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Power stations, ships and air traffic are among the most potent greenhouse gas emitters and are primarily responsible for global warming. Iron salt aerosols (ISAs), composed partly of iron and chloride, exert a cooling effect on climate in several ways. This article aims firstly to examine all direct and indirect natural climate cooling mechanisms...
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... Detailed modeling of MR processes is beyond the scope of our study. We refer the interested reader to the existing literature (Ming et al 2022a, Abernethy et al 2023, Ming et al 2022b, Wang et al 2022, Oeste et al 2017, Xiong et al 2023, Li et al 2023. Due to the lack of data on MR costs and energy requirements, we consider a generic MR characterized by the following two factors: i) a cost per ton of removed methane c [$/tCH4] and ii) a maximum annual removal potential p [tCH4/year]. ...
... For instance, it was shown that existing active methane oxidation technologies such as thermo-catalysts, photo-catalysts, electro-catalysts and biofilters are too energy-intensive to oxidize methane cost-effectively at atmospheric concentrations (Abernethy et al 2023) but that they could be deployed over methane point sources, where the concentration is very high (Nisbet-Jones et al 2021). Non-linear costs are also to be expected for methods that enhance atmospheric methane oxidation by increasing OH (Wang et al 2022) or Cl sinks (Oeste et al 2017, Li et al 2023. For instance, recent research (Li et al 2023) indicates that chlorine emissions must exceed a threshold of 90 Tg/year in order to reduce atmospheric methane concentration. ...
Methane is the second most important anthropogenic greenhouse gas causing warming after carbon dioxide, and the emission reductions potentials are known to be limited due to the difficulty of abating agricultural methane. We explore in this study the emerging option of atmospheric methane removal (MR) that could complement carbon dioxide removal (CDR) in mitigation pathways. MR is technologically very challenging and potentially very expensive, so the main question is at which cost per ton of methane removed is MR more cost effective than CDR. To address this question, we use an intertemporal optimization climate-GHG-energy model to evaluate the MR cost and removal potential thresholds that would allow us to meet a given climate target with the same or a lower abatement cost and allowing for equal or higher gross CO2 emissions than if CDR through BECCS were an option. We also compare the effects of MR and CDR on the cost-effective mitigation pathways achieving four different climate targets. Using the ACC2-GET integrated carbon cycle, atmospheric chemistry, climate and energy system model, we consider a generic MR technology characterized by a given unit cost and a maximal removal potential. We show that to totally replace bioenergy based CDR with MR, the MR potential should reach at least 180 to 320 MtCH4 per year, i.e., between 50% and 90% of current anthropogenic methane emissions, with maximum unit cost between 10,000 and 34,000 $/tCH4, depending on the climate target. Finally, we found that replacing CDR by MR reshapes the intergenerational distribution of climate mitigation efforts by delaying further the mitigation burden.
... One untested proposal involves iron salt aerosol (ISA). This potential approach involves lofting iron-based particles into the troposphere (e.g., from ships or towers) to catalytically produce chlorine radicals (Oeste, 2009;Oeste et al., 2017), mimicking a natural phenomenon proposed to occur when mineral dust combines with sea spray aerosols . Discussing natural analogues of this process and the current state of research, this paper presents a roadmap for research and development that is needed to understand whether ISA enhancement of the chlorine radical sink may be a feasible, scalable, and safe approach for atmospheric methane removal. ...
... For example, the mixing of the iron and sea salt within the aerosol is modeled to occur instantaneously (Meidan et al., 2024;; however, in reality it would likely take hours to days, leading the global model to overestimate the rate of chlorine radical production. Furthermore, the ISA mechanism is likely to occur faster in high-NO x environments (Oeste et al., 2017) but could be less efficient in high-sulfate environments (Bondy et al., 2017;Chen et al., 2020;Legrand et al., 2017;Pio and Lopes, 1998), and both NO x and sulfate may be co-emitted with iron (e.g., from a ship plume). Thus, models that instantaneously dilute emissions across the grid dimensions may misrepresent the ISA mechanism. ...
... Current studies assume that the chlorine radicals released from the photochemical reaction with iron will react (e.g., with methane) to form hydrochloric acid, which will then be reabsorbed back into the aerosol and thus recycled Oeste et al., 2017). It is unclear under which atmospheric conditions this cycle occurs, but if some chlorine radicals are lost then the cycling would be less efficient. ...
The escalating climate crisis requires rapid action to reduce the concentrations of atmospheric greenhouse gases and lower global surface temperatures. Methane will play a critical role in near-term warming due to its high radiative forcing and short atmospheric lifetime. Methane emissions have accelerated in recent years, and there is significant risk and uncertainty associated with the future growth in natural emissions. The largest natural sink of methane occurs through oxidation reactions with atmospheric hydroxyl and chlorine radicals. Enhanced atmospheric oxidation could be a potential approach to remove atmospheric methane. One method proposes the addition of iron salt aerosol (ISA) to the atmosphere, mimicking a natural process proposed to occur when mineral dust mixes with chloride from sea spray to form iron chlorides, which are photolyzed by sunlight to produce chlorine radicals. Under the right conditions, lofting ISA into the atmosphere could potentially reduce atmospheric methane concentrations and lower global surface temperatures. Recognizing that potential atmospheric methane removal must only be considered an additive measure – in addition to, not replacing, crucial anthropogenic greenhouse gas emission reductions and carbon dioxide removal – roadmaps can be a valuable tool to organize and streamline interdisciplinary and multifaceted research to efficiently move towards understanding whether an approach may be viable and socially acceptable or if it is nonviable and further research should be deprioritized. Here we present a 5-year research roadmap to explore whether ISA enhancement of the chlorine radical sink could be a viable and socially acceptable atmospheric methane removal approach.
... A variety of other dispersion methods might be possible with modest engineering improvements, including guyed masts (up to ∼500 m), tethered balloons, and kites like those used for airborne wind power generation [55][56][57][58]. Aerosols could also be lofted in the heated gases from existing smokestacks [59]. We model airplanes because they are a proven technology. ...
The photocatalytic decomposition of atmospheric methane (CH4) and nitrous oxide (N2O) could be valuable tools for mitigating climate change; however, to date, few photocatalyst deployment strategies have had their costs modeled. Here, we construct basic cost models of three photocatalytic CH4 and N2O decomposition systems: (1) a ground-based solar system with natural airflow over photocatalyst-painted rooftops, (2) a ground-based LED-lit system with fan-driven airflow, and (3) an aerosol-based solar system on solid particles dispersed in the atmosphere. Each model takes as inputs the photocatalyst’s apparent quantum yield (AQY; a measure of how efficiently photons drive a desired chemical reaction) and the local CH4 or N2O concentration. Each model calculates an overall rate of greenhouse gas (GHG) drawdown and returns a levelized cost of GHG removal per equivalent ton of carbon dioxide (tCO2e). Based on prior studies of atmospheric carbon dioxide removal, we adopt 100/tCO2e only for AQYs of >10% for CH4 and >1% for N2O. Dispersing photocatalytic aerosols in the troposphere could be cost-effective with AQYs of >0.4% for ambient CH4 or >0.04% for ambient N2O. However, the mass of aerosols required is large and their side effects and social acceptability are uncertain. We note that, for any system, AQYs on the order of 1% will likely be extremely challenging to achieve with such dilute reagents.
... More specifically, iron aerosols in the atmosphere catalyze the production of chlorine gas from sea salt aerosols through a complex photochemical process involving the release of Fe (III) chlorides, sunlight activation, and subsequent reoxidation, ultimately contributing to atmospheric halogen release (Wittmer et al 2015, Wittmer andZetzsch 2017). This opens up the possibility of decreasing atmospheric methane by increasing the atmospheric production of chlorine (Cl) by adding iron aerosols (Dietrich Oeste et al 2017). Laboratory studies show that when sea salt and iron aerosols interact, iron chloride species are formed and can generate chlorine molecules (Cl 2 ) through photochemical reactions (Wittmer et al 2015, Wittmer and Zetzsch 2017. ...
Keeping global surface temperatures below international climate targets will require substantial measures to control atmospheric CO2 and CH4 concentrations. Recent studies have focused on interventions to decrease CH4 through enhanced atmospheric oxidation. Here for the first time using a set of models, we evaluate the effect of adding iron aerosols to the atmosphere to enhance molecular chlorine production, and thus enhance the atmospheric oxidation of methane and reduce its concentration. Using different iron emission sensitivity scenarios, we examine the potential role and impact of enhanced iron emissions on direct interactions with solar radiation, and on the chemical and radiative response of methane. Our results show that the impact of iron emissions on CH4 depends sensitively on the location of the iron emissions. In all emission regions there is a threshold in the amount of iron that must be added to remove methane. Below this threshold CH4 increases. Even once that threshold is reached, the iron-aerosol driven chlorine-enhanced impacts on climate are complex. The radiative forcing of both methane and ozone are decreased in the most efficient regions but the direct effect due to the addition of absorbing iron aerosols tends to warm the planet. Adding any anthropogenic aerosol may also cool the planet due to aerosol cloud interactions, although these are very uncertain, and here we focus on the unique properties of adding iron aerosols. If the added emissions have a similar distribution as current shipping emissions, our study shows that the amount of iron aerosols that must be added before methane decreases is 2.5 times the current shipping emissions of iron aerosols, or 6 Tg Fe yr⁻¹ in the most ideal case examined here. Our study suggests that the photoactive fraction of iron aerosols is a key variable controlling the impact of iron additions and poorly understood. More studies of the sensitivity of when, where and how iron aerosols are added should be conducted. Before seriously considering this method, additional impacts on the atmospheric chemistry, climate, environmental impacts and air pollution should be carefully assessed in future studies since they are likely to be important.
... Reducing methane emissions and extracting methane from the atmosphere may both contribute to lowering the concentration of atmospheric methane. One methane abatement method that has been proposed, based on natural analogues [4], is the use of Cl radicals in air. According to models, such an approach would effectively remove methane and may also reduce harmful tropospheric ozone air pollution [5]. ...
Methane, a potent greenhouse gas, is a significant contributor to global warming, with future increases in its abundance potentially leading to an increase of more than 1∘C by 2050 beyond other greenhouse gases if left unaddressed. To remain within the crucial target of limiting global warming to 1.5 ∘C, it is imperative to evaluate the potential of methane removal techniques. This study presents a scoping analysis of different catalytic technologies (thermal, photochemical and electrochemical) and materials to evaluate potential limitations and energy requirements. An analysis of mass transport and reaction rates is conducted for atmospheric methane conversion system configurations. For the vast majority of catalytic technologies, the reaction rates limit the conversion which motivates future efforts for catalyst development. An analysis of energy requirements for atmospheric methane conversion shows minimum energy configurations for various catalytic technologies within classic tube or parallel plate architectures that have analogs to ventilation and industrial fins. Methane concentrations ranging from 2 ppm (ambient) to 1000 ppm (sources, such as wetlands, fossil-fuel extraction sites, landfills etc) are examined. The study finds that electrocatalysis offers the most energy efficient approach (∼0.2 GJ tonne⁻¹ CO2e) for new installations in turbulent ducts, with a total energy intensity < 1 GJ tonne⁻¹ CO2e. Photocatalytic methane removal catalysts are moderately more energy intensive (∼2 GJ tonne⁻¹ CO2e), but could derive much of their energy input from ‘free’ solar energy sources. Thermal systems are shown to be excessively energy intensive ( > 100 GJ tonne⁻¹), while combining photovoltaics with electrochemical catalysts (∼1 GJ tonne⁻¹ CO2e) have comparable energy intensity to photocatalytic methane removal catalysts.
... It has been suggested that atmospheric methane might be removed by injecting iron salts, which, according to its advocates, would come with the added benefit of producing reflective clouds and a capacity to increase marine bio-productivity (Oeste et al. 2017). This idea is currently being studied but has received minimal coverage from sources other than those exploring it, and their claims are, therefore, hard to verify. ...
The frozen elements of the high North are thawing as the region warms much faster than the global mean. The dangers of sea level rise due to melting glacier ice, increased concentrations of greenhouse gases from thawing permafrost, and alterations in the key high latitude physical systems spurred many authors, and more recently international agencies and supra-state actors, to investigate “emergency measures” that might help conserve the frozen North. However, the efficacy and feasibility of many of these ideas remains highly uncertain, and some might come with significant risks, or could be even outright dangerous to the ecosystems and people of the North. To date, no review has evaluated all suggested schemes. The objectives of this first phase literature survey (which can be found in a separate compendium (https://doi.org/10.5281/zenodo.10602506), are to consider all proposed interventions in a common evaluation space, and identify knowledge gaps in active conservation proposals. We found 61 interventions with a high latitude focus, across atmosphere, land, oceans, ice and industry domains. We grade them on a simple three-point evaluation system across 12 different categories. From this initial review we can identify which ideas scored low marks on most categories and are therefore likely not worthwhile pursuing; some groups of interventions, like traditional land-based mitigation efforts, score relatively highly while ocean-based and sea ice measures, score lower and have higher uncertainties overall. This review will provide the basis for a further in-depth expert assessment that will form phase two of the project over the next few years sponsored by University of the Arctic.
... and there is renewed interest in large-scale scientific assessment of this CDR method (Buesseler et al., 2023;Emerson, 2019;NASEM, 2021;Oeste et al., 2017;Yoon et al., 2018). Simulations with biogeochemical model project that continuous basin-scale or globally-applied OIF could sequester around 2-4 Gt CO 2 year −1 (Aumont & Bopp, 2006;Fu & Wang, 2022;Oschlies et al., 2010;Tagliabue et al., 2023;Zahariev et al., 2008). ...
... A recent review by the National Academy of Sciences Engineering and Medicine (NASEM) concluded that substantially more research is needed to fully assess OIF and called for 290 million US$ within 10 years (NASEM, 2021). Indeed, there are already emerging efforts to explore new ideas for OIF implementation and establish OIF field research (Buesseler et al., 2023;Emerson, 2019;Oeste et al., 2017;Yoon et al., 2018). Although our informed back-of-the-envelope approach is less internally consistent than biogeochemical modeling, it enabled critically important guidance for these emerging efforts by mapping (cost-)efficiency for OIF south of 60°S. ...
... We found relatively pronounced gradients in OIF (cost-)efficiency, suggesting that any iron fertilizer would require precise injection to maximize OIF efficiency. This finding argues against recent suggestions to distribute iron through atmospheric transport (Emerson, 2019;Oeste et al., 2017) since it seems unlikely that high precision would be achievable by such means. Buesseler et al. (2023) and Yoon et al. (2018) have proposed potential OIF locations in the Southern Ocean based on nutrient conditions and considered much of the Southern Ocean area for future OIF research, including the open Southern Ocean. ...
Ocean iron fertilization (OIF) aims to remove carbon dioxide (CO2) from the atmosphere by stimulating phytoplankton carbon‐fixation and subsequent deep ocean carbon sequestration in iron‐limited oceanic regions. Transdisciplinary assessments of OIF have revealed overwhelming challenges around the detection and verification of carbon sequestration and wide‐ranging environmental side‐effects, thereby dampening enthusiasm for OIF. Here, we utilize five requirements that strongly influence whether OIF can lead to atmospheric CO2 removal (CDR): The requirement (a) to use preformed nutrients from the lower overturning circulation cell; (b) for prevailing iron‐limitation; (c) for sufficient underwater light for photosynthesis; (d) for efficient carbon sequestration; (e) for sufficient air‐sea CO2 transfer. We systematically evaluate these requirements using observational, experimental, and numerical data in an “informed back‐of‐the‐envelope approach” to generate circumpolar maps of OIF (cost‐)efficiency south of 60°S. Results suggest that (cost‐)efficient CDR is restricted to locations on the Antarctic Shelf. Here, CDR costs can be <100 US/tonne CO2 in offshore regions of the Southern Ocean, where mesoscale OIF experiments have previously been conducted. However, sensitivity analyses underscore that (cost‐)efficiency is in all cases associated with large variability and are thus difficult to predict, which reflects our insufficient understanding of the relevant biogeochemical and physical processes. While OIF implementation on Antarctic shelves appears most (cost‐)efficient, it raises legal questions because regions close to Antarctica fall under three overlapping layers of international law. Furthermore, the constraints set by (cost‐)efficiency reduce the area suitable for OIF, thereby likely reducing its maximum CDR potential.
... ISA's methane depletion activity was observed and measured in atmospheric chemistry research laboratories around 10 years ago. It was described in several papers (Wittmer et al., 2015;Wittmer & Zetzsch, 2017;Oeste et al., 2017). In 2023 the same chemical mechanism in mineral dust was measured in field trials in the Caribbean and Cape Verde islands marine boundary layer (van Herpen, 2023). ...
The intensifying impacts of climate change are exceeding projections and amplifying the risk of catastrophic harm to the environment and society throughout the 21st century. Planned and proposed rates of emissions reduction and removal are not proceeding at a pace or magnitude to meet either the 1.5°C or 2.0°C targets of the Paris Agreement. Moreover, the impacts, damage and loss occurring at today’s 1.2°C of global warming are already significantly disrupting the environment and society. Relying exclusively on greenhouse gas (GHG) emissions reduction and removal without including climate cooling options is thus proving incompatible with responsible planetary stewardship. Multiple approaches to exerting a cooling influence have the potential to contribute to offset at least some of the projected climate disruption if deployed in the near term. Employed thoughtfully, such approaches could be used to limit global warming to well below 1° C, a level that has led to large reductions in sea ice, destabilization of ice sheets, loss of biodiversity, and transformation of ecosystems. An effective plan for avoiding “dangerous anthropogenic interference with the climate system,” would include: a) early deployment of one or more direct cooling influence(s), initially focused on offsetting amplified polar warming; b) accelerated reductions in emissions of CO2, methane and other short-lived warming agents; and c) building capacity to remove legacy GHG loadings from the atmosphere. Only the application of emergency cooling “tourniquets,” researched and applied reasonably soon to a “bleeding” Earth, have the potential to slow or reverse ongoing and increasingly severe climate disruption.
... One untested proposal involves iron salt aerosols (ISA). This potential approach involves lofting iron-based particles into the troposphere (e.g., from ships or towers) to catalytically produce chlorine radicals (Oeste, 2009;Oeste et al., 2017), mimicking a natural phenomenon proposed to occur when mineral dust combines with sea spray aerosols . Discussing natural analogues of this process and the current state of research, this paper presents a roadmap for research and development that is needed to understand whether ISA enhancement of the chlorine radical sink may be a feasible, scalable, and safe approach for atmospheric methane removal. ...
... For example, the mixing of the iron and sea salt within the aerosol is modeled to occur instantaneously (Meidan et al., submitted;van Herpen et al., 2023); however, in reality it would likely take hours to days, leading the global model to overestimate the rate of chlorine radical production. Furthermore, the ISA mechanism is likely to occur faster in high NO x environments (Oeste et al., 2017) but could be less efficient in high sulfate environments (Bondy et al., 2017;Chen et al., 2020;Legrand et al., 2017;Pio et al., 1998), and both NO x and sulfate may be co-emitted with iron (e.g. from a ship plume). Thus, models that instantaneously dilute emissions across the grid dimensions may misrepresent the ISA mechanism. ...
... Current studies assume that the chlorine radicals released from the photochemical reaction with iron will react (e.g., with methane) to form hydrochloric acid, which will then be reabsorbed back into the aerosol and thus recycled Oeste et al., 2017). It is unclear under which atmospheric conditions this cycle occurs, but if some chlorine radicals are lost (e.g. to reactions with ozone instead), then the cycling would be less efficient. ...
... One method being explored is to release iron-fortied sea water aerosols into the atmosphere, resulting in CH 4 oxidation with hydroxyl radicals. 116 The wider impacts (e.g. on animal health) of such an approach need to be assessed. ...
Chemistry needs to play a central role in achieving ‘net zero’ emissions of greenhouse gases (GHGs) into the atmosphere to prevent changes to the climate that will have catastrophic impacts for humanity and for many ecosystems on the planet. International action to limit global warming to 1.5 °C has framed as a key goal the reduction of global emissions to as close to zero as possible by 2050, with any remaining emissions re-absorbed from the atmosphere. Chemistry underpins innovative approaches to reducing emission of the key GHGs, comprising CO2, CH4, N2O and fluorinated gases, and to the recapture of gases already in the atmosphere. Rapid progress is needed in the application of green and sustainable chemistry and material circularity principles in developing these approaches worldwide. Of critical importance will be the incorporation of systems thinking, recognition of planetary boundaries that define safe operating spaces for Earth systems, and an overall reorientation of chemistry towards its roles in stewardship of the Earth's material resources and in sustainability for people and the planet.
