essential in quantifying CO 2 removed from the atmosphere, and for optimizing the carbonation process. Methods for measurement of carbonate content are well established, but have yet to be applied to materials produced by this system (i.e., almost solely Ca(OH) 2 and CaCO 3 ). Five carbonate content analysis techniques (loss on ignition, LOI; thermogravimetric analysis, TGA; combustion analysis of carbon via infrared absorption, CAC-IR; volumetric calcimetry; and quantitative Fourier transform infrared spectroscopy, FTIR) were investigated for their measurement accuracy and precision over a range of carbonate contents. Sample throughput and levelized cost of analysis were considered in addition to accuracy and precision. LOI and CAC-IR proved favorable against equal consideration of the four factors. Weighting for accuracy and precision, LOI was favorable. Standard operating procedures, including established accuracy and precision levels, for viable carbonate content quantification techniques should be developed, tested, and presented to assure carbon credit buyers, the scientific community, and the public on the validity of carbon credits generated by lime carbonation DAC.

Ocean alkalinity enhancement (OAE) is considered a promising marine carbon dioxide removal (mCDR) option and may contribute to climate change mitigation. Life cycle assessment (LCA) is used to assess OAE environmentally but faces limitations in capturing marine impacts. Improving the assessment of OAE in LCA requires a detailed understanding of its marine environment impact pathways to develop sub-compartmentalised and regionalised characterisation factors (CFs). We demonstrate how such pathways can be identified. We build on Woods et al. (2021), who propose a qualitative framework to identify key components of impact pathways, and Richter et al. (2024), who provide guidance on framework development in a multidisciplinary context. We develop a methodological approach that allows to qualitatively identify the marine environmental impact pathways of OAE and determine which components are integrated in LCIA models or missing, as an initial phase toward developing CFs for life cycle impact assessment (LCIA). Our methodological approach includes: (1) the selection of literature on OAE and its impacts on marine ecosystems; (2) the identification of the marine elementary flows in the life cycle inventory and the fate, exposure, and effect processes; (3) the inputs from LCIA, oceanography, and technology experts; (4) the review of LCIA models to examine which elements of the marine environmental impact pathways are represented or lacking; (5) the definition of research priorities to advance the assessment of OAE’s environmental impacts within LCA. We identified three impact categories associated with OAE marine environmental impacts pathways: marine ecotoxicity, marine eutrophication, and ocean acidification. Existing LCIA models only partially capture these pathways and require adaptation for assessing comprehensively OAE. Research priorities include conducting additional experiments on the ecotoxicological and eutrophic effects of OAE deployment in marine environments, and the effect of added alkalinity on a broader range of marine calcifiers. Several of our recommendations are also relevant to enhance marine technologies’ assessments in LCA more broadly, such as improving the ocean’s representation in models, modelling direct release to offshore marine waters, and broadening the elementary flows’ coverage for marine eutrophication. We present a methodology to identify marine environmental impact pathways of OAE, providing a first phase toward developing CFs. The methodological approach might be adapted to other marine technologies, where identifying impact pathways require a multidisciplinary approach that combines the LCA field with oceanography and engineering expertise.

Carbon dioxide removal (CDR) must scale rapidly to meet climate targets, yet durable pathways remain limited in cost and scalability. Nickel mine tailings provide a globally abundant resource for reaction with atmospheric carbon dioxide and the recovery of critical metals. Here we show that long-term CDR cost of an integrated acid-leach, bipolar membrane electrodialysis (BPMED), and aqueous carbonation system are ~ US$200 tCO₂⁻¹, enabling > 100 MtCO₂ yr⁻¹ of durable removal while shifting nickel production to net-negative emissions. We apply a prospective techno-economic assessment that combines cost reductions from technology learning with modelled changes in the background socio-technical context in which the process operates. However, $22 billion of cumulative investment is required to breakeven by 2045, the functional equivalent of an additional US$154 tCO₂⁻¹, but annual profits could be on the order of $5–20 billion in the latter half of the century. Sensitivity analysis identifies BPMED performance as key to feasibility which offers opportunities for technology innovation. These results establish tailings-based mineralization as a scalable CDR pathway that integrates mining into global climate mitigation strategies.

The production of ikaite, a metastable calcium carbonate hydrate, offers a promising pathway for atmospheric CO2 removal through ocean alkalinity enhancement. This study explores the feasibility of ikaite production through a three-step process, involving calcite (CaCO3) dissolution under elevated CO2 pressure, CO2 degassing from the calcium carbonate rich solution, and subsequent crystallisation. Here, a mathematical model was developed and validated against experimental data, and the effect of key operational parameters was examined. The calcite loading/dosage, particle size and CO2 pressure for dissolution, seed loading and particle size for crystallisation, and degassing pressure as critical factors have significant impact on process efficiencies. Under optimal conditions, involving CO2 pressures of 2 bar for dissolution, 0.01 bar for degassing, and 0.001 bar for crystallisation, with seed loading of 5 kg/m3 and seed particle sizes of 3 mu m, the process achieved steady state ikaite production of 1.64 kg/m3 from a calcite feed of 0.83 kg/m3 . This investigation demonstrates the technical viability of ikaite production through CO2 pressure swing and informs its future development as a potential contributor to climate change mitigation.

Enhanced weathering (EW) with agriculture uses crushed silicate rocks to drive carbon dioxide removal (CDR)1,2. If widely adopted on farmlands, it could help achieve net-zero emissions by 20502, 3-4. Here we show, with a detailed US state-specific carbon cycle analysis constrained by resource provision, that EW deployed on agricultural land could sequester 0.16-0.30 GtCO2 yr-1 by 2050, rising to 0.25-0.49 GtCO2 yr-1 by 2070. Geochemical assessment of rivers and oceans suggests effective transport of dissolved products from EW from soils, offering CDR on intergenerational timescales. Our analysis further indicates that EW may temporarily help lower ground-level ozone and concentrations of secondary aerosols in agricultural regions. Geospatially mapped CDR costs show heterogeneity across the USA, reflecting a combination of cropland distance from basalt source regions, timing of EW deployment and evolving CDR rates. CDR costs are highest in the first two decades before declining to about US$100-150 tCO2-1 by 2050, including for states that contribute most to total national CDR. Although EW cannot be a substitute for emission reductions, our assessment strengthens the case for EW as an overlooked practical innovation for helping the USA meet net-zero 2050 goals5,6. Public awareness of EW and equity impacts of EW deployment across the USA require further exploration7,8 and we note that mobilizing an EW industry at the necessary scale could take decades.

The commercial carbon dioxide removal ecosystem is growing rapidly, but the supporting talent pipeline remains underdeveloped. Universities will have a key role in training the workforce for this expanding sector.

Negative emission technologies (NETs) are an integral part of most climate change mitigation scenarios limiting global warming to 1.5 °C above pre-industrial levels. Several different NETs have been proposed, including ocean alkalinization and direct CO2 removal which have been considered as methods with high carbon removal potential. In ocean alkalinization partial pressure of CO2 sea surface is reduced by spreading alkaline material and in direct removal of CO2 it is extracted from sea water and transported to permanent reservoir. To date, most studies on ocean-based NETs with Earth System Models have been based on idealized scenarios where atmospheric carbon is either simply removed by prescribed amount or some NET is deployed at magnitudes that would be extremely challenging to reach if any economic, technical, or political constraints were considered.In this work, we present Earth System Model simulations using a more realistic global deployment scenario for ocean alkalinization with CaO dispersed at ocean surface in the exclusive economic zones of US, Europe, and China. The dispersion scenario is based on current excess capacities in the lime and cement industries in these three regions, and high-end projections on how they could evolve until 2100. We use the high-overshoot SSP5-3.4-OS as the socioeconomic background scenario. We simulate the deployment scenarios with several Earth System Models. We will show results from simulations with alkalinity enhancement deployment initiated in 2030 and 2040. Furthermore, we compare these results with simulations of direct removal of CO2. Here, the direct removal is calculated from the added alkalinity using approximation for CO2 uptake factor using the relation between alkalinity and dissolved inorganic carbon.The results show that the CO2 is being removed from the atmosphere to oceans after the alkalinity deployment. Compared to the control simulation the global CO2 concentration is reduced by about 7 ppm in the deployment scenario starting in 2030 and about 4 ppm in the deployment scenario starting 2040 by end of the century. For real life deployment the efficacy and detectability of the alkalinity enhancement is a major concern. We will show that the temperature change in the earlier deployment scenario (higher removal potential) cannot be distinguished from the annual variability illustrating the problem in detectability. Furthermore, the simulations show the deployment must be constrained in regions with low oceanic transport to inhibit the precipitation of CaCO3 to retain the CO2 removal potential.Using a more realistic scenario for ocean alkalinization we can give a more realistic assessment of its climate effects and explore new research questions such as detectability of local changes in pH or carbon fluxes with slowly increasing deployment rates. In the realistic deployment scenario, ocean alkalinization decreases the CO2 concentration but does not produce a large signal in the temperature. Therefore, this method can be seen as having potential but its role in removing carbon from the atmosphere is limited, according to these scenarios. Furthermore, the wider effects on the Earth system still require more analysis.

Metal oxides such as lime (CaO and Ca(OH)2) or magnesium oxide (MgO) react spontaneously with CO2 in the air, under ambient conditions, to form stable carbonate minerals. They are therefore, being used as reactive materials to remove carbon dioxide from the atmosphere to help prevent climate change. In these technologies 'thin' layers of calcium or magnesium oxides/hydroxides are spread over an area of land or inside tiered structures to contact the material with CO2 in the air. The proposed thickness of these layers varies by orders of magnitude between theoretical studies, from 3 to 100 mm, however, there is no published data describing the rates of carbonation as a function of layer thickness for lime. This study monitored the carbonation reaction of 2.5, 5, 10, 25 and 50 mm layers of CaO and Ca(OH)2 in ambient temperatures and concentrations of CO2. The results show that repeated spreading of thin layers (<10 mm every 5-10 days) resulted in the largest removal rate per spatial area (>2 t CO2 ha-1 day-1). However, given that the production costs of zero carbon lime may be substantially greater than the cost of land, it may be more economical to maximise conversion through extended periods between applications.

This OceanNETs synthesis report offers a condensed synopsis of major outcomes and key conclusions obtained from the different work packages of the project. It is designed to inform and engage scientists across a wide range of disciplines, thereby contributing to capacity building in the field of marine carbon dioxide removal (CDR). It synthesizes the complex findings of OceanNETs in a condensed format. Alongside the synthesis of key insights, the report provides direct references and links to the full suite of OceanNETs deliverables and peer-reviewed publications, enabling readers to further explore the underlying data, methodologies, and details of the analyses. The introduction, which provides general information about the project, is followed by key messages that convey important lessons learned, points out significant findings, and provides recommendations for possible actions and research activities. Thereafter, the report provides a summary of insights documented in greater detail in OceanNETs deliverables and publications. That part is split into two thematic sections: Section A addresses Society and ONETs, and section B covers Scalability and responses to ONETs (OAE). Each section is further divided into research topics, each of which first provides information on the research approaches and then describes key findings. This synthesis report closes with a series of synopses presented in the form of four ONET research briefs. They provide more complementary and comprehensive insights that go beyond the outcomes of OceanNETs by incorporating results from other relevant publications and reports that often emerged in parallel during the course of the project. The references to OceanNETs publications and deliverables are marked in blue, with deliverables further distinguished from peer-reviewed publications by the inclusion of their deliverable numbers. References to external studies that enrich the overall synthesis are not marked by color. It is noted that few analyses are still in progress and will be published after the project has been closed.

Natural weathering of ultramafic rocks produces laterites that host nickel (Ni) and cobalt (Co) which are critical to a renewable energy transition. Here, we performed sulfuric acid leaching on an ultramafic rock, which produces an iron (Fe)-rich residue that concentrates Ni and Co on laboratory time scales, herein termed artificial laterite. Nickel and Co in the artificial laterite are up to 5 - 7 times enriched relative to the raw material and close to their cutoff grades of natural laterite ores. The most Ni-and Co-rich artificial laterite is dominated by poorly crystalline ferrihydrite as revealed by synchrotron-based powder diffraction (PD) and X-ray absorption spectroscopy (XAS, Fe K-edge). By reacting this artificial laterite with aqueous Fe(II) at circumneutral pH and ambient temperature, release of structurally bound Ni and Co is markedly enhanced. Followed by mild acid extraction, total recovery of Ni and Co is around 80%. Meanwhile, impurities such as coprecipitated silica with ferrihydrite would not negatively impact metal release. Our work demonstrates that acid leaching of ultramafic source rocks can generate artificial laterites that are more reactive than their natural counterparts. Furthermore, leaching of these rocks releases magnesium (Mg), an important cation for carbon mineralization, potentially offsetting carbon emissions during metal extraction.