Ocean Alkalinity Research Articles

Marine carbon dioxide removal by alkalinization should no longer be overlooked

To achieve the Paris climate target, deep emissions reductions have to be complemented with carbon dioxide removal (CDR). However, a portfolio of CDR options is necessary to reduce risks and potential negative side effects. Despite a large theoretical potential, ocean-based CDR such as ocean alkalinity enhancement (OAE) has been omitted in climate change mitigation scenarios so far. In this study, we provide a techno-economic assessment of large-scale OAE using hydrated lime (‘ocean liming’). We address key uncertainties that determine the overall cost of ocean liming (OL) such as the CO2 uptake efficiency per unit of material, distribution strategies avoiding carbonate precipitation which would compromise efficiency, and technology availability (e.g., solar calciners). We find that at economic costs of 130–295 $/tCO2 net-removed, ocean liming could be a competitive CDR option which could make a significant contribution towards the Paris climate target. As the techno-economic assessment identified no showstoppers, we argue for more research on ecosystem impacts, governance, monitoring, reporting, and verification, and technology development and assessment to determine whether ocean liming and other OAE should be considered as part of a broader CDR portfolio.

Investigating the effect of silicate- and calcium-based ocean alkalinity enhancement on diatom silicification

Abstract. Gigatonne-scale atmospheric carbon dioxide removal (CDR) will almost certainly be needed to supplement the emission reductions required to keep global warming between 1.5–2 °C. Ocean alkalinity enhancement (OAE) is an emerging marine CDR method with the addition of pulverised minerals to the surface ocean being one widely considered approach. A concern of this approach is the potential for dissolution products released from minerals to impact phytoplankton communities. We conducted an experiment with 10 pelagic mesocosms (M1–M10) in Raunefjorden, Bergen, Norway, to assess the implications of simulated silicate- and calcium-based mineral OAE on a coastal plankton community. Five mesocosms (M1, M3, M5, M7, and M9) were enriched with silicate (∼ 75 µmol L−1 Na2SiO3), alkalinity along a gradient from 0 to ∼ 600 µmol kg−1, and magnesium in proportion to alkalinity additions. The other five mesocosms (M2, M4, M6, M8, M10) were enriched with alkalinity along the same gradient and calcium in proportion to alkalinity additions. The experiment explored many components of the plankton community, from microbes to fish larvae, and here we report on the influence of simulated mineral based OAE on diatom silicification. Macronutrients (nitrate and phosphate) limited silicification at the onset of the experiment until nutrient additions on day 26. Silicification was significantly greater in the silicate-based mineral treatment, with all genera except Cylindrotheca displaying an increase in silicification as a result of the increased concentration of dissolved silicate. In contrast to the effect of differences in dissolved silicate concentrations between the two mineral treatments, increases in alkalinity only influenced the silicification of two genera, Pseudo-nitzschia and Nitzschia. The four other genera (Arcocellulus, Cylindrotheca, Skeletonema, and Thalassiosira) investigated here displayed no significant changes in silicification as a result of alkalinity increases between 0 and 600 µmol kg−1 above natural levels. In summary, our findings illustrate that the enhancement of alkalinity via simulated silicate- and calcium-based methods has limited genus-specific impacts on the silicification of diatoms. This research underscores the importance of understanding the full breadth of different OAE approaches, their risks, co-benefits, and potential for interactive effects.

Ocean alkalinity enhancement using sodium carbonate salts does not lead to measurable changes in Fe dynamics in a mesocosm experiment

Abstract. The addition of carbonate minerals to seawater through an artificial ocean alkalinity enhancement (OAE) process increases the concentrations of hydroxide, bicarbonate, and carbonate ions. This leads to changes in the pH and the buffering capacity of the seawater. Consequently, OAE could have relevant effects on marine organisms and in the speciation and concentration of trace metals that are essential for their physiology. During September and October 2021, a mesocosm experiment was carried out in the coastal waters of Gran Canaria (Spain), consisting on the controlled variation of total alkalinity (TA). Different concentrations of carbonate salts (NaHCO3 and Na2CO3) previously homogenized were added to each mesocosm to achieve an alkalinity gradient between Δ0 to Δ2400 µmol L−1. The lowest point of the gradient was 2400 µmol kg−1, being the natural alkalinity of the medium, and the highest point was 4800 µmol kg−1. Iron (Fe) speciation was monitored during this experiment to analyse total dissolved iron (TdFe, unfiltered samples), dissolved iron (dFe, filtered through a 0.2 µm pore size filter), soluble iron (sFe, filtered through a 0.02 µm pore size filter), dissolved labile iron (dFe′), iron-binding ligands (LFe), and their conditional stability constants (KFeL′) because of change due to OAE and the experimental conditions in each mesocosm. Observed iron concentrations were within the expected range for coastal waters, with no significant increases due to OAE. However, there were variations in Fe size fractionation during the experiment. This could potentially be due to chemical changes caused by OAE, but such an effect is masked by the stronger biological interactions. In terms of size fractionation, sFe was below 1.0 nmol L−1, dFe concentrations were within 0.5–4.0 nmol L−1, and TdFe was within 1.5–7.5 nmol L−1. Our results show that over 99 % of Fe was complexed, mainly by L1 and L2 ligands with kFe′L′ ranging between 10.92 ± 0.11 and 12.68 ± 0.32, with LFe ranging from 1.51 ± 0.18 to 12.3 ± 1.8 nmol L−1. Our data on iron size fractionation, concentration, and iron-binding ligands substantiate that the introduction of sodium salts in this mesocosm experiment did not modify iron dynamics. As a consequence, phytoplankton remained unaffected by alterations in this crucial element.

Carbon Outwelling and Uptake Along a Tidal Glacier‐Lagoon‐Ocean Continuum

AbstractTidewater glaciers are highly vulnerable to climate change due to warming from both atmospheric and seawater sources. Most tidewater glaciers are rapidly retreating, but little is known about how glacial melting modifies coastal biogeochemical cycles. Here, we investigate carbonate and nutrient dynamics and fluxes in an expanding proglacial tidal lagoon connected to Europe's largest glacier in Iceland (Vatnajökull). The lagoon N:P:Si ratios (2:1:30) imply a system deficient in nitrogen. The large variations in the freshwater endmembers highlighted the complexity of resolving sources and transformations. The lagoon acted as a sink of dissolved inorganic carbon (DIC). Floating chamber incubations revealed a CO2 uptake of 26 ± 15 mmol m−2 d−1. Lagoon waters near the glacier had a 170% higher CO2 uptake than near the lagoon mouth, likely driven by primary production stimulated by nitrogen‐rich bottom water upwelling. The lateral DIC and total alkalinity (TA) flux rates (outwelling) from the lagoon to the ocean were −1.5 ± 0.1 (export to ocean) and 23 ± 5 mmol m−2 d−1 (import into the lagoon) respectively. All samples were undersaturated with respect to aragonite due to glacial meltwater dilution of TA and CO2 uptake. This implies dilution of oceanic alkalinity, lowering the nearshore buffering capacity against ocean acidification.

Regional potential of coastal ocean alkalinization with olivine within 100 years

The spreading of crushed olivine-rich rocks in coastal seas to accelerate weathering reactions sequesters atmospheric CO2 and reduces atmospheric CO2 concentrations. Their weathering rates depend on different factors, including temperature and the reaction surface area. Therefore, this study investigates the variations in olivine-based enhanced weathering rates across 13 regional coasts worldwide. In addition, it assesses the CO2 sequestration within 100 years and evaluates the maximum net-sequestration potential based on varying environmental conditions. Simulations were conducted using the geochemical thermodynamic equilibrium modeling software PHREEQC. A sensitivity analysis was performed, exploring various combinations of influencing parameters, including grain size, seawater temperature, and chemistry. The findings reveal significant variation in CO2 sequestration, ranging from 0.13 to 0.94 metric tons (t) of CO2 per ton of distributed olivine-rich rocks over 100 years. Warmer coastal regions exhibit higher CO2 sequestration capacities than temperate regions, with a difference of 0.4 t CO2/t olivine distributed. Sensitivity analysis shows that smaller grain sizes (10 µm) exhibit higher net CO2 sequestration rates (0.87 t/t) in olivine-based enhanced weathering across all conditions, attributed to their larger reactive surface area. However, in warmer seawater temperatures, olivine with slightly larger grain sizes (50 and 100 µm) displays still larger net CO2 sequestration rates (0.97 and 0.92 t/t), optimizing the efficiency of CO2 sequestration while reducing grinding energy requirements. While relying on a simplified sensitivity analysis that does not capture the full complexity of real-world environmental dynamics, this study contributes to understanding the variability and optimization of enhanced weathering for CO2 sequestration, supporting its potential as a sustainable CO2 removal strategy.

Alkaline earth metal ions affect the dissolution of kaolinite-bearing olivine and its carbon storage effects

Olivine dissolution applications in coastal environments hold great promise for both ocean alkalinity enhancement and carbon dioxide storage. Yet the dissolution mechanism of olivine and the influence of metal ions, such as Mg2+ and Ca2+, remain unclear. Moreover, the weathering products, such as clay minerals are usually mixed with olivine, and the influence of the associated minerals on the dissolution also unexplored. In this study, an 80-day dissolution experiment was performed in Ca2+-free artificial seawater (ASW-Ca) and ASW lacking both Ca2+ and Mg2+ (ASW-CaMg), to elucidate the mechanism of olivine dissolution and the impact of Ca2+ and Mg2+ on the presence of an associated clay mineral, kaolinite. To evaluate the mineral properties before and after dissolution, X-ray diffraction spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy analyses were used. Etch pits were observed on olivine's surface as well as its reduced crystallinity. The atomic ratio of Mg/Si in olivine decreased from 1.49 ± 0.56 in the raw samples to 1.29 ± 0.53 in the samples obtained from ASW-CaMg after undergoing the 80-day dissolution, whereas it was much higher, at 1.85 ± 0.32, in the samples from ASW-Ca. The increasing Mg/Si was attributed to the Mg2+ adsorption on the surface of olivine, which inhibited the release of structural Mg2+. Hence, Mg2+ was a key environmental factor influencing the process of olivine dissolution, and estuaries, which have low Mg2+ content and low pH may be the promising areas for the olivine application. Moreover, in ASW-CaMg, the total alkalinity (TA), dissolved inorganic carbon (DIC) concentration, and charge concentration of alkaline earth metal ions increased by 3392 ± 28, 2922 ± 3, and 3740 ± 91 μmol kg−1, respectively, compared to the data from the blank experiment. Notably, TA and DIC had strong linear relationships with the alkaline earth metal ions (Mg2+ and Ca2+). The release of free alkaline earth metal cations during olivine dissolution was a controlling factor for long-term carbon dioxide storage in ASW-CaMg. Finally, during the 80-day dissolution experiment, no dissolution of the associated kaolinite was observed, however, kaolinite may influence the olivine dissolution by adsorbing the released Si.

The effects of bathymetry on the long-term carbon cycle and CCD

The shape of the ocean floor (bathymetry) and the overlaying sediments provide the largest carbon sink throughout Earth's history, supporting ~one to two orders of magnitude more carbon storage than the oceans and atmosphere combined. While accumulation and erosion of these sediments are bathymetry dependent (e.g., due to pressure, temperature, salinity, ion concentration, and available productivity), no systemic study has quantified how global and basin scale bathymetry, controlled by the evolution of tectonics and mantle convection, affects the long-term carbon cycle. We reconstruct bathymetry spanning the last 80 Myr to describe steady-state changes in ocean chemistry within the Earth system model LOSCAR. We find that both bathymetry reconstructions and representative synthetic tests show that ocean alkalinity, calcite saturation state, and the carbonate compensation depth (CCD) are strongly dependent on changes in shallow bathymetry (ocean floor ≤600 m) and on the distribution of the deep marine regions (>1,000 m). Limiting Cenozoic evolution to bathymetry alone leads to predicted CCD variations spanning 500 m, 33 to 50% of the total observed variations in the paleoproxy records. Our results suggest that neglecting bathymetric changes leads to significant misattribution to uncertain carbon cycle parameters (e.g., atmospheric CO2 and water column temperature) and processes (e.g., biological pump efficiency and silicate-carbonate riverine flux). To illustrate this point, we use our updated bathymetry for an Early Paleogene C cycle case study. We obtain carbonate riverine flux estimates that suggest a reversal of the weathering trend with respect to present-day, contrasting with previous studies, but consistent with proxy records and tectonic reconstructions.

Influence of ocean alkalinity enhancement with olivine or steel slag on a coastal plankton community in Tasmania

Abstract. Ocean alkalinity enhancement (OAE) aims to increase atmospheric CO2 sequestration in the oceans through the acceleration of chemical rock weathering. This could be achieved by grinding rocks containing alkaline minerals and adding the rock powder to the surface ocean where it dissolves and chemically locks CO2 in seawater as bicarbonate. However, CO2 sequestration during dissolution coincides with the release of potentially bioactive chemicals and may induce side effects. Here, we used 53 L microcosms to test how coastal plankton communities from Tasmania respond to OAE with olivine (mainly Mg2SiO4) or steel slag (mainly CaO and Ca(OH)2) as alkalinity sources. Three microcosms were left unperturbed and served as a control, three were enriched with olivine powder (1.9 g L−1), and three were enriched with steel slag powder (0.038 g L−1). Olivine and steel slag powders were of similar grain size. Olivine was added in a higher amount than the steel slag with the aim of compensating for the lower efficiency of olivine to deliver alkalinity over the 3-week experiment. Phytoplankton and zooplankton community responses as well as some biogeochemical parameters were monitored. Olivine and steel slag additions increased total alkalinity by 29 and 361 µmol kg−1, respectively, corresponding to a respective theoretical increase of 0.9 % and 14.8 % of the seawater storage capacity for atmospheric CO2. Olivine and steel slag released silicate nutrients into the seawater, but steel slag released considerably more and also significant amounts of phosphate. After 21 d, no significant difference was found in dissolved iron concentrations (>100 nmol L−1) in the treatments and the control. The slag addition increased dissolved manganese concentrations (771 nmol L−1), while olivine increased dissolved nickel concentrations (37 nmol L−1). There was no significant difference in total chlorophyll-a concentrations between the treatments and the control, likely due to nitrogen limitation of the phytoplankton community. However, flow cytometry results indicated an increase in the cellular abundance of several smaller (∼<20 µm) phytoplankton groups in the olivine treatment. The abundance of larger phytoplankton (∼>20 µm) decreased much more in the control than in the treatments after day 10. Furthermore, the maximum quantum yields of photosystem II (Fv/Fm) were higher in slag and olivine treatments, suggesting that mineral additions increased photosynthetic performance. The zooplankton community composition was also affected, with the most notable changes being observed in the dinoflagellate Noctiluca scintillans and the appendicularian Oikopleura sp. in the olivine treatment. Overall, the steel slag used here was more efficient for CO2 removal with OAE than the olivine over the 3-week timescale of the experiment. Furthermore, the steel slag appeared to induce less change in the plankton community than the olivine when comparing the CO2 removal potential of both minerals with the level of environmental impact that they caused.

Electrochemical ocean iron fertilization and alkalinity enhancement approach toward CO2 sequestration

Achieving net-zero emissions by 2050 requires the development of effective negative emission techniques, including ocean-based approaches for CO2 sequestration. However, the implementation and testing of marine CO2 removal (mCDR) techniques such as ocean iron fertilization (OIF) or ocean alkalinity enhancement (OAE) face significant challenges. Herein, a novel self-operating electrochemical technology is presented that not only combines OIF and OAE, but also recovers hydrogen gas (H2) from seawater, hence offering a promising solution for achieving quantifiable and transparent large-scale mCDR. Experimental results show that the electrochemical OIF (EOIF) can not only increase the concentration of ferrous iron (Fe+2) by 0–0.5 mg/L, but also significantly increases the seawater pH by 8% (i.e., a 25% decrease in the hydrogen ions concentration). The release of iron (Fe+2/Fe+3) can be regulated by adjusting the magnitude of the electric current and its form (e.g., pulsed current and polarity reversal), as well as by optimizing the electrode material and geometry. In certain ocean regions, enhanced iron concentrations stimulate the naturally occurring biological carbon pump (BCP), leading to increased phytoplankton growth, CO2 uptake, and subsequent export of carbon to the deep ocean. Simultaneously, the system increases seawater alkalinity and the buffer capacity, enhancing CO2 solubility and storage in the shallow ocean through the solubility pump. The obtained measurements demonstrate the scalability of EOIF and its ability to operate using solar energy at a lower cost. Overall, the proposed EOIF technology offers a practical, effective, and sustainable solution for addressing climate change on a large scale.

The carbonate pump feedback on alkalinity and the carbon cycle in the 21st century and beyond

Abstract. Ocean acidification is likely to impact all stages of the ocean carbonate pump, i.e. the production, export, dissolution and burial of biogenic CaCO3. However, the associated feedback on anthropogenic carbon uptake and ocean acidification has received little attention. It has previously been shown that Earth system model (ESM) carbonate pump parameterizations can affect and drive biases in the representation of ocean alkalinity, which is critical to the uptake of atmospheric carbon and provides buffering capacity towards associated acidification. In the sixth phase of the Coupled Model Intercomparison Project (CMIP6), we show divergent responses of CaCO3 export at 100 m this century, with anomalies by 2100 ranging from −74 % to +23 % under a high-emission scenario. The greatest export declines are projected by ESMs that consider pelagic CaCO3 production to depend on the local calcite/aragonite saturation state. Despite the potential effects of other processes on alkalinity, there is a robust negative correlation between anomalies in CaCO3 export and salinity-normalized surface alkalinity across the CMIP6 ensemble. Motivated by this relationship and the uncertainty in CaCO3 export projections across ESMs, we perform idealized simulations with an ocean biogeochemical model and confirm a limited impact of carbonate pump anomalies on 21st century ocean carbon uptake and acidification. However, we highlight a potentially abrupt shift, between 2100 and 2300, in the dissolution of CaCO3 from deep to subsurface waters when the global-scale mean calcite saturation state reaches about 1.23 at 500 m (likely when atmospheric CO2 reaches 900–1100 ppm). During this shift, upper ocean acidification due to anthropogenic carbon uptake induces deep ocean acidification driven by a substantial reduction in CaCO3 deep dissolution following its decreased export at depth. Although the effect of a diminished carbonate pump on global ocean carbon uptake and surface ocean acidification remains limited until 2300, it can have a large impact on regional air–sea carbon fluxes, particularly in the Southern Ocean.

Earth system responses to carbon dioxide removal as exemplified by ocean alkalinity enhancement: tradeoffs and lags

Carbon dioxide removal (CDR) is discussed for offsetting residual greenhouse gas emissions or even reversing climate change. All emissions scenarios of the Intergovernmental Panel on Climate Change that meet the ‘well below 2 °C’ warming target of the Paris Agreement include CDR. Ocean alkalinity enhancement (OAE) may be one possible CDR where the carbon uptake of the ocean is increased by artificial alkalinity addition. Here, we investigate the effect of OAE on modelled carbon reservoirs and fluxes in two observationally-constrained large perturbed parameter ensembles. OAE is assumed to be technically successful and deployed as an additional CDR in the SSP5-3.4 temperature overshoot scenario. Tradeoffs involving feedbacks with atmospheric CO2 result in a low efficiency of an alkalinity-driven atmospheric CO2 reduction of −0.35 [−0.37 to −0.33] mol C per mol alkalinity addition (skill-weighted mean and 68% c.i.). The realized atmospheric CO2 reduction, and correspondingly the efficiency, is more than two times smaller than the direct alkalinity-driven enhancement of ocean uptake. The alkalinity-driven ocean carbon uptake is partly offset by the release of carbon from the land biosphere and a reduced ocean carbon sink in response to lowered atmospheric CO2 under OAE. In a second step we use the Bern3D-LPX model in CO2 peak-decline simulations to address hysteresis and temporal lags of surface air temperature change (ΔSAT) in an idealized scenario where ΔSAT increases to ~2 °C and then declines to ~1.5 °C as result of CDR. ΔSAT lags the decline in CO2-forcing by 18 [14–22] years, depending close to linearly on the equilibrium climate sensitivity of the respective ensemble member. These tradeoffs and lags are an inherent feature of the Earth system response to changes in atmospheric CO2 and will therefore be equally important for other CDR methods.

Sustainable carbon sequestration via olivine based ocean alkalinity enhancement in the east and South China Sea: Adhering to environmental norms for nickel and chromium

Enhancing silicate weathering to increase oceanic alkalinity, thereby facilitating the absorption of atmospheric carbon dioxide (CO2), is considered a highly promising technique for carbon sequestration. This study aims to evaluate the feasibility and potential of olivine-based ocean alkalinity enhancement (OAE) for the removal of atmospheric CO2 and its storage in seawater as bicarbonates in the East and South China Seas (ESCS). A particular focus is placed on the potential ecological impacts arising from the release of nickel (Ni) and chromium (Cr) during the olivine weathering process. We considered two extreme scenarios: one where Ni and Cr are entirely retained in seawater, and another where they are completely deposited in sediments. These scenarios respectively represent the maximum permissible concentrations of Ni and Cr in seawater and sediments during the OAE process. Current marine environmental quality standards (EQS) were utilized as the threshold limits for Ni and Cr in both seawater and sediment, with concentrations exceeding these EQS potentially leading to significant adverse effects on marine life. When all released Ni is retained in seawater, the allowable dosage of olivine varies from 0.05 to 13.7 kg/m2 (depending on olivine particle size, temperature, and water depth); when all released Ni is captured by sediment, the permissible addition of olivine ranges from 0.21 to 2.1 kg/m2 (depending on mixing depth). Given the low solubility of Cr, it is not necessary to consider the scenario where Cr exceeds the limit in seawater. The allowable amount of Cr entirely retained in sediments ranges from 0.69 to 47.2 kg/m2.In most scenarios, the accumulation of metals in sediments preferentially exceeds the corresponding threshold value rather than remaining in seawater. Therefore, we recommend using alkalization equipment to fully dissolve olivine before discharging into the sea, enabling a larger-scale application of olivine without significant negative ecological impacts.

Moderate greenhouse climate and rapid carbonate formation after Marinoan snowball Earth

When the Marinoan snowball Earth deglaciated in response to high atmospheric carbon dioxide (CO2) concentrations, the planet warmed rapidly. It is commonly hypothesized that the ensuing supergreenhouse climate then declined slowly over hundreds of thousands of years through continental weathering. However, how the ocean affected atmospheric CO2 in the snowball Earth aftermath has never been quantified. Here we show that the ocean’s carbon cycle drives the supergreenhouse climate evolution via a set of different mechanisms, triggering scenarios ranging from a rapid decline to an intensification of the supergreenhouse climate. We further identify the rapid formation of carbonate sediments from pre-existing ocean alkalinity as a possible explanation for the enigmatic origin of Marinoan cap dolostones. This work demonstrates that a moderate and relatively short-lived supergreenhouse climate following the Marinoan snowball Earth is a plausible scenario that is in accordance with geological data, challenging the previous hypothesis.

Deglacial Pulse of Neutralized Carbon From the Pacific Seafloor: A Natural Analog for Ocean Alkalinity Enhancement?

AbstractThe ocean carbon reservoir controls atmospheric carbon dioxide (CO2) on millennial timescales. Radiocarbon (14C) anomalies in eastern North Pacific sediments suggest a significant release of geologic 14C‐free carbon at the end of the last ice age but without evidence of ocean acidification. Using inverse carbon cycle modeling optimized with reconstructed atmospheric CO2 and 14C/C, we develop first‐order constraints on geologic carbon and alkalinity release over the last 17.5 thousand years. We construct scenarios allowing the release of 850–2,400 Pg C, with a maximum release rate of 1.3 Pg C yr−1, all of which require an approximate equimolar alkalinity release. These neutralized carbon addition scenarios have minimal impacts on the simulated marine carbon cycle and atmospheric CO2, thereby demonstrating safe and effective ocean carbon storage. This deglacial phenomenon could serve as a natural analog to the successful implementation of gigaton‐scale ocean alkalinity enhancement, a promising marine carbon dioxide removal method.

Pathways for marine carbon dioxide removal using electrochemical acid-base generation

Research over the past decade has resulted in various methods for removing CO2 from the atmosphere using seawater and electrochemically generated acids and bases. This Perspective aims to present a unified framework for comparing these approaches. Specifically, these methods can all be seen as falling into one of two categories: those that result in a net increase in ocean alkalinity and use the “ocean as a sponge” for atmospheric CO2 (ocean alkalinity enhancement, or OAE) and those that cycle ocean alkalinity and use the “ocean as a pump” for atmospheric CO2 (ocean alkalinity cycling, or OAC). In this Perspective, approaches for marine carbon dioxide removal (mCDR) using electrochemistry are compared using this framework, and the similarities and differences of these two categories are explored.

Implementation of marine CO2 removal for climate mitigation: The challenges of additionality, predictability, and governability

Achieving net zero CO2 emissions requires gigatonne-scale atmospheric CO2 removal (CDR) to balance residual emissions that are extremely difficult to eliminate. Marine CDR (mCDR) methods are seen increasingly as potentially important additions to a global portfolio of climate policy actions. The most widely considered mCDR methods are coastal blue carbon and seaweed farming that primarily depend on biological manipulations; ocean iron fertilisation, ocean alkalinity enhancement, and direct ocean capture that depend on chemical manipulations; and artificial upwelling that depends on physical manipulation of the ocean system. It is currently highly uncertain which, if any, of these approaches might be implemented at sufficient scale to make a meaningful contribution to net zero. Here, we derive a framework based on additionality, predictability, and governability to assess implementation challenges for these mCDR methods. We argue that additionality, the net increase of CO2 sequestration due to mCDR relative to the baseline state, will be harder to determine for those mCDR methods with relatively large inherent complexity, and therefore higher potential for unpredictable impacts, both climatic and non-climatic. Predictability is inherently lower for mCDR methods that depend on biology than for methods relying on chemical or physical manipulations. Furthermore, predictability is lower for methods that require manipulation of multiple components of the ocean system. The predictability of an mCDR method also affects its governability, as highly complex mCDR methods with uncertain outcomes and greater likelihood of unintended consequences will require more monitoring and regulation, both for risk management and verified carbon accounting. We argue that systematic assessment of additionality, predictability, and governability of mCDR approaches increases their chances of leading to a net climatic benefit and informs political decision-making around their potential implementation.

Public engagement and collaboration for carbon dioxide removal: lessons from a project in the Dominican Republic

Despite an increase in literature on public perceptions of carbon dioxide removal (CDR), there remains a paucity of evidence describing the social and developmental processes involved in the implementation of projects in-situ. This research illustrates a case study documenting a planned research project for coastal enhanced weathering—a form of ocean alkalinity enhancement—in a remote, rural area of the Northwestern Dominican Republic, a Small Island Developing State particularly at risk from climate change impacts. This paper is a collaboration between the company responsible for the project (Vesta) and researchers located in the Dominican Republic and the United Kingdom, We draw upon 2 years' worth of surveys, interviews, focus groups, group information sessions, and reflexive documentation by the Dominican Republic researchers, to present a first-hand account of local community responses to the planned research project and to coastal enhanced weathering and climate change more broadly. We discuss themes of climate vulnerability, justice, and adaptive capacity through the lens of the collaborative governance and social diffusion principles that the project was designed with. We also reflect on a program of outreach and participatory activities which was established to support community development in the areas surrounding the field trial site, as informed by exploration of community needs drawn from the research.

Global‐Scale Evaluation of Coastal Ocean Alkalinity Enhancement in a Fully Coupled Earth System Model

AbstractThe Paris Agreement plans for “net‐zero” carbon dioxide (CO2) emissions during the second half of the 21st century. However, reducing emissions from some sectors is challenging, and “net‐zero” permits carbon dioxide removal (CDR) activities. One CDR scheme is ocean alkalinity enhancement (OAE), which proposes dissolving basic minerals into seawater to increase its buffering capacity for CO2. While modeling studies have often investigated OAE at basin or global scale, some proposals focus on readily accessible coastal shelves, with TA added through the dissolution of seafloor olivine sands. Critically, by settling and dissolving sands on shallow seafloors, this retains the added TA in near‐surface waters in direct contact with atmospheric CO2. To investigate this, we add dissolved TA at a rate of ∼29 Teq y−1 to the global shelves (<100m) of an Earth system model (UKESM1) running a high emissions scenario. As UKESM1 is fully coupled, wider effects of OAE‐mediated increase in ocean CO2 uptake –e.g. atmospheric xCO2, air temperature and marine pH– are fully quantified. Applying OAE from 2020 to 2100 decreases atmospheric xCO2 ∼10 ppm, and increases air‐to‐sea CO2 uptake ∼8%. In‐line with other studies, CO2 uptake per unit of TA added occurs at a rate of ∼0.8 mol C (mol TA)−1. Significantly for monitoring, advection of added TA results in ∼50% of CO2 uptake occurring remotely from OAE operations, and the model also exhibits noticeable land carbon reservoir changes. While practical uncertainties and model representation caveats remain, this analysis estimates the effectiveness of this specific OAE scheme to assist with net‐zero planning.

Phytoplankton response to increased nickel in the context of ocean alkalinity enhancement

Abstract. Ocean alkalinity enhancement (OAE) is considered one of the most promising approaches to actively remove carbon dioxide (CO2) from the atmosphere by accelerating the natural process of rock weathering. This approach involves introducing alkaline substances sourced from natural mineral deposits, such as olivine, basalt, and carbonates or obtained from industrial waste products such as steel slag, into seawater and dispersing them over coastal areas. Some of these natural and industrial substances contain trace metals, which would be released into the oceans along with the alkalinity enhancement. The trace metals could serve as micronutrients for marine organisms at low concentrations but could potentially become toxic at high concentrations, adversely affecting marine biota. To comprehensively assess the feasibility of OAE, it is crucial to understand how the phytoplankton, which forms the base of marine food webs, responds to ocean alkalinization and associated trace metal perturbations. As one of the most abundant metals in OAE source materials, understanding the impacts of nickel (Ni) on the phytoplankton is critical for OAE assessment. In this study, we investigated the influence of nickel (Ni) on three representative phytoplankton species over a gradient of nine Ni concentrations (from 0 to 100 µmol L−1 with 12 µmol L−1 synthetic organic ligand). The impacts of elevated Ni varied among the tested phytoplankton species. The coccolithophore Emiliania huxleyi and the dinoflagellate Amphidinium carterae exhibited a growth rate inhibition of about 30 % and 20 %, respectively, at the highest Ni concentrations. The half maximal inhibitory concentration (IC50, at which the growth rate is inhibited by 50 %) of both species exceeded the tested range of Ni. This suggests that both species were only mildly affected by the elevated Ni concentrations. In contrast, the diatom Thalassiosira weissflogii displayed a considerably higher sensitivity to Ni, with a 60 % growth rate inhibition at the highest Ni concentration and an IC50 value of 63.9 µmol L−1. In conclusion, the variability in phytoplankton sensitivity to Ni exposure suggests that for OAE applications with Ni-rich materials caution is required and critical toxic thresholds for Ni must be avoided.

Submarine volcanism along shallow ridges did not drive Cryogenian cap carbonate formation

Abstract The termination of Neoproterozoic “Snowball Earth” glaciations is marked globally by laterally extensive neritic cap carbonates directly overlying glacial diamictites. The formation of these unique deposits on deglaciation calls for anomalously high calcium carbonate saturation. A popular mechanism to account for the source of requisite ocean alkalinity is the shallow-ridge hypothesis, in which initial spreading ridges surrounding fragments of Rodinia, assumed to be dominated by volcanic margins, were formed at sea level. The shallow ridges are inferred to have promoted widespread deposition and alteration of glassy hyaloclastite—a source of alkalinity. We test this hypothesis by quantifying the prevalence of shallow ridges along Pangea's passive continental margins, and by assessing Neoproterozoic reconstructions of tectonic plates. We find that the most frequently occurring depth range for incipient mid-ocean ridges is 2.1 ± 0.4 km. Ridges with initial elevations of approximately sea level are rare and have anomalous crustal thicknesses >14 km that only occur proximal to large igneous provinces (LIPs). Hyaloclastite is uncommon on mid-ocean ridges as it is generally restricted to water depths of <200 m for tholeiitic basalts, instead forming mostly on intraplate seamounts. Additionally, ocean drilling recently found hyaloclastite to be insignificant along the outer Vøring Plateau (offshore Norway)—an exemplar of a volcanic margin. Reconstructions of Rodinia and associated LIPs demonstrate that volcanic margins potentially hosting minor hyaloclastites were scarce during the late Neoproterozoic. We conclude that the shallow-ridge hypothesis fails to explain the formation of cap carbonates and suggest that other mechanisms such as enhanced continental weathering may be largely responsible.