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.