How does methane removal work, and how is it different from carbon dioxide removal?

The term “methane removal” is shorthand for “accelerating the natural transformation — or oxidation– of methane that has been emitted into the atmosphere into carbon dioxide and water.” Methane Action is helping to foster the development of an evidence basis for methane removal and ensuring that it is advanced in scientifically and environmentally sound, safe, and sustainable ways.  

There is a growing body of research and technological development around carbon dioxide (CO2) removal  or CDR. As the Intergovernmental Panel on Climate Change explains in their Sixth Assessment Report, “Carbon dioxide removal (CDR) refers to anthropogenic activities that deliberately remove CO2 from the atmosphere and durably store it in geological, terrestrial or ocean reservoirs, or in products. Carbon dioxide is removed from the atmosphere by enhancing biological or geochemical carbon sinks or by direct capture of CO2 from air. Emission pathways that limit global warming to 1.5°C or 2°C typically assume the use of CDR approaches in combination with greenhouse gas (GHG) emissions reductions. CDR approaches could be used to compensate for residual emissions from sectors that are difficult or costly to decarbonize. CDR could also be implemented at a large scale to generate global net negative CO2 emissions (i.e., anthropogenic CO2 removals exceeding anthropogenic emissions), which could compensate for earlier emissions as a way to meet long-term climate stabilization goals after a temperature overshoot.”

Practices or technologies that remove CO2 are often referred to as “negative emissions technologies.” Alternatively, these practices are sometimes referred to more broadly as “greenhouse gas removal” if they involve removing gases other than CO2. Methane removal is therefore a type of “greenhouse gas removal” or “negative emission” technology.

Methane removal both resembles and differs from CDR in several important ways. Methane removal resembles CDR in that, like CDR, it can be achieved by enhancing existing natural processes that remove the greenhouse gas from the atmosphere. But, once oxidized, methane doesn’t need to be sequestered or stored to keep it from being reemitted because it is then transformed into water vapor and CO2.  This transformation is a big net gain for the climate, since methane more than 80 times more potent than CO2 over 20 years. Since methane is so much more powerful than CO2 as a climate forcer, it takes much less methane removal to have the same impact on the climate as CDR.

Unlike CDR, some types of methane removal don’t require capturing methane by filtering large volumes of air with air moving devices — which is a good thing because the concentration of methane in the atmosphere is about 200 times lower than carbon dioxide. Capturing methane at that level of concentration would be very energy-intensive. Nor is it necessary to do so, because methane can be oxidized freely by enhancing natural sinks or by the use of catalysts that oxidize methane.  

Methane removal technologies may be applied in closed systems such as direct air capture machines or photocatalytic reactors. They can also be applied in the open, at or near large methane emissions sources like coal mines or rice paddies, or in the ambient air.  Delivery mechanisms may be active or passive, ranging from catalytic coatings on vehicles and buildings, to solar updraft chimneys that move large volumes of air through passive convection, to active air moving devices and diffusion of catalysts. Either way, energy requirements for methane removal are much smaller than for CDR.

All methane in the atmosphere eventually breaks down to water vapor and CO2 through natural oxidation. But since current methane emissions are so high, methane is getting added to the atmosphere much faster than it can be oxidized naturally. In fact, methane concentrations are now more than 3 times higher than at any time in the last 800,000 years, and have become a major factor in climate change. But methane removal technologies could speed up natural processes sufficiently to counteract high emissions levels and lower methane concentrations. Coupling effective methane removal technologies with aggressive cuts in methane emissions could enable us to restore atmospheric methane to preindustrial levels rapidly.  

Developing methane removal capabilities would also equip us to respond to possible feedback loops or methane “bursts” from natural sources, such as thawing permafrost or warming wetlands. Those feedback loops are potentially very large sources of biogenic methane emissions that we wouldn’t be able stop once they started.  It’s unknown whether or when such large releases of methane would occur, though some scientists believe they might happen soon, and may even be underway already. If or when they do occur,  methane removal technologies could enable us to mitigate them, breaking the emissions down before they further accelerated global warming.

There are a broad range of methane removal technologies under development.  They include photocatalysts, metal catalysts associated with zeolites and porous polymer networks, biological methane removal such as biotrickling filters and soil management approaches, and iron salt aerosol methods or catalysts enhancing natural methane sinks which accelerate atmospheric oxidation of methane.   

These technologies are in various stages of development. Each needs further research on cost, technological efficiency, scaling and energy requirements, co-benefits and any potentially negative impacts. Each requires appropriate governance, safety assessment, environmental impact assessment and research and development to advance toward testing and deployment.

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