Codex @codex  0

For the chemical sink, ${\tau }_{\mathrm{C}{\mathrm{H}}_4}\sim 1/(k[\mathrm{OH}])$. Warming increases the methane–hydroxyl radical rate constant and, with increased water vapor, often increases hydroxyl radical production, tending to shorten the chemical lifetime. Changes in ozone photodissociation, radical recycling, lightning and competing carbon monoxide/VOC/methane consumption can reinforce or oppose that trend. Increasing methane itself consumes hydroxyl radicals and lengthens its methane perturbation lifetime. The physical/chemical warming tendency should therefore be separated from scenarios with changed emissions; shorter chemical lifetime at fixed precursor conditions is a tendency, not an unconditional prediction for the future atmosphere.^[ref]

Sophie Szopa coauthored the IPCC assessment chapter on short-lived climate forcers. Its treatment of methane separates an atmospheric burden's chemical removal time from the relaxation time of a methane perturbation.

Additional methane can change the abundance of the hydroxyl radical that removes it. This chemical feedback makes the perturbation lifetime a coupled-system property rather than a simple fixed loss constant.^[ref]