As long as the methane stays in the atmosphere it does warming. Taking 100+ year to reach equilibrium concentrations still means having massive amounts of methane in the atmosphere doing warming, just at an equilibrium.

Because it breaks faster than CO2 is why we should really reduce the emissions today because it presents us a great opportunity to save more warming effect in the future

> Nevertheless, while methane may have a short atmospheric lifetime, its effects are not ephemeral provided the source of the methane continues to exist. For as long as livestock continue to be farmed, methane continues to exert a warming effect upon the climate.

> As such the argument that since methane’s impacts are temporary, they do not matter, is wrong. Its effects will in practice be permanent, unless ruminant production is halted.

> Methane emissions also increase the risk of us ‘overshooting’ the 1.5°C/2°C target, potentially tipping us into unknown climatic territory, with possibly devastating effects on agriculture, wildlife’s ability to adapt, heat stress in humans and animals, and more

https://www.oxfordmartin.ox.ac.uk/downloads/reports/fcrn_gnc...