It depends what you mean by an existential threat.

I think there's a reasonable chance that global warming (combined with other factors; biosphere degradation, resource depletion, unsustainable farming, lack of fresh water, increasing war over increasingly limited resources, ect), may cause our current civilization to collapse.

If our civilization collapses, what are the odds that we'll recover, and eventually get back up to where we are now? I don't know, but if our civilization collapses and we're left without modern tools in a world in the middle of an ongoing mass extinction that we started, things start to look really dodgy. In any case, we don't know what percentage of intelligent species go from being merely intelligent to having an advanced technology; it could be that we just passed The Great Filter in the past 200 years or so (say, at the moment the industrial revolution started), in which case losing that advance and passing back through it in the other direction would dramatically lower our chances of becoming a space-faring civilization.

Of course, if we reach a sufficiently high level of technology before the other problems I talked about kick in, then they're all solvable.