What is our assessment?

Based on the potential for harmful impacts, the risks of using AOE to destroy atmospheric methane outweigh its uncertain benefits. Because of this, as well as the fact that there are other effective solutions to reduce methane emissions already available, this climate solution is Not Recommended.

What is it?

AOE of methane is a technology designed to accelerate the natural decay of methane to CO₂  by increasing the concentration of hydroxyl and chlorine radicals in the air. In the presence of sunlight and oxygen, these molecules convert methane to CO₂. This solution aims to increase the atmospheric concentration of these molecules by injecting precursors, such as iron salts and hydrogen peroxide, into the air.

Does it work?

Methane is a potent greenhouse gas, more than 80 times stronger than CO₂ at trapping heat on a 20-year basis. Under natural conditions, it persists in the atmosphere for about 10 years before it converts into CO₂. Therefore, artificially accelerating methane conversion reduces its disproportionate warming impact. 

There is evidence that the concentration of hydroxyl radicals affects the rate at which methane is converted into CO₂ in the atmosphere. However, research into AOE for methane removal is still in its early stages and limited to a few modeling and laboratory studies. There are currently no real-world examples of atmospheric methane removal. The effectiveness of the solution is unknown and uncertain. Based on current research, no one knows if it is possible to remove atmospheric methane at a meaningful scale in a safe and cost-effective manner. 

Why are we excited?

Because methane is such a potent greenhouse gas, any actions to reduce its concentration in the atmosphere would be emergency brake solutions with immediate and disproportionate climate benefits. In addition, unlike direct air capture or carbon capture and storage, there is no need to capture or store the gas that the process produces. 

Why are we concerned?

AOE for methane removal is an untested technology designed to alter atmospheric chemistry that presents novel and potentially uncontrollable risks to Earth systems and ecosystem processes. Hydroxyl and chlorine radicals are highly reactive molecules, and they do not react solely with methane. When they react with other atmospheric constituents they can generate other, even stronger, climate pollutants such as nitrous oxide as well as other air pollutants such as PM2.5, carbon monoxide, nitrogen dioxide, and ground-level ozone, and they could deplete stratospheric ozone. Some proposed AOE methods, such as atmospheric injection of iron salt aerosols, create chlorine radicals. The chlorine- and iron-containing byproducts of these aerosols could adversely affect ocean chemistry and food webs when they are deposited on the ocean surface (see Deploy Ocean Fertilization). 

Other serious concerns include technical feasibility, scalability, cost, monitoring, reporting and verification, and governance. For example, in order for methane in the atmosphere to be reduced at climate-relevant scales, the production of chlorine or hydroxyl radicals would need to be magnitudes greater than the current global production. Costs have not been estimated, but they would likely be high. New tools for monitoring atmospheric methane would need to be developed to quantify the amounts of methane removed for accurate accounting and verification. Similar to stratospheric aerosol injection, deployment of atmospheric methane removal could pose geopolitical, legal, and ethical challenges. In addition, it could distract from or delay action on other methane reduction approaches, such as managing oil and gas methane, managing coal mine methane, improving landfill management, and increasing centralized composting.