There are fundamental research areas that would significantly increase the precision of our scientific understanding around the effectiveness and attractiveness of any approach to addressing atmospheric methane. These areas need significant additional funding and focus, and if invested in would benefit the broader space of advancing climate science, and better understanding climate response solutions due to the interconnectedness of earth systems.
Specific to surface-based methane removal methods:
The driver of methane’s short lifetime is a set of methane sinks. The vast majority of these sinks by volume is the oxidative capacity of the atmosphere, driven by hydroxyl (~90%) and chlorine (1-5%) radicals. Atmospheric oxidation is a complex set of atmospheric chemistry processes that drive the atmospheric “cleanup” of multiple gasses that have health and climate impacts, including methane, carbon monoxide, non-methane volatile organic compounds (NMVOCs), and ozone. Because oxidation capacity is relatively fixed, the volume of emissions of any of these gasses can impact the atmospheric lifetime of other reactive gasses.
Better understanding of the atmospheric oxidative capacity would have two major benefits to advance methane removal technologies: 1) better estimates of changing sources or sinks give confidence about whether there are rising methane emissions from natural sources, a primary motivator for atmospheric methane removal, and 2) being able to verify the impacts of interventions gives confidence in their effectiveness. The oxidative capacity can be better understood through improved field observations, laboratory experiments, and expanded capabilities of earth system models.
There are many open scientific questions around the atmosphere’s oxidative capacity, and advancing the science in the field will improve our climate modeling and projection ability. The estimated contributions of hydroxyl and chlorine need refinement. Mineral dusts, air pollution, cloud cover, and other atmospheric conditions may affect the relative contributions; these effects must be better characterized. As emissions of reactive gasses (including carbon monoxide, hydrogen, ozone, and NMVOCs) consumed by either hydroxyl or chlorine change, the available capacity for oxidizing methane will be affected. Climatic changes or deliberate intervention through solar radiation management may alter the flux of UV light through the atmosphere in unknown ways, further impacting the oxidative capacity of the atmosphere, as oxidation reactions in this context are powered by UV light. There are also significant opportunities to improve our understanding of the soil, aquatic, and forest-based methane sinks, as these are only lightly modeled and monitored, and also directly impact the two major benefits noted above.
When considering action on atmospheric methane, it is extremely valuable to better constrain sinks, and improve predictive capabilities around the evolving risks of natural system feedbacks and tipping elements, as they will be important inputs into considerations around future potential deployment of any methane removal approach that proves potentially feasible.
Both natural and anthropogenic methane sources are fundamentally more challenging to model and track than the equivalents for carbon dioxide. Methane sources are a variety of biological processes, both natural and anthropogenic, and leakage from fossil fuel operations. An estimated ~40% of overall methane emissions currently come from natural sources such as wetlands, termites, and other natural biological processes, though there’s increasing evidence and risk of these sources increasing emissions as a result of climate-change-induced feedbacks. A further ~30% come from diffuse anthropogenic sources (<200ppm) including rice paddies, and animal agriculture. Widely distributed sources like these are even harder to monitor with existing techniques than high concentration methane emissions from point sources.
There are currently broad modeled ranges related to methane emissions sources, and many gaps in measurement and monitoring, leading to considerable lack of clarity as to the level and trajectory of emissions from tropical wetlands, arctic regions, freshwater, coal beds, oil & gas operations, agriculture, wastewater, landfills and more. While there are several satellite and aircraft-based measurement and monitoring systems coming online in the next few years (e.g., MethaneSAT, ClimateTRACE, CarbonMapper, GHGSAT) for concentrated sources, there are significant opportunities to improve the measurement of highly diffuse and intermittent methane sources such as wetlands, freshwater ecosystems, distributed agriculture and arctic hotspots.
In order to better measure and monitor natural analogues or small-scale pilots of any approach, it is critical to expand observation networks to support global, frequent, long-term monitoring from space, air, and ground of methane concentration and fluxes, 13C and 2H isotopic ratios, OH, Cl, N2O, NOx, and other precursors/products of methane reactions. Additionally this area would benefit from low-cost, lightweight, ~10 ppb sensitive, in situ and remote methane sensors for atmospheric, aquatic and terrestrial monitoring.
All surface-based catalyst methods (e.g., Atmospheric Methane Filters, Catalytically Active Surfaces) must contend with the challenge of getting enough volume of air into contact with the active catalyst in the right conditions and for enough time to breakdown a significant portion of the extremely dilute (2ppm) methane that volume contains, while keeping energy and resource use low enough to remain climate-beneficial and cost-plausible. This constitutes a significant mass transfer problem that will need to be addressed in order to render surface-based catalysts feasible - both in terms of passively or extremely energetically favorably moving large volumes of air, and in terms of generating sufficient residence time with extremely low pressure drop across the active catalyst.