ideas like existential risk are highly novel
Why is that so? The end of the world is a strong element in major religions and is a popular theme in literature and movies. The global warming meme made the idea that human activity can have significant planet-wide consequences be universally accepted.
Existential risk due to astronomical or technological causes, as opposed to divine intervention, is pretty novel. No one thinks global warming will end humanity.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.