Atmospheric methane removal is a critical research area to develop, and establish if there exist safe, effective methods to accelerate the destruction of methane once in the atmosphere, especially from elevated natural system emissions from methane-emitting natural feedbacks and climate tipping points.

Iron Salt Aerosols (ISA) is one approach being studied to enhance atmospheric methane oxidation, in order to achieve atmospheric methane removal. ISA is potentially a solution that could scale at low cost, however scientific questions abound at this stage, and whether it’s safe, effective, and efficient has not yet been determined.

There is extremely limited research aimed at understanding and potentially enhancing various methane oxidation pathways today. One of the few currently active research tracks is to explore the chlorine pathway via iron salt aerosols (ISA) as a temporary enhancement to methane oxidation. Iron salt aerosols, which are currently introduced to the atmosphere naturally via mineral dust as well as anthropogenically via coal-burning and steel-manufacturing, are hypothesized to catalyze the formation of chlorine radicals in the atmosphere in specific conditions within the marine boundary layer. These radicals then oxidize methane into hydrochloric acid, water, and CO₂, reducing the warming impact of methane by ~75x compared to its oxidation product of CO₂.  Adding aerosolized iron salts in specific marine environments may be a path to temporarily and substantially increase the oxidative capacity of the chlorine pathway. 

The research on ISA is in the early stages, and it is still unclear whether it would have the desired effect, have no negative climate or health downsides, be technically feasible, be economically feasible, or be practically deployable. While ISA and other atmospheric methane oxidation approaches may prove to have the potential to mitigate significant methane, there are major uncertainties regarding how deploying iron aerosols at large scales could affect the global Earth system, local ecosystems, or human health, which must be studied. Some potential side effects include the generation of harmful aerosols and gases (e.g., chlorine), unintentional cloud formation, and algae bloom in environments where iron is a limiting nutrient. Chlorine, in particular, is a tightly-regulated ozone-depleting substance, and increased atmospheric concentrations could lead to reduced ozone in the stratosphere (bad), where the ozone layer absorbs ultraviolet radiation from the sun, or in the troposphere (good?), where ozone is a pollutant with deleterious effects on human health. Meanwhile, in the right conditions, ozone and water can form hydroxyl radicals, which can further oxidize methane.

It is possible that there are additional unforeseen side effects of temporarily increasing iron aerosols and chlorine concentrations in the marine boundary layer, and it is currently unknown whether these combined side effects would lead to net warming or net cooling, and how they may impact human and ecosystem health.

Any potential for a significant greenhouse gas sink like carbon dioxide removal or ISA must be carefully communicated and governed to avoid becoming an emission mitigation deterrent or deployed unsafely. Specifically, we don’t yet know if ISA will work, and we must avoid a tradeoff of mitigating methane, CO₂, and other GHG sources. 

The Path Forward

Increasing the oxidative power of the atmosphere through ISA appears to present a potential pathway to address rising natural methane emissions, and historical emissions, as well as the potential of massive unexpected methane releases, but many critical questions remain. Given these unknowns, Spark’s position is to fund carefully designed research tracks in such a way that we systematically address and characterize ISA’s largest uncertainties, while carefully communicating about its possible role in the climate solution ecosystem, and beginning to engage stakeholders around effective governance. 

Meanwhile, we are doing a landscape analysis to uncover other promising atmospheric and low-concentration point-source removal and oxidation pathways, and will be supporting researchers in those spaces and exploring mechanisms to draw additional researchers to them.  

Current open scientific questions include:

• What’s the real world catalytic efficiency in different conditions? 
• What are the atmospheric factors that could affect the rate and scale of this approach?
• What side-reactions could introduce deleterious health effects at scale, in what conditions? 
• What side-reactions could introduce direct or indirect climate impacts if deployed at scale? 
• At various scales, what are the bounds for cost per ton and why?
• What are the practical limits of modeling and measurability of the expected effects?

Making progress on these scientific questions at this stage can be achieved through lab-based work, computational modeling, and atmospheric measurement of existing natural phenomena.