An existential threat to us is, as the name implies, a threat to our existence. Unknown domino effects which may or may not have changes so large that it causes

An existential threat to us is, as the name implies, a threat to our existence. Unknown domino effects which may or may not have changes so large that it causes massive swathes of humans to die off, forever changing the future of our species, seems to fit that description. How much more does it need to be existential to you?