I don't deny climate change, but what makes it a _an existential threat to humanity_? I understand that to mean wiping out every last human. Is that what is meant? If so, that I am definitely sceptical to.
No doubt it can cause mass starvation on a scale we have never seen before. But that is some else than an existential threat to humanity, unless exactly everyone starve to death.
True, humanity will likely survive. But our current civilization (and most of us) likely not.
You also have to account for the conflict that will arise if hundreds of millions of people will no longer be able to grow any crops where they live. They won’t just stay out and starve in silence.
With climate change droughts become more common and the capability of agriculture to feed all those people breaks down.