I'm sure you can find some individual scientists who want to get in front of a camera tot ell us it's the end of the world, just like you can some individual scientists who say it's all crap. I want to see what's in the IPCC's 99.9th percentile of things going wrong.
The last scenario talks about, if I recall correctly, the oceans becoming more welcoming to bacteria that produce hydrogen sulfide. Sorry, I don't remember the details, but will update tonight from the book at home if I remember.
Thumbnail sketch of the event: something (possibly massive volcanic eruptions near in time to a significant meteorite impact) deoxygenated the water enough to cause a substantial marine life die-off. The death of so much marine life in such a short time encouraged sulfate-reducing bacteria, which would have released significant amounts of hydrogen sulfide into the atmosphere. Hydrogen sulfide would have killed plant (and, therefore, animal) life and weakened the ozone layer, exposing the suriving flora and fauna to further UV radiation.
"96% of all marine species[5] and 70% of terrestrial vertebrate species becoming extinct."
There is some concern regarding a risk (I have no idea how big the risk is) that anthropogenic global warming could "tip the scales" of the global ecology towards the conditions that allow for a Permian-Triassic-style cascade.
> Assuming everyone understands that, it's an interesting case study in psychology as to why the second one might be scarier than the first one.
It doesn't make sense to base decisions on a worst case scenario without taking probability into account.