> I finally have some confidence that I understand how methane lifetimes work, and that for my purposes it can be summarized as:
Methane emissions decay gradually, with an average lifetime of about 12 years (“perturbation lifetime”, which is what matters for climate purposes).
This will increase by roughly 35% if methane concentrations double, or decrease roughly 25% if concentrations return to pre-industrial levels.
I was a bit confused by this sentence (which is a direct excerpt from the piece). Reading the whole article, the `this` refers to the methane lifetime. Oddly (to me), the lifetime methane in the atmosphere increases with the amount of methane in the atmosphere. That's what the author has been working to understand. Very interesting!
The opposite is true, that the higher the methane concentration, the lower the rate of effect would be to degrade all atmospheric methane. The rates of methane accumulation and its degradation are inversely proportional beyond the limit of the atmosphere to degrade it.
The reality is more complicated because the cut off is very blurry - absorbsion does still increase as methane increases, just not fast enough to keep up, and it falls further behind the more methane we put out.
And it seems like there are methods for capturing methane: https://news.mit.edu/2022/dirt-cheap-solution-common-clay-ma...
That half life depends on concentration is not surprising to me; ethanol's half life in the blood also depends on it's concentration and the reason is rather straight forward: the liver has limited amounts of enzymes needed to process booze.
What is surprising to me, though, is that there is a mechanism that has such a massive effect at the extremely low concentrations of methane that are present in the atmosphere. Sure, OH is rare, but I'd guess is generated in large amounts in the upper atmosphere (UV + H2O -> OH- + O+ + momentum to keep them away from each other).
Does anyone here have any hard math on this?
H2O + γ -> OH• + H•
Rather than:
H2O + γ -> OH- + H+
Oh, and whichever way, H2O splits into OH and H, not OH and O, but i assume that was a typo!
https://en.wikipedia.org/wiki/Hydroxyl_radical “Notation: The unpaired electron of the hydroxyl radical is officially represented by a middle dot, •, beside the O.”
Note that article uses a superscripted dot <sup>•</sup>OH, and the dot is usually prefixed in the article (presumably so as to put the dot next to the O that doesn’t have a full shell?).
However the article sometimes suffixes the dot, to put it beside the R organic radical. Weird.