Lennart Thomas Bach

Marine carbon dioxide removal (mCDR) removes marine CO ₂ from seawater to enable additional uptake of anthropogenic marine CO ₂ by the ocean. However, this uptake is very slow, limiting mCDR efficiency on years to century timescales. Furthermore, it occurs over vast spatial scales, making estimates with real world measurements unfeasible and robust estimates with conventional ocean models expensive. Here, we develop a novel approach that uses virtual particle tracking of mCDR-perturbed absorbers, which accumulate atmospheric marine CO ₂ along their trajectories. Benchmarking against state-of-the-art ocean model simulations shows that absorbers can reproduce the global structures of mCDR efficiency estimated by a low-resolution (1°) model (±8%) and mCDR efficiency from 12 selected sites estimated with a high-resolution (0.1°) model (±4%), while being 10 ² and 10 ⁶ times computationally cheaper. This advance enabled assessing mCDR efficiency on all coastlines around the world at 4-12 km resolution. This high-resolution global assessment found that 10-year mCDR efficiency was generally high (efficiency>80%) across (sub)tropical coasts, but more limited around Antarctica and parts in the North Atlantic, the Mediterranean, North-East-Asian coastlines, and some Southern Hemisphere mid-latitudes. Furthermore, dense nearshore sampling reveals 30-50% efficiency differences over tens to hundreds of kilometres, showing that coastal mCDR efficiency estimates depend on high-resolution assessments. Through integration into accessible online tools, our study provides a transparent and spatially explicit assessment of where coastal mCDR interventions are most likely to provide efficient atmospheric marine CO ₂ uptake.

Ocean iron fertilisation has re-emerged as a candidate marine carbon dioxide removal approach, leading to proposals for larger and longer field experiments. Here, I argue that iron fertilisation research may be trapped in a dilemma of scales. The three critical bottlenecks – (1) predicting plankton community responses and carbon export, (2) quantifying nutrient reallocation and carbon additionality, and (3) assessing deep-ocean oxygen loss – occur at scales larger than those accessible even to proposed meso-scale experiments. While such studies would undoubtedly improve our understanding of ocean biogeochemistry, their outcomes may remain contingent on ecological conditions and too limited to predict deployment-scale effectiveness and environmental risk. Rather than continuing to scale experiments in space and time, I propose a complementary strategy that scales replication by conducting many experiments across diverse ecological and oceanographic settings. This approach may more likely identify reproducible patterns, improve model parameterisations, and support credible estimates of fertilisation performance and environmental consequences.

Ocean alkalinity enhancement (OAE) aims to mitigate climate change by increasing the chemical capacity of seawater to store anthropogenic CO ₂ . OAE can be implemented through multiple pathways, each of which intentionally modifies marine carbonate chemistry through increases in total alkalinity (TA). Experimental research has only recently begun to assess how such TA perturbations (ΔTA) and associated carbonate chemistry changes affect ocean ecosystems. Meaningful assessments need context on how ΔTA‐induced by different OAE pathways would evolve over time and in magnitude. Here, we use a dilution equation, a regional model, and a global model to explore how marine organisms would be exposed to ΔTA under realistic constraints. We find that a more extreme ΔTA of>1,000 μmol kg ⁻¹ , a perturbation common in OAE experiments, only occurs for minutes in a minuscule fraction of the OAE‐perturbed seawater volume. In contrast, ΔTA between 1 and 100 μmol kg ⁻¹ is a ubiquitous perturbation range for OAE under real‐world constraints, yet are not commonly in focus of environmental OAE assessments. These results suggest that there is a disconnect between real‐world ΔTA that can plausibly be invoked by OAE and the experimental ΔTA range frequently used in the environmental OAE assessment. While “unrealistic” ΔTA can provide mechanistic insights into an organism or ecosystem response to carbonate chemistry changes, they can also cause overestimation of OAE effects, if the unrealistic ΔTA is not contextualized appropriately. Our results can be used to improve the contextualization of OAE studies, thereby making the interpretation of ΔTA effects on the environment more robust.

The Peruvian Upwelling is known for its exceptionally high surface water productivity and the presence of one of the world's largest Oxygen Minimum Zones. The upwelling of silicate-rich subsurface waters typically supports diatom-dominated primary productivity in this region. However, warmer surface waters and subsequent changes in stratification and nutrient supply can cause a shift in plankton communities from diatoms to dinoflagellates and silicoflagellates, which affects the silicon (Si) and carbon (C) cycles. In 2017, we investigated the Si cycle in a field experiment off the coast of Peru. Pelagic mesocosms (∼55 000 L) were deployed for 50 d from February to April to simulate upwelling conditions, which coincided with a coastal El Niño. This unique setting allowed us to study the evolution of stable silicon isotopes in seawater ( δ ³⁰ Si _dSi ) and its direct comparison to the produced biogenic material ( δ ³⁰ Si _bSi ) without the influence of unaccountable water mass mixing. On day 12, approximately 40 % of the surface water of the mesocosms was replenished with nitrate-depleted deep water (low N : Si and N : P ratios), which strongly influenced the phytoplankton community. Prior to the addition of the deep water, the phytoplankton community was dominated by diatoms but shifted towards a pronounced dominance of flagellates, including silicoflagellates. At the beginning of the experiment, when diatoms dominated the phytoplankton community, the δ ³⁰ Si _dSi distribution in the surface water (+1.4 ‰ to +2.5 ‰) was within the same range as observed in previous seawater studies in the Peruvian upwelling. After deep water addition, low N : Si (0.02 to 0.2 mol mol ⁻¹ ), strongly deviating from the preferred 1:1 ratio for diatoms, favored silicoflagellate (and dinoflagellate) growth and resulted in higher δ ³⁰ Si _dSi values (up to +4.1 ‰) in the surface waters. The strong increase in δ ³⁰ Si _dSi was associated with low δ ³⁰ Si _bSi values (−0.26 ‰ to +0.65 ‰) caused by high Si isotope fractionation factors between seawater and silicoflagellates. For the first time, the field experiment allowed us to estimate the Si isotope fractionation factor for silicoflagellates (εsilicos=-3.54 ‰ ± 0.56 ‰), which is remarkably high compared to diatoms (−1.1 ‰) and may provide a novel tool to study changes in the present and past marine silicon cycle. However, this value should be confirmed through culture experiments before it can be established as a reliable proxy.

Marine Carbon Dioxide Removal (mCDR) approaches are increasingly considered for climate change mitigation as a supplement to rapid emissions reduction. However, it is still unclear if mCDR approaches could be effective, safe, and accountable. A critical requirement for mCDR to work, potentially, is robust monitoring, reporting, and verification (MRV) of greenhouse gas(es) removed through mCDR activities. MRV frameworks and protocols are currently developed in a scattered manner by individual stakeholders, so that the principles they are built upon may or may not appeal to the broader international community that is interested in the ocean commons. International agreement and consolidation on MRV for mCDR seem crucial to legitimize and validate mCDR, considering that the ocean is a globally interconnected fluid and that activities by some may affect many others. Here, we undertake a step toward consolidation of MRV by consulting the international scientific community. We established a global network of scientists organized into 6 “continental” nodes, each of which addressed the same set of MRV-related questions and whose thoughts were equally weighted in the synthesis. Our consultation shows that while there are many converging views on MRV (e.g., the importance of modeling for MRV), there are also differences in the regional MRV priorities (e.g., the importance of regional vs global models). The areas of consensus and divergence identified herein may be instrumental in the design of more widely accepted MRV frameworks, informed equally by scientists from 6 continents.

Eutrophication and global warming are key drivers of oxygen loss, also termed deoxygenation, in coastal ecosystems worldwide. Artificial reoxygenation has been suggested as a local or regional solution to increase oxygen concentrations and improve water quality by various parties, including water managers and industry. Three main approaches have been proposed: (1) bubbling with air with the aim to destratify and mix the water column (2) injection of pure oxygen and (3) pumping of oxygenated water to greater water depths (downwelling). In this review, we summarize the results of recent field trials and other implementations of artificial reoxygenation in coastal systems, which, to date, only involve small bays and estuaries. We also discuss potential benefits and risks. While the recent trials indicate that reoxygenation of the water column can be achieved, low oxygen conditions returned rapidly within days to months of discontinuing operations. This illustrates that artificial reoxygenation typically only provides a temporary solution to deoxygenation. Potential side effects of artificial reoxygenation could include enhanced emissions of the greenhouse gas carbon dioxide and, upon bubbling and destratification in shallow waters, also of methane. Additionally, downwelling could lead to warming and an associated increased oxygen demand near the seafloor. Reoxygenation will not necessarily reduce the nutrient availability for phytoplankton, implying that water quality may remain poor. We recommend a careful, case-by-case assessment of the suitability of artificial reoxygenation in coastal systems prior to implementation and monitoring before, during and after each intervention. Any field trials should involve all relevant parties, including scientists and local communities, and results should be reported with full transparency. While in the short-term, artificial reoxygenation may be useful to alleviate oxygen loss in some coastal systems, long-term improvements in the oxygen levels and quality of coastal waters require reductions in nutrient inputs and greenhouse gas emissions.

Coastal Enhanced Weathering (CEW) is a marine Carbon Dioxide Removal (CDR) method that adds ground alkaline minerals to shallow regions of the ocean in order to increase seawater alkalinity, i.e., its capacity for storing atmospheric CO₂ as bicarbonate. While CEW is promising with regard to cost and scalability, it is an uncontained, “open-system” style of CDR, and presents significant challenges to effective measurement, reporting, and verification (MRV) of the process. In particular, quantifying how much alkalinity is released from an added amount of mineral is challenging as the minerals dissolve and release alkalinity over wide spatial and long temporal scales. Such quantification is further complicated by the fact that dissolved alkalinity is rapidly diluted below detectable concentrations. Here, we propose an approach to measure alkalinity formation that relies on solid sediment tracers to track mineral grains underwater and quantify how much they have dissolved over time. The amount of dissolution at any given point in time is proportional to the amount of alkalinity released. Thus, the approach aims to overcome the near impossible detection of alkalinity accumulation in the dissolved phase by tracking the loss of alkaline material in the solid phase. We describe a test of the fundamental aspects of this method including the measurement of the mineral and tracer content of a sediment sample, and the extent to which those measurements correlate with changes in seawater chemistry. We found that olivine dissolution significantly increased alkalinity in seawater, compared to control incubators. X-Ray Diffraction (XRD) was able to quantify the change in the olivine in the sediment (albeit with large relative errors) and automated particle counting methods were able to enumerate tracer particles when illuminated under UV-A light. These results serve as a proof-of-principle (and starting point) to further explore a promising way to measure alkalinity addition to the ocean resulting from CEW deployments.

Coastal Enhanced Weathering (CEW) is a marine Carbon Dioxide Removal (CDR) method that adds ground alkaline minerals to shallow regions of the ocean in order to increase seawater alkalinity, i.e., its capacity for storing atmospheric CO₂ as bicarbonate. While CEW is promising with regard to cost and scalability, it is an uncontained, “open-system” style of CDR, and presents significant challenges to effective measurement, reporting, and verification (MRV) of the process. In particular, quantifying how much alkalinity is released from an added amount of mineral is challenging as the minerals dissolve and release alkalinity over wide spatial and long temporal scales. Such quantification is further complicated by the fact that dissolved alkalinity is rapidly diluted below detectable concentrations. Here, we propose an approach to measure alkalinity formation that relies on solid sediment tracers to track mineral grains underwater and quantify how much they have dissolved over time. The amount of dissolution at any given point in time is proportional to the amount of alkalinity released. Thus, the approach aims to overcome the near impossible detection of alkalinity accumulation in the dissolved phase by tracking the loss of alkaline material in the solid phase. We describe a test of the fundamental aspects of this method including the measurement of the mineral and tracer content of a sediment sample, and the extent to which those measurements correlate with changes in seawater chemistry. We found that olivine dissolution significantly increased alkalinity in seawater, compared to control incubators. X-Ray Diffraction (XRD) was able to quantify the change in the olivine in the sediment (albeit with large relative errors) and automated particle counting methods were able to enumerate tracer particles when illuminated under UV-A light. These results serve as a proof-of-principle (and starting point) to further explore a promising way to measure alkalinity addition to the ocean resulting from CEW deployments.