I don't claim an existential risk to humanity, I claim there's an existential risk to our current global civilization which manages to feed, cloth and shelter almost 8 billion people in largely tolerable conditions and which will need to find room for another 1.5 billion before global population peaks. If parts of the world where large numbers of people live become heat stressed to the extent that living in them is impossible without air conditioning and agriculture starts to become a lot less productive in currently fertile areas then the secondary effects of billions of people trying to find somewhere else to live in a short space of time are likely to be not good. I don't think we face extinction. I do think we face having to transition to a low carbon lifestyle. We can either do that in a peaceful way and maintain and even increase the standard of living for the majority of the human race or we can do nothing and face decades of major disruption that kills huge numbers of people.