[mlyle]

If you release the same amount of methane per year for a couple dozen years, the methane reaches a steady state where it has a constant value, but CO2 continues to rise.

This is because the methane is being converted to CO2 at a relatively quick clip.

The timescale for CO2 is larger-- centuries.

[hkt]

Methane - CH4 - degrades to carbon dioxide in the atmosphere. A quick clip, however, it is not: it can take 20 years, during which that single carbon atom yields 20x the global warming potential as part of a methane molecule than it does as part of a carbon dioxide molecule.

[tremon]

it can take 20 years, during which that single carbon atom yields 20x the global warming potential

Actually, it's more than that. The 20x global warming potential of CH4 is based on a 100-year average, which is the yardstick unit of measurement in the scientific literature. But as you say, methane degrades in 20 years so its average-20x contribution happens within those 20 years:

> The IPCC reports that the global warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe [..] and 105 times the effect when accounting for aerosol interactions

-- from https://en.wikipedia.org/wiki/Atmospheric_methane?useskin=ti...

[mlyle]

The point is, if emissions are steady state for a bit more than 20 years, the amount of CH4 remains constant. It will be removed from the atmosphere at the same rate it is added. It takes centuries or more for CO2 to reach equilibrium.

Indeed, this means most of the warming you get from emitting a molecule of CH4 is after it has decayed to the more stable CO2. Of course, sharp increases in methane are of concern for dynamic effects, from things like permafrost thaw.