Would methane levels moderate without methane removal technologies?

To some degree, they would, though not enough to avoid further contributing to global warming.  It’s a somewhat complex equation:  As clean energy replaces fossil fuels, methane emissions from the fossil fuel sector, as well as carbon dioxide and other GHG emissions, will fall somewhat.  At the same time, lowering fossil fuel emissions will also lower current rates of atmospheric methane oxidation, which will raise atmospheric methane concentrations.  Methane has a relatively short half-life in the atmosphere and theoretically it would fall to pre-industrial levels eventually if emissions suddenly ceased.  But as a practical matter, methane emissions won’t suddenly cease.  On the contrary, methane emissions from hard-to-mitigate sources, both anthropogenic and natural, are growing fast.  To reverse the rapid, continuing rise in atmospheric methane and to avoid significant methane-induced global warming will require active methane removal measures.

Fossil fuel emissions comprise about 20% of methane emissions, half of which is from coal mines.  The International Energy Agency report Net Zero by 2050 calls for an immediate end to oil and gas exploration.  It points out “early results from satellite data show leaky oil and gas industry infrastructure is responsible for far more of the methane in the atmosphere than previously thought.”

That’s not even counting methane emissions from the mouths of coal mines, which are significant.  Disused or abandoned coal mines and oil and gas wells continue releasing methane long after production stops.  Thankfully, that problem is on the US policy agenda, though it’s not a priority everywhere fossil fuels are extracted. 

As fossil fuel production and consumption fall and get replaced with clean energy, methane emissions may decrease by 10%.  Nitrogen oxide and sulfur dioxide pollution will also fall as a result, which will be a welcome improvement for air quality and public health. 

But at the same time, cutting nitrogen oxides and sulfur oxides pollution will also have a countervailing side-effect of reducing atmospheric methane oxidation.  That’s because nitrogen oxides happen to stimulate production of hydroxyl (°OH)  radicals in the atmosphere, which drive 90% of natural methane oxidation. To some degree, sulfates produced by SOx also artificially suppress methane emissions. So decreasing NOx and SOx pollution, while welcome, will tend to increase atmospheric methane. Since sulfate aerosols have a cooling effect, their reduction will have a warming effect.

Methane emissions have a relatively short lifetime in the atmosphere of about 10-12 years and largely disappear within 30 years. Carbon dioxide can persist in the atmosphere for hundreds of years. But since methane is a far more potent greenhouse gas than CO2, with 84 times the global warming potential over 20 years, a ton of methane still traps significantly more heat than a ton of CO2. In other words, methane packs a much bigger warming punch despite its relatively short lifetime.

That’s not to say we should prioritize cutting methane emissions over CO2.  We need to cut both; in fact, the two are linked.  As CO2 emissions rise, so do methane emissions; and reciprocally, as methane emissions rise, more CO2 is released.  So there’s a twin imperative to cut methane and CO2 emissions alike as deeply and rapidly as we can, while also investing in greenhouse gas removal technologies, including methane removal technologies.

Stopping new oil and gas exploration, replacing fossil fuels with clean energy, and capping disused oil and gas wells are all critical for bringing methane levels as well as CO2 levels down.  But they will not be sufficient by themselves. To avoid methane-induced warming and prevent high methane concentrations from triggering feedback loops requires a both-and approach:  both reducing methane emissions from all sources, and directly removing methane from the atmosphere.

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