It's not an existential crisis. That's not what the science says. There's no evidence that climate change will make humans extinct or end civilization. If that's not what you mean by existential crisis, then don't use that word because that's what it means.
It's anticipated that at least millions, more likely hundreds of millions of humans will die to the consequences of climate change including effects of extreme weather, flooding and (especially) famine. This won't extinct all of humanity but can be expected to severely impact civilization(s), with a chance of this impact being fatal to individual cultures.
For a prospectus of what could happen, I point to the large-scale devastation of (ancient) civilization attributed to the Sea Peoples (https://www.history.com/news/who-were-the-sea-peoples). They're believed to have toppled the Hittite Empire and severely weakened the Egyptian one. "some historians believe they had been displaced from their homeland by famine or natural disasters." -- the parallels to the anticipated effects of climate change should be clear.
Large-scale political unrest, very possibly including a WW3, is something I feel justified in considering "an existential crisis." Your mileage may vary.
100's of millions is not necessarily an existential crisis because it already happened in the 1900's due to war, disease, and famine. Even if that absolute scale of deaths happens again because of climate change, it will be proportionally smaller due to the global population being an order of magnitude larger.
Do you have a reference to that prediction of it probably killing 100's of millions of people? On it's own, it doesn't really mean much because what time scale and what's the no-climate-change number to compare to?
Famine, meteorites, large scale war, atomic bombs, infectious disease, the plague - all survivable by society at large. With your definition almost nothing qualifies as existential crisis.
That's just being pedantic and obviously not what the GP's post means. If it were, then walking in the garden is an existential crisis because you're bound to step on a bug and that individual will cease to exist.
