The skyworld I'm currently working on used to be an Earthlike planet until a supernatural disaster struck. Dense, translucent gas billowed from geysers on the surface until the ocean of fog covered all terrain below about 1 kilometer of elevation. This would have happened in the span of several years.
The gas is inert, non-toxic, and not an actual chemical, though the density is probably around 0.5 g/cm^3. The most anomalous property is that objects immersed in the gas experience a passage of time about 30 times faster than objects outside. Due to the density, I think Earth's atmosphere would get displaced upwards a bit(?)
The civilizations at the time of the disaster are at a medieval level of technology and the planet has a similar fraction of land above one kilometer as does Earth. Some polities will doubtlessly be able to evacuate and bring domesticated and wild animals with them. What I'm interested in is whether enough ecosystems will survive in the long-term to sustain these high altitude civilizations. Will alpine vegetation be enough to support ecosystems when most vegetation dies? Will animals with the mobility to reach high elevation and animals introduced be able to form a new ecosystem in the short time frame? And will humans be able to live off of it?