It's important to say explicitly what "not forever" is here. Half life of methane in the atmosphere is 9 years[0]. Then it decomposes into CO₂ and water; the latter is suspected to be a potent greenhouse gas at that altitude[1].

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[0] - https://en.wikipedia.org/wiki/Atmospheric_methane#cite_ref-C...

[1] - https://en.wikipedia.org/wiki/Atmospheric_methane#Atmospheri...