True runaway (i.e. oceans boiling / Venus) cannot happen on Earth unless you significantly increase incoming radiation stream (or alternatively halve the planet's albedo).

The runaway effect is scary b/c at certain temperature (~400K) atmosphere consisting predominantly of water vapor looses its ability to radiate out more heat up until 1600K.

[0] https://www.nature.com/articles/ngeo1892 (see fig. 2b) (edit: the figure: https://imgur.com/a/ytoEXzd)

edit #2: I've measured some pixels and the starting runaway temp is closer to 315K / 42C, damn