The runaway fashion isn't obvious to me. Once the ice is gone the albedo won't get worse, and atmospheric methane decays in some decades.

Oh my god, the ice "being gone" is basically the definition of catastrophe. If we get that far then it's way, way, way too late. Saying "once the is is gone the albedo won't get worse" is kind of like saying "eventually the plane that hits the mountain has to stop, and the crash can't get any worse".

And yes, atmospheric methane does decay in decades but the point is that before it does, it depletes sea ice and warms the ocean. The absolute worst-case scenario in a runaway climate model is something like the clathrate gun. [2]

> "One of the most eminent climate scientists in the world, Peter Wadhams, believes an ice-free Arctic will occur one summer in the next few years and that it will likely increase by 50% the warming caused by the CO2 produced by human activity" [1]

[1] : https://www.lifeworth.com/deepadaptation.pdf (sorry, not the primary source)

[2]: https://en.wikipedia.org/wiki/Clathrate_gun_hypothesis

Depends on what you mean I guess with runaway. Dinosaur age tropicals everywhere? Maybe not. Runaway as in deserts making their way into where they were not before, winters disappearing in many places we are used to having them, coastal cities having to pack up and move. Yeah, I can easily see that over a century or two.